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North American Continent-Ocean Transects Program (1989)

Chapter: 6. The 23 Transects: Synopses, Findings, and Problems

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Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
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Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 35
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 36
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 37
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 38
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 39
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 40
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 41
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 42
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 43
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 44
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 45
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 46
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 47
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 48
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 49
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 50
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 51
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 52
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 53
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 54
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 55
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 56
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 57
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 58
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 59
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 60
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 61
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 62
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 63
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 64
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 65
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 66
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 67
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 68
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 69
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 70
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 71
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 72
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 73
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 74
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 75
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 76
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 77
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 78
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 79
Suggested Citation:"6. The 23 Transects: Synopses, Findings, and Problems." National Research Council. 1989. North American Continent-Ocean Transects Program. Washington, DC: The National Academies Press. doi: 10.17226/1521.
×
Page 80

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6 The 23 Transects: Synopses, Findings, and Problems TRANSECT A2 KODIAK TO EUSEOEWIM, A[ASEA Synopsis and Findings Transect A2 (Figure 1) extends NW from oceanic lithosphere of the Pacific plate across the convergent plate boundary of North America at the Aleutian Trench. Land- ward of the trench in western continental Alaska, A2 contains two tectonic regimes of sequential development (Figure 4~. The Chugach-Prince William terrane is an amal- garn of displaced terranes with Cretaceous and Paleogene ages of attachment to one another. The peninsular terrane includes the modern Aleutian arc which dates as far back as 42 mybp and comprises magmatic arc and forearc zones that are constructed partly on the older terrane assembly. The magmatic arc is on the Peninsular terrane, and part of the forearc is on the Chugach-Prince William terrane (Figure 4~. Cratonal North America does not exist in A2. Recent studies of the Aleutian forearc in A2 have led to major strides in un- derstanding the processes of the growth of this forearc. Teleseismic earthquakes are essentially absent between the trench axis and the edge of the shelf. Landward of the shelf edge, teleseismic earthquakes are concentrated in a zone that dips only 4° continentward beneath Kodiak Island at the inner side of the forearc. Northwest of the island, the zone dives at 35°- 40° to the roots of the volcanic arc. Thus, in the zone of most active tectonism at the edge of the North American plate, the rocks have insufficient strength and the coupling between plates is not enough to produce frequent large earthquakes. In seismic reflection sections that cross the Aleutian Trench axis and landward slope of the trench slope, a 2-km-thick and 2~km-Iong, little-disrupted sequence of stratiform sediment is subducted beneath a major decollement at the deformation front. The absence of significant stratal disruption (quarter wavelength or ~resolution" in the seismic record equals about 50 m) at depths greater than 4 km requires low friction. Such low friction is possible if pore fluids cannot readily drain from the subducted sequences and instead become pressured to near lithostatic levels. Given weak rocks in 34

35 I 160 156O arctic Ocean-Canada Basin :~ 136° 65° ~ (~) hi-. NO r t h ~ . .. :::: l VO L C A N O F S O F T H E | ..-.~..~,...~.... an: "-] 7 ~ ALF'JTI pry MAGMATIC ARCH CHUGACH PR I ~ CE it\ WILLIAM TERRACE = PC] ~ INSULAR TERRANE 1 YAK UTAT TERRA;'I- 60° OTHER TERR.~IES ~~ ~ soul . - i ~ a e By kanji ~k T.~ ~ Bunk FIGURE 4 Map of Alaska showing posiitons of corridors A2 and A3. i' .-1 Charlotte ~ walls of major thrust faults in the forearc, the lack of teleseismicity is not surprising, and the low friction along the plate boundary thrust faults may even extend to the depths indicated by the zone of infrequent teleseismic earthquakes. At the deformation front of the forearc, a decollement divides the subducted and accreted ocean basin sediment. The accreted sediment is tectonically thickened by tilting, folding, and imbricate thrusting to about twice its original thickness. Along the seismic section that A2 follows, the original 1.5 km- to 3-km-thick offscraped section is thickened to perhaps 4 to 6 km. Further thickening of the forearc requires another process because maximum landward rotation and active faulting are not observed in the mid- and upper-sIope areas. Since the seismic refraction dataindicate 10- to 12- km-thick sedimentary and metasedimentary rock beneath the forearc basins (between Kodiak island and the rr agmatic arc, Figure 2), we appeal to underplating to form about 5 km of the forearc thickness above the subduction zone. This contention is supported by onIand studies that indicate an underplated origin for the Cretaceous accretionary complex that crops out across most of Kodiak Island; early Tertiary rocks have been interpreted similarly. We propose that the Cretaceous

36 and early Tertiary underplated rocks were once about 10 km deep and were uplifted by younger underplating of Oligocene to Quaternary rocks. Underplated sedimentary rock is the most abundant protolith in the Aleutian forearc in A2. Sediment attached to the toe of the forearc by frontal accretion and sediment of forearc basins do not appear to be as readily preserved. The absence of frontal accretionary materials suggests their removal by tectonic erosion of the front of the forearc. An ancient example is the tectonic erosion along the Border Ranges fault, which is the contact between older terranes and the Aleutian forearc (Figure 4) where the front of the Permian to Early Cretaceous margin in the Peninsular Range has disappeared. Sirn~larly, it is proposed that the middle Eocene to middle Miocene margin front has disappeared within the Aleutian forearc, causing juxtaposition of Early Eocene and Pliocene accreted sediment along the midsIope of the present forearc. The foundation for the landward part of the Aleutian arc and parts of A2 north of the Aleutian volcanic chain (the backarc zone) is made up of five major terranes (Figure 4~. These include Precambrian, Paleozoic, Mesozoic, and in the seawardmost O ~1 ~ ~ ~ 1~ L ~ 1 _ 1 ~ · 1 1 ~ - r"~,~ rocks, r~pr~r~g con~nenta~, oceanic, suDaucr~on zone, and magmatic arc origins. The three northernmost terranes, all north of the modern volcanic chain, assembled to one another at their present latitude by Early Cretaceous time. Their boundaries, however, took up further displacement, mainly strike-slip, later in the Cretaceous, and they attached as a composite to terranes north of A2 in Late Creta- ceous time (Figure 4~. In contrast, the Peninsular terrane, which lies south of these in the Alaska Peninsula (Figure 4), was apparently far to the south in the Cretaceous and must have arrived at present latitudes in post-Cretaceous time. Moreover, the most seaward terrane (Chugach-Prince William), which is the Aleutian forearc, was apparently also well south (30° latitude) of its current position as late as Oligocene time. It contains Cretaceous rocks at its inner reaches; the rocks are generally younger seaward. The history of the northward movement and attachment of the southern two terranes is poorly known, as are the latitudinal positions of the earlier, mid-Cenozoic parts of today's Aleutian forearc. Moreover, the pre-mid-Cretaceous evolutions of all rocks within A2 probably represent histories at other, probably distant, sites elsewhere in the Pacific realm. TRANSECT A3 GULF OF ALASKA TO THE ARCTIC OCEAN Synopsis and Findings Transect A3 (Figure 1) traverses mainland Alaska between bordering oceanic lithospheres the Pacific plate at an active margin on the south, and the Amerasia plate of the Arctic Ocean at a passive margin on the north. Between the margins, Alaska in A3 is composed entirely of displaced terranes and at least two collapsed basins floored by oceanic lithosphere. The edge of Proterozoic North America lies well east of A3 (Figure 4~. The active margin in the Gulf of Alaska is fronted by a complex zone of right- oblique subduction. The toe of the continent is an accretionary wedge, the Aleutian forearc, comprised of thick Cenozoic cIastic sediments derived from Late Mesozoic accretionary wedges and accreted terranes in the Coast Ranges of northwestern North America. In the eastern Gulf of Alaska the accretionary wedge buttresses against the

37 Yakutat terrane (Figure 4~. In the western part of the Gulf, it buttresses against the Pacific plate. The transition between these structural domains is a zone of southeast- verging thrust folds. The Yakutat block, consisting of slightly deformed southward- prograding Cenozoic ciastic rocks overlying a continentalized Late Mesozoic accre- tionary prism on the east and PaTeogene oceanic crust on the west, is an example of a terrane now undergoing accretion to the continent's leacling edge. Pacific-North American displacements southeast of A3 are mainly strike-sTip and confined to the Queen Charlotte fault in Transect B1 (Figure 1~. Between B1 and A3, faults splay into the continent to the northwest and take up oblique convergence as the plate boundary curves from NW to SW around the Gulf of Alaska (Figure 4~. The forearc in the active margin in A3 is about 200-km wide, and comprises the Chugach, Prince William, and Yakutat terranes. In the western part of the Gulf of Alaska the forearc widens to 400 km, and there is a gap in the associated volcanic arc. The continental side of the forearc is apparently a north-dipping accretionary surface, the Border Ranges fault zone (Figure 4), against the south side of the amalgamated Peninsular and ATexander-Wrangellia terranes (the Talkeetna superterrane). This mainly early- and mid-Tertiary fault is a major splay of the plate boundary zone. Other major splays, which in general are younger southward and down section, lie within the forearc prism (accretionary wedge). North of the forearc and Border Ranges fault zone. there is a i.500-km-wide region , , ~ in A3 comprising displaced terranes with locally intervening wedges of deformed Cre- taceous flysch. The flysch probably represents wedges of ocean-floor sediment accreted from now-consumed oceanic lithospheres that separated terranes before their collisions. Attachment ages are mainly Cretaceous, and attachment structures are, in general, mid- and late-Cretaceous paleoobduction and paleosubduction zones and latest Cre- taceous and Tertiary right-sTip faults. The terranes include protoliths of highly varied tectonic kindred, including arcs, oceanic plateaus, and deep seated metamorphic rocks of continental affinity. A number of them have had large northward transport in or since the Cretaceous. Some terranes, such as the Talkeetna superterrane, apparently arrived in Alaska as a complete amalgam, but others may have accreted individually. The post-attachment disruption of terranes by right-slip faulting of the diffuse plate bounciary zone as far north as the southern foothills of the Brooks Range (Figure 4) creates difficulties in deciphering the earlier tectonic evolution. The northern region of the transect contains the deep Yukon-Koyukuk basin, of Cretaceous age, which was probably founded on oceanic basement and which is bordered by abducted ophiolite at both margins. North of the basin is the Brooks Range, which is underlain by a continental fragment, the North Slope subterrane of the Arctic Alaska terrane. This subterrane consists of little-deformed shelf and shelf basin deposits of Late Devonian to Neocomian age resting on deformed early Paleozoic sedimentary rocks. It is overthrust from the south by coeval outer shelf and slope deposits, Precambrian to Devonian metasedimentary rocks, and by Jurassic(?) ophiolite. North of the range is a north-vergent foreland thrust belt and foredeep of Cretaceous and Tertiary age which was deposited on the North Slope subterrane. Lace Early Cretaceous strata of the foredeep succession prograded northward across the passive margin that forms the boundary between the North Slope subterrane and the oceanic Amerasia plate of the Arctic Ocean. The progradational succession across this boundary, which is the present northern margin of Northern America, locally exceeds 13 km in thickness (Figure 4~. The Arctic Alaska terranes may have been derived from Arctic Canada and have

38 undergone about 70° counterclockwise rotation during the opening of the Canada Basin by seafloor spreading. The rifted northern margin of this terrane may represent predrift extension within northwestern North America. The thrusting at the southern margin of this terrane appears to be a consequence of convergence between the northward- drifting plates of the paTeo-Pacific, with its entrained lithotectonic terranes, and North America. TRANSECT B1 INTE1~ONTANE BELT (S1lEENA MOUNTAINS) TO INSULAR BELT (QUEEN CHARLOTTE ISLANDS) Synopsis and Findings Transect B1 (Figure 1) displays the structure of the active margin of the North American continent at and near the Queen Charlotte Islands of British Columbia and near the triple junction among the Pacific, Juan de Fuca (or Explorer), and North American plates (Figure 5~. The transect extends northeast of the active margin through the Insular and Coast Plutonic Belts to the Intermontane Belt (Figure 5) within the region of ancient displaced terranes. The Queen Charlotte fault is a major transpressive boundary between northwest- transTating, underthrusting oceanic lithosphere of the Pacific plate and the transitional lithosphere of North America that is comprised of terranes of Mesozoic attachment ages (Figure 5~. The obliquity of displacement is about 20. from strike-slip. Transpression apparently evolved from a transtensional mode in this area at about 6 mybp. The Queen Charlotte fault forms a backstop to an active accretionary wedge on the Pacific plate. Several active NW-striking strike-sTip faults occur inboard and parallel to the Queen Charlotte, implying that displacements are currently distributed at this active margin. The kinematic change at 6 mybp was associated with large uplift of the continental edge in the Queen Charlotte Islands and subsidence to the rear in the Hecate Strait (Figure 5~. South of the Queen Charlotte Islands are the triple junction and the Neogene Juan de Fuca plate, which underthrusts the margin with near normality (Figure 5~. The earlier tectonic evolution of the B1 region is registered in three principal stages: terrane accretion in the Mesozoic, transgression and magmatism in Cretaceous to Eocene times, and transtension from the Eocene to 6 mybp. The first stage represents tectonic progradation of the continental margin relative to nuclear Precambrian North America (farther east from B1, Figure 1), whereas the second, third, and modern stages record diverse effects of active margin tectonism on the earlier accreted terranes. Transect B1 includes four major terranes (Figure 5~; from the present continental edge eastward, they are the Wrangellia, Alexander, Stikine, and Cache Creek terranes. The first three apparently arrived at the edge of North America in sequence, from inboard out. The Stikine, composed of Late Paleozoic arcs, was attached to the Cache Creek, a subduction complex, in Late Triassic time, before the terrane pair collided with North America in mid-Jurassic time. The Alexander terrane, which consists of Late Paleozoic and Early Mesozoic arc or rift-related rocks, collided with the Stikine, then at the western edge of North America, later in Jurassic time. The Wrangellia terrane, composed of Late Paleozoic arc and Triassic oceanic plateau rocks, then underthrust and stuck to the Alexander in the Late Jurassic. The Coast Plutonic

39 a) \~\~\ I \ `~\ <° \\ I . . (' ~ 0 200 , 1 1 North PaCifi O ~ ~ ~ ~ Crato e Fuca \Plate | ~~o.W American ~ ° ~: ' ~ A=___ / ~ ~ '9~ \ B3 I ~ b) on \ I .. At\ W ~ · :~K <I Juan de Fuca ~ co lex cipix ''\ ~ / J \ Plate | ~4~ / ,, \ , ,, \, , ~ : , / / / // Cal cover 200 \ FIGURE 5 Map of western Canada and northwestern United States showing positions of corridors B1, B2, and B3.

40 belt (Figure 5) probably formed along the suture zone of the Alexander ancI Stikine terranes and may contain collision-related magmatic and metamorphic rocks. Post-accretionary active margin displacements appear to be concentrated in the Coast Plutonic Belt. These include probably eastward thrusting of the Coast Plutonic Belt onto rocks of the Intermontane Belt and the development of a major belt-parallel ductile clextral shear zone, the Work Channel Lineament of Cretaceous through Eocene ages of activity (Figure 5~. The vertical motions unroofed rocks of the western Coast Plutonic Belt as much as 25 km at rates up to 2 mm/yr. Magmatism, perhaps anatectic, and crustal thickening occurred to the east in the Intermontane Belt during this stage. During the third stage of Oligocene and Miocene duration, block faulting and magmatism were widespread in the Intermontane Belt. These are interpreted to have resulted from dextral transtensional motions at the plate boundary. TRANSECT B2 JUAN DE FUCA PLATE TO ALBERTA PLAINS Synopsis and Findings Transect B2 (Figure 1) spans the southern Canadian cordillera that lie between the oceanic Juan de Fuca plate on the west and cratonic North America in Alberta on the east. The cordillera are the transitional zone of western North America, and their structure includes the cumulative effects of 2 by of earth history. Modern tectonic features of the cordillera include the Cascade magmatic arc (Figure 5) and an accretionary wedge at the continent's leading edge offshore of Vancouver Island. Both features are relater! to the subduction below North America of the Late Neogene oceanic lithosphere of the Juan de Fuca plate, which is a remnant of the former, once extensive Farallon plate. The Neogene tectonic features are superposed on older structures of the cordillera that permit division of the cordillera into five strike-parallel belts (Figure 5~; from east to west, they are called Rocky Mountain, Omineca Crystalline, Intermontane, Coast Plutonic, and Insular belts. The Rocky Mountain Belt contains micI-Proterozoic to Tertiary mainly sedimentary strata on continental crust, the earliest being rift deposits that were followed by continental shelf and slope deposits of Cambrian to Jurassic age, and last, foreland basin deposits of Late Jurassic to Early Tertiary age. The Intermontane and Insular belts are mainly low-grade metamorphic and unmetamorphosed sedimentary and volcanic strata and comagmatic granitic rock of mainly Late Paleozoic to Tertiary ages. The older parts of these are derived largely from intraoceanic magmatic arcs and ocean basin deposits, and the younger, from continental margin arcs and erogenic ciastic basin deposits. Permian to Jurassic strata and Cretaceous intrusions in these two belts yield paleomagnetic and/or paTeontological data which imply that the rocks did not form in the positions they now occupy in the continental margin. The Intermontane and Insular belts contain most of the displaced terranes of the Canadian Cordillera. The Omineca and Coast belts are welts of high-grade metamorphic and granitic rocks of mid-Mesozoic to Tertiary ages that developed in regions of tectonic overlap and/or crustal thickening and incorporated the little metamorphosed strata in the flanking belts. The Canadian Cordillera in a general way is a two-sided orogen. East-directed Late Cretaceous-Paleocene thrusts occur in its eastern part; west-directed Tertiary

41 to Recent thrusts occur in its westernmost part associated with subduction of Pacific Ocean crust. The two belts of metamorphic and granitic rocks form the cores of smaller asymmetric but typically two-sided orogens and show complex polyphase deformation, mainly Jurassic in the east and Cretaceous in the west. Late Cretaceous and Early Tertiary dextral wrench faults, the latter related to widespread extension faults, occur in the central cordillera. Deep seis~ruc reflection studies are restricted to the structurally simple eastern and western margins. Gravity and seismic reflection studies along the line of the transect show a thickening of the crust from about 40 to 45 km under the Alberta Plains to a maximum of 50 to 55 km near the Rocky Mountain Trench, which separates Rocky Mountain and Omineca belts. West from here, the crust thins progressively to about 35 km under the Intermontane Belt and about 20 km under the Insular Belt. The region crossed by Transect B2 evolved in the following four main stages: (1) Rifting of the ancestral North American supercontinent in mid-Proterozoic (Belt-Purcell) and /or late-Proterozoic (Windermere) time. (2) ~ I ~ ~ Development and continuation of a long-lived, Cambrian to Jurassic passive margin, part ot which Is preserved In the eastern Cordillera, and arc magmatism at sites an unknown distance west of the margin. (3) Onset of convergence at or near the western edge of the sialic continent that began in Jurassic time with accretion of the Kootenay terrane (Figure 5) to North America. Subsequently, an assemblage of noncontinental terranes of the Intermontane and Omineca belts accreted to the Kootenay. Convergence continued into Cretaceous and Paleocene time, causing collision of the displaced terranes of the Insular Belt and development of the Rocky Mountain foreland thrust belt and related foreland basins. (4) Development of today's tectonic features by oblique convergence. These in- clude Eocene dextral wrench fault systems and normal faults with east-west extension. Part of the dextral slip may have occurred as early as Late Cretaceous time and indicates earlier phases of oblique convergence. The terrane assemblages of the Intermontane and Insular belts record tectonic histories that differ substantially from that of Precambrian North America. They indicate convergence in Late Paleozoic and early Mesozoic times within the Panthal- lasic/ancestral Pacific Ocean basin and in the Jurassic, near the margin of North America before attachment to the sialic continent. Upper Paleozoic and Lower Meso- zaic volcanic and sedimentary strata that are in terranes of the Intermontane Belt formed in offshore(?) arcs, related subduction complexes and backarc basins. The three terranes of the Intermontane Belt (Quesnel, Slide Mountain, Cache Creek) were together by earliest Jurassic time, and subsequently were accreted to Kootenay terrane in late Early Jurassic time at a suture in the Omineca Crystalline Belt. To the west, in the Insular belt, Paleozoic arc rocks, Triassic rift rocks and Lower Jurassic arc rocks form the extensive Wrangellia Terrane (Figure 5) which arrived with attached small oceanic ancl arc terranes. These were accreted to the new continental margin in Early to mid-Cretaceous time. The suture lies within the Coast Plutonic Belt, where voluminous bodies of granitic rock and local calcalkaline volcanics presumably record crustal convergence until the present day. Speculatively, the southern Canadian Cordillera can be interpreted to show several stages in the formation of continental crust, from the thick (40 to 55) km) old conti- nental crust in the east, through intermediate (30 to 35 km) crust beneath terranes

42 in the Intermontane Belt accreted to North America in the Jurassic, to thin (20 km) crust beneath terranes accreted in the Cretaceous in the west. Problems 1. Precambrian Rifting and Drifting: It is not clear whether supercontinent drifting followed rifting in mid-Proterozoic (Belt-Purcell, cat 1500 Ma) and/or in Late Proterozoic (Windermere, cat 800 Ma) times. The configuration of the Windermere succession suggests that drifting occurred in the Late Proterozoic, but the significance of the earlier basin development is uncertain. 2. Kootenay Terrane: The degree of allochthoneity of the Kootenay Terrane and the nature and significance of its Tower and mid-Paleozoic tectonic events are not understood. Comparable Reformational, metamorphic, and magmatic events do not occur in North American continental margin strata in B2, implying the Kootenay is allochthonous. However, similar rocks occur along much of the Cordillera, and it is difficult to argue that the Kootenay is greatly exotic. 3. Early Permian Accretion: The Kootenay and Slide Mountain terranes were joined in Early Permian time by thrusting. Is this a phase of the Sonoma orogeny that occurred in Early Triassic and perhaps earlier times to the south in Nevada (transects ClandC2~? 4. Permo-Mesozoic Plate Tectonics: What is the cause of the change between convergence recorded by intraoceanic magmatic arcs in the Late Paleozoic and Early Mesozoic, and convergence recorded by apparent collisions and arcs on the continental margin starting in late Early Jurassic time? Is this due to major changes of absolute plate motions, recorded elsewhere by the North Atiantic opening, or a change from old, heavy subducting oceanic crust to young, light crust? 5. Interpretation of Superposed Deformation: It is difficult to distinguish the effects of collisions of accreted terranes and subsequent, continuing deformation on the same site because of tectonic burial and crustal softening, from behind-the-arc shortening. 6. Question of Anatexis: Are some granitic rocks and their extrusive equivalents generated by tectonic burial and anatexis of transitional and continental crust, aside from those generated in subduction zones? 7. Dip of Id Subduction Zones: Dip directions of possible subduction zones prior to Cretaceous collision in the Coast Plutonic Belt are not known. 8. Strike-Slip Displacements: What is the magnitude of possible postaccre- tionary or intraplate dextral strike-slip movements within the Canadian segment of the Cordillera? Recent paleomagnetic results suggest that mid-Cretaceous plutons within the southern Coast Plutonic Belt and northern Cascades were intruded and cooled at least 2400 km to the south of their present positions. What was the locus of this movement? There is structural evidence using offsets of markers for dextral displacements in the northern Canadian Cordillera in excess of 1000 km, but at the latitude of the transect the only clearly recognizable strike-slip fault is the Fraser River Fault System with offset of 80 to 110 km. 9. Deep Structure: It is impossible without further seismic reflection work to do other than speculate on the deeper crustal structure in Interrnontane and Coast Plutonic belts. In the former, there is evidence for a least four deformations: local, probably Triassic-Jurassic deformation (in lower crust of the early Mesozoic arc?; west-directed, mid-Jurassic(?) deformation on the west side of the Omineca Crystalline

43 Belt (Louis Creek Fault and kindred structures); east-directed Late Jurassic structures on east side of Intermontane Belt (thrusts near Cache Creek); and Early Tertiary transtensional deformation. What is the nature of the deep fabric of the Intermontane Belt, and what proportion of it is contributed by each of these structures seen at the surface? 10. Depth of Strike-Slip Faults: What is the termination at depth of such major strike-slip faults as the Fraser River fault system? TRANSECT B3 JUAN DE FUCA SPREADING RIDGE TO MONTANA THRUST BELT Synopsis and Findings Transect B3 (Figure 1) comprises two corridors. The northern (B3n) extends from the subduction zone between the Juan de Fuca and North American plates on the west, across the cordillera of northern Washington and Idaho to cratonal North America in Montana. The southern (B3s) runs east from the spreading ridge between the oceanic Pacific and Juan de Fuca plates across the Columbia Plateau of southern Washington to the foreland thrust belt in Idaho. The objective of the two corridors is to display differences in the structure of the continental edge and the cordillera along strike. Active tectonic regimes (Figure 5) are the eastward subduction of the Juan de Fuca plate below and accretion of sediment to the western edge and base of the modern continent. The Cascade magmatic arc lies on the continent east of the subduction zone, and behind it is a broad zone of modern extensional faulting, uplift, and magmatism that may represent backarc rifting. The modern tectonic regimes are superposed on mid-Cenozoic and earlier tectonic provinces that are on land and extend west of the present shoreline. These occur in a broad region of ancient displaced terranes from the Puzet Lowlands east to ~ , ~ . ... . · . . . . . . . .. . . · .. . , · ~ . , . eastern Washington and western Idaho and to the boundary With Precambrian North America and its suprajacent exotic(?) and thrusted foreland cover (Figure 5~. Ages of attachment of the displaced terranes to North America and to one another and the ages of foreland thrusting are Late Mesozoic and Early Cenozoic. This reflects convergence of oceanic and other lithospheres below western North America for most of Mesozoic and Cenozoic time. During this active margin phase, the leading edge of continental North America has grown westward and thickened greatly as a forearc by two repetitive processes: the collision of terranes and the underplating by ocean-plate sediments and under- lying oceanic crust. Examples in the northern corridor are as follows. In the Late Cretaceous, a thick (up to 15 km) pile of nappes, derived from the edge of North America, was driven westward onto the Wrangellia terrane (Ban, Figure 5) as it collided with the continental margin. In the Eocene a thick (>15 km) terrane of seamount basalt was accreted to the expanded continent. Subsequent convergence has carried submarine-fan and ocean-plate sediments beneath these accreted terranes, and panels of sediments and oceanic crust have been sliced off the descending plate and plated as imbricate stacks and duplexes on the base of the tectonically prograding continental front. Active Scraping and frontal accretion continue at present. In B3s, the Puget Lowland (arc-trench gap) is underlain by an 8 km-thick slab of Paleogene

44 seamount basalts that was accreted in the Eocene. This slab overlies a zone up to 25 km thick consisting of imbricated fault-bound panels of submarine fan turbidites and oceanic lithosphere. The sediments, originally deposited along the continental slope, were subducted and underplated beneath the accreted seamount terrane. As in Ban, 40-km-thick crust west of the magmatic arc comprises both accreted terranes and underplated materials. These interpretations about the structure and constitution of the continental edge are prompted by geophysical data indicating that the Moho is 35 to 40 km creep where it intersects the descending Juan de Fuca plate beneath the forearc region about 200 km east of the deformation front just west of the subduction zone. Thus, young continental crust of full thickness has developed in the last 100 my near a consuming plate boundary. The unexposed basement beneath the magmatic arc and the western and central Columbia Plateau basalt is interpreted to consist of the southeastern continuation of the thrust and nappe system that is exposed in the western Cascade Mountains (Figure 5~. A major fault or fault zone separates this composite terrane from the separate and distinct Wallowa-Seven Devils terrane that underlies the eastern plateau. The Wallowa-Seven Devils terrane was juxtaposed with Precambrian sialic crust of North America along a steep, crustal-scale fault zone that was last active in mid-Cretaceous time. The westernmost extent of sialic Precambrian North America lies buried west of the Kettle and Okanogan domes in eastern Washington (Figure 5~. The clomes are structural cuIrn~nations that expose amphibolite-grade rocks that are interpreted to belong to the Precambrian continent. They are overlain by terranes of oceanic and arc- related rocks that were thrust eastward across the former continental edge, probably in mid-Jurassic time. The domes are unroofed by Tertiary low-angle extensional faults, some of which reactivated earlier thrusts. The westward-tapering wedge of Precambrian North American crust beneath eastern Washington may include a cryptic west-dipping thrust that extends east an the sole fault of the I`ate Jurassic to Late Cretaceous Montana or Rocky Mountains- FoothilIs thrust belt (Figure 5~. In eastern Washington, the hanging wall may include, together with Precambrian crystalline basement, all of the exposed Belt. Supergroup of mid-Proterozoic age at this latitude. The footwall probably consists of basement in Washington and basement together with Belt and platformal Paleozoic cover in Montana due to ramping of the sole fault to horizons within the stratified rocks. In the frontal part of the thrust belt, in Ban, only Paleozoic cover is imbricated. This differs significantly from the thrust belt geometry in the southern Canadian cordillera (Transect B2) where Paleozoic and Proterozoic strata are widely imbricated together above a sole fault that is localized near the top of crystalline basement. Problems 1. Locally in the frontal part of the accretionary wedge, thrusts and ramp folds verge landward rather than in a more typical seaward direction. The mechanical parameters or physical properties responsible for landward vergence are unknown. 2. The northern corridor crosses several major strike-slip faults (e.g., San Juan, Straight Creek) that dip vertically at the surface and that have estimated displacements of tens of kilometers. It is unknown whether the faults continue vertically downward!

45 to end at the Moho, offset the Moho, or flatten into mid-crustal zones of dislocation or detachment. 3. In the northern corridor, the western edge of Precambrian sialic North America is not well located. Strontium-isotope data are inconclusive, and most of the crystalline rocks west of known Proterozoic North American sedimentary rocks are poorly dated. 4. The identity and structural configuration of rocks responsible for a positive gravity anomaly west of the Purcell anticlinorium are unknown. The Belt Supergroup is possibly underlain by either a parautochthonous wedge of Paleozoic carbonates, or a displaced slice of Precambrian crystalline basement. 5. The nature and orientation of the terrane boundary beneath the central Columbia Plateau are unknown, as are the amount and sense of displacement along what must be a major fault zone. TRANSECT C1 MENDOCINO TRIPLE JUNCTION TO NORTH AMERICAN CRATON Synopsis and Findings Transect C1 (Figure 1) crosses the cordillera of the western United States from the Mendocino triple junction off the coast of northern California to the craton in Wyoming. The tectonic record of C1 (Figure 6) contains events throughout Phanero- zoic time and comprises the following stages: (1) rifting and drifting at the end of the Proterozoic; (2) maintenance of a passive margin together with episodic deformation in Paleozoic and deformation plus accretion in Early Triassic times; and (3) onset of active margin tectonism that has existed to the present and includes accretion of terranes, continental arc magmatism, foreland thrusting, and distributed strike-slip deformation with senses of slip and obliquity that have varied with time and place. East of the Cascade arc and its recently extinct southerly prolongation, active tectonism is manifested by a broad region of regionalized uplift as great as 1.5 km in the last 10 my or less. The region includes the Sierra Nevada, Basin-Range, Colorado Plateau, and Rocky Mountains (Figure 6~. The uplift is apparently related to shallowing of the asthenosphere and advection of mantIe-generated magma. Within the uplifted region, the Basin-Range province, which presents the most conspicuous deformation, is a broad region of rifting, volcanism, high heat flow, high seismicity, and low-velocity upper mantle. Extension in the Basin-Range, regionally east-west, is highly heterogeneous. It is probably at a maximum in eastern Nevada where mid- crustal rocks (core complexes) are exhumed by low angle normal faults. These may also be sites of maximum volume addition to the crust by mantle-derived magma. The Neogene uplift and extension of the cordillera may be related to backarc rifting and to disuse plate boundary shear linked to the San Andrew transform. The backarc regime began as far back as 35 mybp whereas the transform-related regime is <10 my. The older Phanerozoic tectonic evolution in C1 begins with rifting of the Precam- brian supercontinent in two or more episodes in the Late Proterozoic, consummated by the onset of drifting and subsidence of an unusually broad shelf at about 500 mybp. The present edge of Proterozoic North America probably lies in west central Nevada (Figure 6) below thick sedimentary and tectonic cover; the edge in C1 may be closest in western North America to the original 600 my drift juncture, reflecting a rn~nimum of tectonic erosion of the continental margin at this latitude.

46 'I -my : ~ ~c, I\ Cenozoic cover `\ terranes it, \N >~ ;~ Sonora i: Hi :~; ~ ~ I FIGURE 6 Map of western United States showing positions of corridors C1, C2, and C3. A passive margin was maintained from Cambrian to m-Triassic time, although repetitive collisional events occurred at the margin which emplaced major allochthons of oceanic and bas~nal rocks eastward at least 100 km across the continental slope and outer shelf. The older of the passive margin allochthons (Figure 6) was emplaced in Early Mississippian time, the later in the Early Triassic. Both passive margin orogenies probably represent arc-continent collisions, although the magnitude of arc translation before collision is disputed. The active margin to western North America in C1 began in mid-lYiassic time and continues today. In the early Mesozoic, a continental magmatic arc developed above the earlier terranes in C1 (and above Proterozoic North America to the south in Transect C2) and above a subduction zone that was probably in the present Great Valley (Figure 6~. Active margin events west of the continental arc have been mainly the accretion of terranes and phases of strike-slip faulting and rifting among the ter- ranes. East of the continental arc, events consisted of extensive foreland thrusting and

47 moderate magmatism but only minor unroofing in the Mesozoic and major extension and rifting in later Cenozoic time. In the Coast Ranges (Figure 6), the oldest recognized event is the Middle Jurassic (165 my) formation of the Coast Range ophiolite, probably in a backarc basin of an offshore arc. The arc and its cover were thrust above the edge of North America (then the western flank of the continental arc) in the Nevadan orogeny at about 150 mybp. Attachment of Eranciscan-type terranes outboard of the Coast Range ophiolite may have followed the Nevadan orogeny. The discrete Early ant] mid-Cretaceous ages of blueschists in such terranes suggest sequential collision of preassembled terranes rather than continuous accretion at the continental edge. Attachment of early Franciscan terranes, mainly melange' was followecl by right-slip faulting between 50 and 85 mybp within the terrane assembly, perhaps in response to passage of the Kula-Farallon triple junction. Such faulting further dismembered the Franciscan melanges. Unclerthrusting and attachment of younger Franciscan terranes resumed at 38 mybp. The Sierra Nevada and Klamath Mountains (Figure 6) witnessed copious continen- tal arc magmatism from Late Triassic through Cretaceous times and record collisional tectonism. Accretion of several terranes to early Mesozoic North America in the Kla- maths occurred between 180 and 150 mybp. The accretionary surfaces for each were subplanar east-dipping thrusts. The final phase of such accretion occurred during the Nevadan orogeny. It is uncertain whether all the terranes assembled elsewhere and then collided with North America in the Nevadan orogeny, or each terrane attached in sequence from east to west with the Nevadan orogeny as the culminating event. In the Sierra Nevada, terranes were also accreted to the edge of early Mesozoic North Amer- ica during or before the 150 mybp Nevadan orogeny, probably above a west-dipping thrust. Obduction of the Coast Range ophiolite onto earlier terranes of the Sierran foothills climaxed this sequence. Terranes of the Sierra and Klamaths are not easily correlated. On the eastern side of the Sierran-Klamath continental arc in C1, active margin tectonism caused an unusually broad realm of thin-skinned foreland contraction in Early Jurassic to Eocene time. The realm extends from the backside of the arc east to the hinge between the craton and subsiding shelf of Paleozoic North America (Figure 6~. Within the reaIm, thrust belts occur in several discrete strands with varied vergence; such beTts are probably positioned by pre-existing declivities in and above the top of the basement. Between strands there is probably a throughgoing detachment that connects displacements across the foreland. There is little recognized in~rolvement of basement. Minor piutonism, perhaps anatectic, occurred in two short episodes (160 and 80 mybp) during foreland contraction. Displacements in the cordilleran foreland thrust belts in C1 have little evident contemporaneity with collisional events to the west at the continental edge, although timing of events is not well known in either realm. The cordilleran foreland realm differs from others in the worId by minimal hinterward increase in unroofing and metamorphism. The classic Laramide orogeny, which is restricted to central and eastern Wyoming in C1, differs from the foreland contraction just discussed. The Laramide is thick- skinned, brings basement over cover, and occurred in the PaTeogene mainly after contractile events to the west. The Laramide may differ also in tectonic origin and relate to rotation of the Colorado Plateau. . . . .~.

48 TRANSECT C2 CENTRAL CALIFORNIA OFFSHORE TO COLORADO PLATEAU Synopsis and Findings Voluminous batholithic activity of the continental arc in the Sierra Nevada that was related to Franciscan subduction began between the axial Great Valley and western Sierra metamorphic belt at about 130 Ma with the emplacement of mainly gabbroic to tonalitic plutons. Subsequent magmatism swept eastward and became more granitic with an ~85 Ma termination along the eastern Sierra. Remnants of Triassic and Jurassic batholithic rocks are split obliquely by the major Cretaceous batholithic belt.

49 The Late Cretaceous forearc basin was built in part across the Early Cretaceous miasmatic arc. v Remnants of foreland basins related to the Antler and Sonoma progenies are present in central and western Nevada, and major Jurassic to earliest Tertiary fore- land deposits are present in Utah. Mesozoic foreland deformation was distributed along three well-defined belts in Nevada and Utah, and each may root into a com- mon decollement. There floes not appear to be a direct relation between foreland deformation and marginal accretion in the Mesozoic. Modern transform motion of the San Andreas plate juncture is distributed over a zone of ~100 km width with a possible minor component also hidden within Basin and Range extensional motion. The Salinia sialic crystalline terrane and Late Cretaceous and Early Tertiary accretionary prism complexes are amalgamated onto the western edge of the Pacific plate and are thus sliding northward along the transform juncture. Such phenomena in the tectonic attrition of sialic fragments and earlier ensimatic accretionary complexes are believed to have been a first order process interspersed with accretionary events along the Mesozoic California margin as well. Mid-Cenozoic extensional tectonism displayed in C2 by the Snake Range and related "metamorphic core complexes" offers an opportunity to view mid-crustal pro- cesses in extensional regimes. Fundamental questions raised by such exposures are whether similar structural patterns are currently developing beneath the widespread graben-horst structures of the Basin and Range; do major decollements develop by large-scale pure shear or simple shear; might Cenozoic decollements be reactivated Mesozoic foreland thrusts; and what is the role of magmatism during extension? TRANSECT C3 PACIFIC ABYSSAL PLAIN TO RIO GRANDE RIFT Synopsis and Findings Transect C3 (Figure 1) contains the following modern tectonic provinces, in east- ward succession (Figure 6~: a region of the Pacific plate that comprises a seamount province on abyssal oceanic crust and displacing terranes of the continental border- land, including a batholithic belt in the Peninsular Ranges; a transform rift floored by nascent oceanic crust that is the Pacific-North American plate boundary; the greatly extended Basin and Range province, here in its widest zone in North America and developed mainly in former cratonal lithosphere; and the stable craton. Older terranes exist both east and west of the plate boundary. Their histories of attachment to one another and to Proterozoic North America have been complex, beginning in the Cretaceous. The superposition of the modern and Neogene tectonic provinces on these has yielded extreme structural heterogeneity along C3. The Phanerozoic tectonic evolution of C3 is contested from two main perspec- fives regarding the degree of allochthoneity of terranes outboard of Proterozoic North America. The concept of parautochthoneity derives from geochem~cal and petrologic relations that seemingly link aspects of terranes across the full length of the transect and implies only small displacements. Conversely, the concept of great allochthoneity stems from stratigraphic relationships and paleomagnetic data from some terranes that imply several thousand kilometers of transport. The C3 transect group includes spokesmen from both viewpoints, but the perspective of great allochthoneity prevailed. The principal findings in Corridor C3 are as follows:

50 Precambrian Rocks: Five distinct suites of Precambrian rocks are inferred to exist in C3: (1) stable cratonal rocks east of the Rio Grande Rift; (2) block-faulted and attenuated "cratonal" rocks between the Rio Grande Rift and the left-slip fault of the Mojave-Sonora discontinuity, (3) and (4) exotic Precambrian rocks of a terrane that occurs both between the Mojave-Sonora megashear and the San Andreas fault and west of the San Andreas (Figure 6), and (5) cryptic Precambrian rocks from a terrane in coastal southwestern California (Figure 6) inferred from the geochern~stry of suprajacent rocks. Rifting and Truncation: Gradients in thickness and facies among Lower Paleozoic and latest Precambrian North American continental shelf sediments in Nevada, north of C3, imply Late Proterozoic rifting and the onset of drifting at about 600 mybp. Only a thin sequence of Paleozoic platformal cover strata occurs in Proterozoic North America in C3, however, and the thick subsiding shelf sections present in Nevada are missing. This suggests that a major tectonic truncation of the craton occurred in C3 alter the Paleozoic, possibly associated with Jurassic left slip on the Sonora-Mojave discontinuity. A third phase of rifting and right slip translation exists in the Salton Trough (Figure 4) at the North America-Pacific plate boundary. Detachment faulting and associated basin-range faulting indicate widespread rifting of the craton in C3 and the possibility of drifting or truncation at some future tune. Terrane Amalgamation, Accretion, and Dispersion: The stratigraphy of sedimen- tary overlap sequences and the distribution of stitching plutons chronicles a history of amalgamation, accretion, and dispersion of displaced terranes. The history is a contin- uous series of tectonic events surrounding the coastwise translation of terranes that has been ongoing for the past 100 Ma. Detritus derived from the continent lies within all the terranes, a circumstance that requires contiguity with the craton. Paleomagnetic data indicate that terranes amalgamated in southern latitudes, generally in present Central America. Two accretion events occurred in the Tertiary: (1) the 55 Ma arrival of the composite Santa Lucia-Orocopia allochthon, and (2) the Miocene accretion of the Baja-Borderiand composite terrane. The modern San Andreas fault system is the youngest displacement zone causing terrane dispersion. ^. .. _% . . ... Problems 1. To what extent did the rifting and drifting in the Caribbean and Gulf of Mexico influence the history of the Bisbee Basin and Chihuahua trough of southern Arizona and northern Mexico? 2. What is the driving force that affects intraplate thrusting, e.g., terrane accretion, subduction polarity, Benioff angle? 3. What ~ the geological framework of northwest Mexico and how does it correlate with rocks of the basin and range? 4. What is the geometry and kinematic history of the Mojave-Sonora megashear in the Mojave Desert area? 5. What is the tectonic significance of the Pelona-Orocopia schist: direction of vergence, age, depositional setting of protolith, and conditions of metamorphism? 6. What is the paleogeography of the Late Mesozoic batholithic rocks of western North America—one exceptionally long continuous magmatic arc, a variety of double volcanic arcs, or many unrelated arcs that are now juxtaposed?

51 7. What are the age and tectonic significance of the McCoy Mount awn Forma- tion? Was it deposited within an intracratonic rift or did open-ocean conditions exist to the southwest? tion? 8. Tertiary volcanic events: what controls their chern~stry and spatial distribu- 9. How deeply rooted is each of the terranes? 10. What kinematic and dynamic models best explain the large-scale rotations recorded for parts of southern California? 11. Are the large-scale northward translations real as required by paleomagnetic data from all the terranes of southern California? 12. What tectonic model best explains the mid-Tertiary extension in the Basin and Range province and the local regions of marked stretching manifest by detachment faults and core complexes? TRANSECTS D1 TO D4 EASTERN CANADA AND NORTH ATLANTIC OCEAN Synopses and findings of each D Transect are discussed separately; the problems occurring in them are treated together. Transect D1: Northern Appalachians: (West sheet) Gren~rille Province, Quebec, to Newfoundland; (East sheet) Rifted margin offshore northeast Newfoundland Synopsis and Findings Transect D1 displays the full width of transitional lithosphere in eastern Canada between the craton composed of unmodified Grenvillian basement and an arm of Atlantic oceanic lithosphere in the southern Labrador Sea (Figures 1 and 7~. The Phanerozoic evolution of the continent-ocean transition in eastern Canada occurred in three phases. The first included rifting, drifting, and the development of a passive continental margin with the lapetus Ocean in Late Precambrian and earliest Paleozoic time. The second phase saw the closing of lapetus and the emergence of the Ap- palachian orogen through collisional tectonics in Ordovician and later Paleozoic times. The third phase began with rifting in the Appalachian orogen late in Triassic time, followed by opening of the Atlantic basin from the Jurassic to the present. The opening of lapetus is recorded in western Newfoundland (Figure 7) by rifting of the one-billion-year-old Grenvillian basement, accumulation of thick cIastic se- quence, and basaltic magmatism in Late Precambrian time. Following the rift-related events, drifting and the thermal subsidence of the continental shelf is indicated by facies changes in the sedimentary cover of Early Cambrian to latest Early Ordovician age. These thicken eastward across the shelf and grade upward from immature to mature siliciciastic rocks to platformal carbonate rocks. Contemporaneous sediments of the slope and rise turbidites, hemipelagites, and sedimentary breccias are now in allochthons above the carbonate shelf strata. Collisional tectonics at and above the passive margin of North America in New- foundIand caused the effacement of the lapetus basin and the protracted development of the Appalachian orogen. They began near the end of Early Ordovician time, prow ably by arc-continent collision. The subduction zone apparently dipped away from

52 ,~ 60° sh° S2° ~ ~ — 0, 2 LL, 1~ ~ 56° it/ Use ~ /—\ rSE.~IU~TS: ~ fN ~ ~ i c EDGE NORTH WAR I CAN CONT I NENT /' SO `~? we a ~ 48° FIGURE 7 Map of eastern Canada showing positions of corridors D1 to D4. North America, which was overridden by exotic terranes by at least 75 km. Subse- quent collisions occurred in Paleozoic time, causing further growth of the Appalachian orogen by deformation of the continental margin and growth of the continent by accretion of terranes. The first indication of instability is indicated by an ancient karst topography developecl across an upwarped carbonate shelf. Later subsidence is recorded first by the deposition of deeper water carbonates across the disturbed bank, then by a flood of cIastic rocks from the east. These were structurally overri(lden by a sequence of contrasting rock assemblages in separate slices. The structurally lowest slices consist of sedimentary rocks from the nearby continental margin, and the highest ophiolitic slices represent farther travelled oceanic crust and mantle. The structural

53 pile propagated from east to west, possibly through accretion successively of landward sections from the subsiding continental margin. The present contacts between the structural slices are marked by thin zones of shale melange. In Newfoundland, Precambrian North America is circumscribed progressively out- ward by three large terranes, all of unknown paleogeography with respect to North America. From west to east, these are the Dunnage, Gander, and Avalon terranes (Figure 7), An additional, more easterly terrane, the Meguma, occurs in mainland Nova Scotia. Island arc rocks of the Dunnage terrane overlie an ophiolitic substrate. The Dunnage is an example of a Lower Paleozoic island arc built upon the crust of Iapetus. The Gander terrane contains a thick elastic sequence that may be a miogeocli- nal apron along the west flank of the continental Avalon terrane. The Gander terrane may have once bordered a continental craton but it is nowhere linked to a craton now. The Avalon terrane contains probable Grenvillian basement, Late Precambrian thick sedimentary and volcanic cover, and overlying Cambrian shales with Atlantic or European faunal affinities. Its cover probably records rifting and subsidence in the growth of lapetus, but it is uncertain whether Avalon existed as a fragment within lapetus or at its eastern margin and whether Avalon emerged from the same or a very different paleolatitude than its current position. Important findings on the relation of surficial and deep crust al structures have e Verged from analysis of seismic reflection sections that extend from the craton across Newfoundland. Crusta] blocks defined by distinctive deep reflection sets do not coincide with major terranes defined at the surface, except for the Gander-Avalon boundary, which is the Dover fault. The vertical Dover fault penetrates the crust, and the Avalon terrane has deep lithospheric underpinnings. Subsurface data confirm that Grenvillian basement extends eastward about 70 km beneath the ophiolitic Dunnage terrane but also indicate the sialic crust of the Gander terrane extends westward to the subsurface edge of Precambrian North America. Thus, a collisional suture exists in the lower crust beneath the Dunnage terrane. The opposing vergences of structures on opposite sides of the Dunnage terrane coupled with its thin-skinned geometry suggest that the collision of Gander and North America was normal to their mutual boundary. In contrast the nature of the Dover fault suggests oblique transport of the Avalon terrane with respect to the Gander. The appearance of cosmopolitan faunas in Middle Ordovician rocks of the Ap- palachians, and in Middle Ordovician rocks of the Caledonides of Scandinavia on the opposite side of lapetus, requires a breakdown of faunal barriers. This implies proxim- ity of terranes across the lapetus tract, and it agrees with the Ordovician destruction of some of its continental margins. Widespread occurrences of Late Silurian and Devonian terrestrial rocks throughout most of the Appalachian-CaTedonides orogen, coupled with the wide extent of middle Paleozoic orogenesis, indicate the almost com- plete destruction of lapetus at this time. Restricted Late Paleozoic marine deposits and limited extent of Late Paleozoic deformation in the Appalachian-Caledonides oro- gen signify the closure of remaining narrow seaways and structural tightening of weak crust al areas. Stratigraphic and sedimentologic analyses of the Appalachian orogen in Newfound- land indicated that it built up from the miogeocline outward by three accretionary events. These are dated stratigraphically as Early to Middle Ordovician, Silurian- Devonian, and Carboniferous-Permian; and these times of accretion coincide with the main erogenic episodes affecting the system, namely Taconian, Acadian, and Allegha- nian, respectively. The boundaries between the first accreted western terranes are

54 subhorizontal to moderately dipping zones marked by ophiolites and melanges, im- plying head-on subduction and abduction. Later boundaries between eastern terranes are steep ductile shears and brittle faults, implying transcurrent movements. Late Paleozoic transcurrent motions may have modified the early Paleozoic boundaries. Accretionary analysis of the Appalachian orogen in Newfoundland indicates Early to Middle Ordovician linkages between the North American miogeocline and Dunnage terrane, and between the Dunnage and Gander terranes. The Avalon terrane was aclded later. It lacks Ordovician linkages and effects of Ordovician Taconian progeny, and its latest time of accretion is defined by stitching to the Gander terrane by Devonian _1_ be_ _ mt ~ 1 · 1 ~ 1l ~ I . ~ p~u~ons. ~ ne ounces between one nest accrerea western terranes are subhorizontal to moderately dipping zones marked by ophiolites and melanges, implying heaxl-on subduction and abduction. The Gander-Avalon terrane boundary is a steep mylonite zone, suggesting transcurrent movement. The third and modern phase of development of the continent-ocean transition in the Grand Banks region (Figure 7) of Canada began with rifting that started in Triassic time with incipient breakup between Africa and North America. A second rifting phase in the Early Cretaceous or the Late Jurassic created the rifted margin east of the Grand Banks in response to plate motions between Iberia and North America. Drifting between northwest Europe and North America created the margins northeast of Newfoundland in Late Cretaceous time. The Mesozoic history of this area is complex because of the variety, proximity, and geometry of plates In continental breakup during the Cretaceous, and because rifting appears to have taken place over a Tong time interval, about 50 Ma. South of Newfoundland, early Mesozoic opening parallels the axis of the latest compressional event related to lapetus closure. North of the Grand Banks, the axis of opening developed two branches at an Early Tertiary triple point in the southern Labrador Sea. The northwestern arm of this triple junction cuts obliquely across older structures. Where Mesozoic-Cenozoic rifting cuts across the Appalachian orogen in northeast Newfoundland and the Grenville structural province of Labrador, some Paleozoic ter- rane boundaries and Precambrian structural features propagated eastward, and their prolongations coincide with offsets in the present margin and major oceanic fracture zones. Most obvious is the prolongation of the Dover fault into the Charlie Fracture Zone (Figure 7~. The importance of the former as a Paleozoic terrane boundary is apparent from the contrasting surface geology of the Avalon and Gander terranes as well as their deep crustal contrasts. Similarly, the prolongation of the Precambrian Grenville structural front is expressed in the Cartwright Fracture Zone (Figure 71. Surprising, the fundamental Avalon-Meguma terrane boundary is not expressed in a rift or transform margin, although it localized the Fundy rift basin and the Orpheus depression. The Tail of the Bank at the Grand Banks transform margin is a modern promon- tory that paraBels the Paleozoic St. Lawrence Promontory of the Appalachian orogen. Furthermore, the "transform" linking the St. Lawrence Promontory to the Quebec Reentrant lies along the prolongation of the older Labrador Trough, which may repre- sent a much older (Aphebian) continental margin. Although inheritance and ancestral controls are implied among these features, there are no actual structural breaks that can be traced from one to another. The Orphan Basin, crossed by D1, is a broad

55 depression, partly filled with sediments, which is bounded to the east by Orphan Knoll (a continental fragment) and to the north by the Charlie Fault Zone, created along the seaward continuation of the Dover fault. To the south lies the Flemish C)ap, another continental fragment similar to that of Orphan Knoll. Thus, the fragmented nature of this margin is quite different from the relatively sharply rifted margin south of the Grand Banks. The subsidence history of this margin is difficult to interpret according to a simple cooling model. Very rapid subsidence occurred in the Eocene, with little or no tectonically controlled subsidence thereafter. This rapid subsidence may be related to decoupling between Rockall Plateau and Orphan Basin, between which the northern transform boundary of the latter developed. The deep structure of Orphan Basin (Figure 7) is characterized by crustal thinning, and by a high velocity lower crustal layer. The thin crust is consistent with extension of the lithosphere during rifting. The lower crustal layer may be a product of magma generation during rifting that has intrudecL or underplated the thinned continental crust. Orphan Knoll separates the Orphan Basin from the deep ocean. It appears to be a large horst, stranded at a distal point with respect to the basin. A fairly sharp transition from continent to ocean occurs to the east of Orphan Knoll. A large positive gravity anomaly lies over the outer shelf, landward of the Orphan Basin. It is important to recognize that this anomaly does not mark the ocean-continent transition which lies several hundred kilometers farther east. Such observations at modern margins suggest caution when using gravity to define ancient ocean-continent transitions. Transect D2: Transform margin south of Grand Banks: Offshore eastern Canada Synopsis and Findings Transect D2 is designed to illustrate the nature of the transitional region formed at a transform fault in a passive margin system. The edge of North America south of the Grand Banks (Figure 7) is of transform origin and formed by shearing in a NW-SE striking plate between the African and North American plates during opening of the Atlantic. The Grand Banks margin traces the northern Signet of these plate motions. Its age is Early-MiddIe Jurassic in the northwest where it joins the rifted Nova Scotian margin (Figure 7) and Early-MiddIe Cretaceous at its southeastern terminus. The northwestern segment of this margin has sunilarities to the adjacent rifted margin oh Nova Scotia. The stratigraphy and depositional environments in the north- western part are similar to those of sediments at the Nova Scotia rifted margin. Thus, the junction between the Nova Scotian rifted margin and the Grand Banks transform margin is complex and diffuse. It is only southeast of the South Whale Basin that the geological style of margin typifies transform tectonics. Continental basement beneath the southern Grand Banks comprises the Meguma and Avalon terranes of the Appalachian orogen. The boundary between the Avalon and Meguma terranes is defined by the Collector magnetic and gravity anomalies and is interpreted as a steep transcurrent fault (Figure 7~. The Meguma terrane consists primarily of Cambro-Ordovician deep water sediments. Basement to the Meguma is thought to be of Grenvillian age.

56 Elongate rift basins which are half graben occur on the Grand Banks. Most reflect reactivation of faults in Avalon crust. They are filled with Triassic nonmarine and paralic cIastic sedunents, Triassic and Lower Jurassic salt and carbonates. These basins remained active sites of rifting until Early-Middle Cretaceous time, as long as the African and North American continents were in contact along the transform. In Late Cretaceous time, the Grand Banks were uplifted, perhaps in response to the onset of rifting between Iberia and the eastern Grand Banks. This produced a major unconformity, above which latest Cretaceous and younger marine cIastic sediments blanket the rift basins and basement. Except for the rift grabens and the northwestern segment of this margin, sediments are thin over much of the Grand Banks and a deep wide sedimentary basin, such as those along the rifted margins, did not develop along the transform margin, although a narrow basin occupies the continental slope. The contact between the Avalon and Meguma terranes appears to have been reactivated either by rifting or by plate reordering on a more global scale. Early-MiddIe Cretaceous volcanism occurred near this boundary on the Grand Banks and also on oceanic crust producing the Fogo Seamounts and the J-Anomaly Ridge along the southeastern part of the transform margin. Farther north and east, the Newfoundland Seamounts of mid-Cretaceous age were created after the separation of Iberia from North America. The relationship between volcanic activity in these different regions is unclear. The Newfoundland Seamounts may be the seaward prolongation of the Avalon-Meguma boundary. The ocean-continent boundary appears to be much sharper across the transform margin than across the rifted margins. Unlike the rifted margins, there is no broad zone over which the crystalline continental crust has been thinned, and there are no deep marginal sedimentary basins on the transform margin. The ocean-continent transition is only 30 km wide in the region of transform motion. At present there is no evidence for basaltic intrusion in the transition region. Landward of this transition there may be a zone of thinned crust, but thinning is not as intense as on the rifted margins. Some of this thinning is due to erosion during the mid-Cretaceous uplift of the Grand Banks region and not to necking of the crust. The oceanic crust adjacent to the transform margin is anomalous: it consists of a very thick layer 2 while layer 3 is either thin or absent. This may be related to the excess volcanism of the region. The narrow ocean-continent transition at the transform margin supports the hypothesis that extensional forces cause crustal thinning at the rifted margins, whereas transform margins exhibit little or no thinning. The absence of a large sedimentary basin along the transform margin may be due to the absence of extension and thinning during rifting. The oceanic crust south of the Grand Banks and east of Nova Scotia contains the Jurassic Quiet Magnetic Zone. Exceptions occur where Seamounts are present. There is no equivalent of the East Coast Magnetic Anomaly along the transform. The free air gravity anomalies across this transform margin are weak by comparison with those across the rifted margin. In general, large positive gravity anomalies, 60 to 100 mGal, are associated with the deep sedimentary basins on rifted margins, while smaller anomalies correspond to platform regions. Such a pattern is consistent with a significant contribution to these anomalies from sediment loading and isostatic adjustment. The small size of the positive gravity anomalies across the transform margin, 40 mGal or less, may therefore reflect the absence of a major basin above the transition zone.

57 Transect D3: Rifted continental margin off Nova Scotia: Offshore eastern Canada Synopsis and Findings The objective of Transect D3 (Figure 1) is to depict the structure of the continent- ocean transition at a typical rifted margin. The contemporary margin off Nova Scotia was formed over the Meguma and Avalon terranes in response to rifting and drifting between Africa and North America (Figure 7~. Rifting began in the Late Triassic and produced basins beneath the present continental shelf. Of these, the Orpheus sub-basin which lies on the Avalon-Meguma boundary is a prominent example (Figure 7~. Regionally, the rift basins are not uniformly distributed throughout the Nova Scotian and southern Grand Banks regions. Most are in the Avalon terrane, rather than the Meguma terrane. Triassic mafic volcanism appears to lie well inland of the present ocean-continent transition. Triassic rifting was accompanied by the deposition of nonmarine cIastics within the rift basins. During the Early Jurassic, thick salt beds developed beneath much of the present shelf and slope. In Early-Middie Jurassic time, a deepening, less restricted marine environment led to the construction of carbonate platforms. During this time, seafloor spreading began east of the rifted North American continent. The rift-drift transition does not appear to be marked by a period of uplift, erosion, or by the development of a clear breakup unconformity. After the onset of seafloor spreading, the margin subsided and over 10 km of Jurassic and younger sediments were deposited on the outer shelf and slope. Variations in morphology, sediment type, and deposition rate throughout the post-rift history of the margin were largely controlled by the paleoenvironment, and by the balance between rates of subsidence, sediment influx, and eustatic sea level changes. These factors caused a major change in sedimentation, from carbonate deposition to cIastic influx in Late Jurassic-Early Cretaceous time. They also produced several regional unconformities within this section and controlled the position of the edge of the shelf. Salt diapirism further altered the stratigraphy of the marginal sedimentary basin and produced the Sedimentary Ridge province beneath the continental slope (Figure 7~. Diapiric growth was most active in Cretaceous time. The Sedimentary Ridge province contains over 10 km of sediment (Figure 7~. Its landward edge is marked by the hinge line, an extensional fault. At this point, the sediments thicken rapidly towards the shelf edge. Landward of the hinge line, there has been progressive oniap of sediments, forming a coastal plain sequence. The Upper Triassic-Lower Cretaceous carbonate banks terminate beneath the shelf as defined seismically, and presumably mark the position of the paleoshelf edge. The seaward extent of the Scotian Basin is not clearly delineated; it gradually merges with the North Atlantic basin across the continental rise. Beneath the continental slope, the Sedimentary Ridge province consists of salt diapirs. Salt diapirs also occur beneath the shelf. Basement to this section includes rocks of the Meguma and Avalon Appalachian terranes whose boundary is probably a steep transcurrent fault (Figure 7~. The Meguma terrane consists principally of deep water sediments of Cambro-Ordovician age. This terrane is believed to extend seaward to the outer shelf region, on the basis of seismic velocities and because Meguma rocks are found in the bottom of deep exploratory wells on the outer shelf of the LaHave platform (Figure 7~. Carboniferous sediments may also occur below Mesozoic strata in the Scotian Basin.

58 The deep structure of the margin is characterized by extensive thinning of the crystalline continental crust by factors of 2 to 3. The thinning intensifies towards the ocean-continent transition. The thinned crust may be a measure of the amount of lithospheric extension that occurred during rifting. The cooling of the lithosphere after the rifting episode can explain the shape of the subsidence history curves determined from deep borehole data. The region of truly transitional crust has probably been both thinned and intruded and has properties that are different from either oceanic or continental material. This transitional region has high seismic velocities of 7.4 km-s-, in a zone about 100-km-wide off Nova Scotia. This velocity, typical of basaltic rocks, is evidence for the intrusion of basaltic magma, which underplated or intruded the thinned continental crust during rifting. The transition zone is also associated with the East Coast Magnetic Anomaly, a prominent marker that can be traced southwards as far as the Blake Plateau. Its cause is unknown, but it is generally accepted that it marks the ocean-continent transition. A large positive gravity anomaly lies over the outer continental shelf and a corre- sponding negative anomaly lies over the continental slope and rise. These anomalies do not coincide with the East Coast Magnetic Anomaly but rather, they follow the mor- phology of the present margin. The gravity anomalies have two causes: (1) changes in crustal thickness, sediment thickness, and water depth near the shelf edge; and (2) the lateral extent over which the toad of sediment and water is isostatically compensated by flexure in the underlying lithosphere. In general the heat flow over oceanic and over continental parts of the rifted margin are similar. There is a large scatter in heat flow values over the Sedimentary Ridge province, which is attributed to the high conductivity salt diapirs surrounded by lower conductivity sediments. The oceanic region seaward of the transition zone represents the oldest oceanic crust, generated in Jurassic time. This region lies within the Quiet Magnetic Zone, so no seafloor spreading anomalies are apparent. There is no evidence for anoma- lous oceanic crustal thicknesses, or oceanic basement highs near the ocean-continent transition in this region. Transect D4: Rifted continental margin off Labrador Synopsis and Findings The continent-ocean transition in Labrador (Figure 1) is the result of continental breakup between Greenland and North America in Late Cretaceous time. This margin formed by rifting of the Precambrian craton. The Nain cratonal province probably extends offshore beneath the region of Transect D4 (Figure 7) It consists primarily of Archean rocks and is older than the Grenville province to the south. Paleozoic rocks are virtually absent from the mainland. Thin Paleozoic sediments have been sampled locally in the bottom of deep boreholes in the continental shelf. The present continental margin formed by rifting beginning in Early Cretaceous time. Precambrian basement rocks were disrupted by normal faults, and subaerial volcanic rocks and nonmarine cIastic sediments filled the rift basins. During mid-Late Cretaceous time a regional unconformity developed (the breakup unconformity?), prob- ably in response to uplift just before the onset of seafloor spreading. Subsequently, continental separation was completed and postrift sediments, mainly fine-grained claw tics, were deposited on the margin. The postrift subsidence of the Labrador margin

59 can be described by models in which the lithosphere is stretched, then thinned during rifting. However, postrift sedimentation on the Labrador continental shelf was delayed for about 10 Ma after the beginning of seafloor spreading because of uplift late in the rift phase. Thinned and intruded continental crust occurs beneath the outer shelf. Deep boreholes have bottomed variously in Precambrian rocks, Paleozoic sediments, and Early Cretaceous voicanics. The lower crust is characterized by a velocity of 7.3 km- s~~; because this velocity is similar to that of oceanic layer 3b, it is difficult to use the velocity structure of the margin to define the ocean-continent transition. The best estimate for the position of the oldest oceanic crust is where the top of oceanic basement can be seen as a clear reflector. However, there remains a broad region beneath the edge of the continental shelf and the continental slope where crustal affinities are unclear. The similarity of the 7.3 km-s-, velocity under the shelf to those of basic intrusive rocks suggests magma rn~gration during rifting and the underplating of the lower crust by basaltic magma intrusion. Alternatively, this high velocity crustal layer may be typical of the adjacent continental crust. There are no measurements of continental crustal thickness or velocity in eastern Labrador, within the Nain province of the Canadian Shield. A prominent free air gravity anomaly exists across this rifted margin. A large positive anomaly lies over the outer continental shelf, and a much smaller negative anomaly lies over the continental slope or rise. There are no equivalents of the East Coast Magnetic Anomaly on the Labrador margin. The oldest oceanic magnetic anomaly that can be identified in this region is anomaly 33 (80 Ma). However, the anomalies are weak and confusing near the margin, leading some to speculate that transitional or continental crust underlies a large part of the Labrador Sea. Near the continental margin, flat-lying oceanic basement horizons exhibit internal reflections that probably represent oceanic basaltic flows interbedded with sediment. This is perhaps the best evidence for the position of the continent-ocean boundary. Transects D1 to D4: Problems 1. What was the fate of the Tower subcrustal lithosphere during Taconian and Acadian overthrusting and collision of the ancient passive margin with an island arc? 2. Why is there little extension observed in the Appalachian orogen by com- parison with the Cordillera? What is the role of transcurrent faults in creating an extensional environment? Are they particularly important in the later stages of oro- genic development? 3. How many outboard terranes, each with a distinct geological history, are present off eastern Canada? Can they be directly related to those to the south? 4. What is the nature of terrane boundaries at depth? How deep do they extend? Does the Meguma terrane have its own lithosphere? 5. Are there differences in the prerift lithospheres off Nova Scotia and Labrador, whose last Reformational events are Paleozoic and Precambrian (Archean), respec- tively? Can these differences be detected in differences in vertical motions, for example, rates and amounts of rift and postrift uplift and subsidence, thermal time constants? 6. Under what thermomechanical conditions does lithospheric rupture occur? What causes the lithosphere to break where it does?

60 7. The mode of isostatic balance across some of the rifted margins is unclear, as large positive gravity anomalies are not balanced by comparable negative anomalies. How can this be explained? 8. What is the role of volcanism in the rifting process? Is it of primary or secondary importance? How can we devise experiments to determine this? Why does volcanism sometimes occur well inland of the continent-ocean boundary. 9. What causes uplift late in the rift stage and the development of regional breakup unconformities? 10. How sharp is the break from continental to oceanic crustal thickness across a transform margin? If there is minor thinning at this margin, what causes it? 11. Why does the oceanic crust adjacent to the transform appear to be anomalous? Are there analogs with oceanic transform faults? 12. What causes the East Coast Magnetic Anomaly? 13. What conditions trigger subduction along a passive margin? TRANSECT E1 ADIRONDACKS TO GEORGES BANE Synopsis and Findings Transect E1 (Figure 1) extends east from the Grenvillian basement of the North American craton in the Adirondack Mountains across the Appalachian orogen of New England to Atlantic lithosphere off Georges Bank (Figure 8~. The two phases of passive margin development first in Late Proterozoic and Early Cambrian times and second in the Mesozoic were similar in E1 to regions north and south (Transects D1 and E2-5~. The intervening evolution of the Appalachian orogen from m~-Ordovician to Permian times, however, had some marked differences in E1 from that of other transects, although features such as the Blue Ridge-Green Moun- ta~n axis and certain gravity and magnetic anomalies extend throughout the orogen (Figure 8~. Evidence for an Acadian event is strong in New York, northern New England and maritime Canada (E1 and D1), but sparse, if not absent, farther south (E2-5) (Figure 1~. The Alleghanian deformation of the Valley and Ridge province (E2 and south) dies out northward, and the last traces of it are believed to occur east of the Catskills in E1 (Figure 83. In Canada, by contrast, a flat-lying veneer of Late Devonian and Carboniferous strata overlies earlier Paleozoic deformed rocks of the New Brunswick platform. The Blue Ridge-Green Mountain axis first became active in the Taconian and coincides with a prominent gravity high extending from Newfoundland to Vermont. Farther south this gravity high lies progressively farther southeast of the Blue Ricige-Green Mountain axis; the separation of the two features is some 60 km in the central and southern Appalachians. This separation may be the result of Late Paleozoic, low-angle thrusting. The above phenomena are consistent with the consolidation of Laurasia as a single landmass during the Devonian. Northern Tapetus had then closed, but its southern part did not close until the end of the Paleozoic. Problems 1. Does the Meguma terrane of Nova Scotia extend southwestward as far as the continental shelf oh New England in Transect E1?

61 76°w 72°w 68°w 64°w f 1 ~ .- . it// \ ~ // \ ~ #~ =o \ ~ =- t\ ~ \ }W ~ C , < ~NB I exposed Ma 3~1 (Jr) basin buried Ma - (Jr) basin [~\~g rocks older 7.\~.\l than 1 bil- lion years sedimentary -I hang basins east coast magnetic anomaly UK Hamburg klippe Iaconic Allochthon\ SG Baltimore Cneiss micra continent HI Hsrtland Terrsne RP Reading Prong SMA S~ur~town Mts. Allochthor Newark Bas FIGURE 8 Hap of eastern united States sbowlug poshlons of corridors E1 to E5. 68°W 36°N 32°N 72 °w ;ln ~R°N

62 2. The geographic limits within =~ -f't~ ~~~ A- a ~~~ O <~_ of Appalachian system are poorly established and necessary for confident assessment of the tectonic origin of each phase. Although the northwesterly limit of the Taconian orogeny is adequately known, the southeastern margin is uncertain, and the Acadian and Alleghanian limits are poorly located. 3. The extents and histories of subsidence, sedimentation, deformation, and metamorphism of sedimentary basins associated with the Taconian and Acadian inter- vals (Ordovician through Devonian) need clarification. 4. Did lapetus exist after the Taconian orogeny, either as a large basin or as a series of small basins of vastly reduced area, or did it close completely in the Taconian? 5. A major obstacle to interpretation of Paleozoic tectonic evolution of the AN palachian system is the paucity of dating of metamorphic events, particularly their P- T histories. 6. The source of the positive gravity anomaly along the Green Mountain axis whose basement exposures consist of lower density rocks is an important unknown. E1 of the overlapping Paleozoic c~ro~enlr '.ll~r~ TRANSECT E2 NEW YORE APPALACHIAN BASIN TO BALTIMORE CANYON TROUGH Synopsis and Findings Transect E2 (Figure 1) extends from the Allegheny Plateau of New York across the Valley and Ridge, Great Valley, Hamburg klippe, Reading Prong, Newark Mesozoic basin, Piedmont, and Atlantic Coastal Plain to the Baltimore Canyon Trough. It has a northward extension to the Manhattan Prong. Its Phanerozoic tectonic history includes three phases: (1) development of Late Proterozoic and Early Cambrian passive margins on the eastern edge of the North American craton and all margins of the Baltimore Gneiss microcontinent (Figure 8) subsequent to continental rifting; (2) collisiona] events (a) of the microcontinent with North America, (b) of island arcs with both the microcontinent and North America during the early Paleozoic and (c) of Africa with the earlier formed composite terrane during the Late Paleozoic; and (3) Mesozoic rifting and drifting. Phases (2) and (3) are emphasized. The first collision (Taconian orogeny) in Transect E2 began in late Middle Ordovi- cian time although an earlier Late Cambrian (Penobscottian) Reformational event is recognized a short distance south of E2. The Taconian event resulted from subduction with southeastward underriding of North America and the closing of a small ocean basin between North America and microcontinent, possibly the Baltimore Gneiss microcon- tinent. The closure led to the development of a foreland basin and the emplacement of thrust sheets, including Middle Proterozoic basement, and recumbent folds. In the northern part of the transect a southeast-dipping subduction zone developed in the Ordovician between the North American craton and an island arc, probably the con- tinuation of the HartIand terrane of the northern Appalachians (Transect E1, Figure 6~. Oceanic rocks, including ophiolite fragments, occur in thrust sheets formed during this event. No Ordovician volcanic rocks are known in the main part of the transect, but volcanic rocks of Cambrian age crop out a short distance to the south. Oceanic rocks have been abducted onto and over the Baltimore Gneiss rn~crocontinent and onto North America in the Philadelphia area. The oceanic rocks are characterized by strong positive Bouguer gravity anomalies, which permit the tracing of the Taconian suture zone beneath deposits of the Atlantic Coastal Plain.

63 The effect of the Middle Paleozoic Acadian orogeny is detected in E2 by the thick wedge of synorogenic and particularly postorogenic (Catskill delta) sediment that underlies the transect from eastern Pennsylvania to its western boundary, and by plutonism and metamorphism in the Manhattan Prong (Figure 84. During the Late Paleozoic Alleghanian orogeny, resulting from the collision of Africa with the previously formed composite terrane, the region underwent a ramp-and-flat type of deformation that resulted in the classic Appalachian folds. Thrust faults are largely blind and are shown mainly by drilling and seismic reflection. It appears that many of these Alleghanian thrust faults experienced later right clextral slip, probably late in the erogenic period. North American basement, although thrust faulted, extends at least as far east as the abducted eugeocTinal rocks. Triassic and Jurassic rifting, in part along older faults, resulted in most marked thinning in the 100 km seaward of Atlantic City, forming the Baltimore Canyon Trough. Seafloor spreading began in the Middle Jurassic; the western edge of the oceanic crust is marked by the East Coast Magnetic Anomaly. As much as 5 km of synrift deposits occur within the onshore Newark basin and beneath the Baltimore Canyon Trough. That thickness decreased landward to less than 2 km at Atiantic City where a thin wedge of Coastal Plain deposits continues another 80 km to the northwest. During the Middle and Late Jurassic, a carbonate shelf-edge platform prograded 40-km-oceanward across the oceanic crust. During the Late Cenozoic, the shelf edge retreated 20 km in response to numerous sea level fluctuations. Tectonic heredity was of major importance in this region. Extensional faults formed during the Late Proterozoic rifting event appear to have become thrust faults during the closing of the early Paleozoic ocean. Such thrust faults can clearly be shown to have been further reactivated by extension during the opening of the Atlantic. Problems 1. Was there a Late Cambrian collisional event in the northern part of the transect as there was farther south? 2. Did Acadian metamorphism, plutonism, and deformation occur in the crypt talline terrane of this transect south of the Manhattan Prong, or, on the other hand, did the Acadian ciastic wedge derive entirely from the "Acadian mountains of the northern Appalachians and the post-Taconian metamorphism and deformation date from the Alleghanian? 3. Differentiation of terranes requires further study of Middle Proterozoic base- ment rocks. Those of the Baltimore Gneiss m~crocontinent clearly differ from those of the Trenton Prong, and both differ from those of the Reading Prong, which are quite similar to those of the Adirondacks and, thereby, are clearly North American. To some, the For~ham Gneiss of the Manhattan Prong appears similar to the rocks of the Reading Prong; to others, not similar at all. All these rocks are apparently of about the same age. If this is true, do the outboard terranes represent rifted blocks that have come back home, or are they slices of 1-Ga basement juxtaposed on lateral faults? Is the Manhattan Prong a- northern extension of the Baltimore Gneiss microcontinent on the other side of the Newark Basin? 4. Where are the syn- and post-orogenic sediments related to Alleghanian moun- tain building? Abundant sediment related to the earlier tectonic episodes is preserved. Is the Alleghanian sediment all erocled away or is the Triassic-Lower Jurassic basin fill the Alleghanian molasse?

64 5. Did the development of the Appalachian orogeny really occur in three dim crete phases from Ordovician (or Cambrian?) to Permian times, or was it in fact a heterogeneous continuum representing the protracted closing of Africa and North America? 6. Why are the terranes west of the Taconian suture and east of the Avalonian terranes in New England and eastern Canada largely absent in E2? TRANSECT E3 PITTSBURGH TO BALTIMORE CANYON TROUGH Synopsis and Findings l Transect E3 (Figure 1) begins in the Appalachian Plateau and crosses the Valley and Ridge, Great Valley, Blue Ridge, Piedmont, Atlantic Coastal Plain, Baltimore Canyon Trough, and East Coast Magnetic Anomaly. Seismic reflection subsurface control was available for all but the basement under the Atlantic Coastal Plain (Figure 8~. Autochthonous North American Grenvillian basement extends in the subsurface under the Atlantic Coastal Plain to the east edge of Richmond, Virginia. At Rich- mond, Grenvillian rocks were overridden along the trace of an east-dipping Taconian suture by the volcanic Carolina terrane. Thick-skinned tectonics involved nappes of Grenvillian basement and Carolina terrane cover which were thrust westward in dextral transpressional movements during the Alleghanian. Subsequent erosion has exhumed the Grenvillian Goochiand and Richmond nappes along and under the west edge of the Atlantic Coastal Plain of Virginia and adjacent states. Along the central Piedmont of Virginia, the west edge of the Goochiand nappe was thrust westward over the Cambrian Chopawamsic voicanics of the Carolina terrane. The west edge of the Chopawamsics bounds the Taconian suture zone consisting of east dipping melange and turbidites. Thus, between the Chopawamsics and the subcrop of the Carolina terrane under the Coastal Plain, the Taconian suture has been breached by erosion over the GoochIand and Richmond Grenvillian basement uplift. At the west edge of the melange-turbidite sequence, deep water North American rift (Late Proterozoic Lynchburg) and rift/drift (Late Proterozoic to Cambrian Evington Group) sequences are in depositional contact with the allochthonous Grenvillian rocks of the Blue Ridge anticlinorium. Taconian metamorphism and ductile deformation affected the entire Virginia Piedmont. The melange sequence contains several Cambrian to Early Ordovician plutons that may have been generated over oceanic crust during subduction of a spreading ridge. The Taconian cIastic wedge in Virginia was largely derived from cannibalized platform drift and deep water rift/drift sequences of the North American continental margin. "Acadians metamorphism, at about 350 Ma, may have locally affected the central Virginia Piedmont. This deformation produced the Catskill delta ciastic sequence in the Valley and Ridge. The "Acadian" of Virginia is younger than the deformation of the type region In the northeastern United States and may reflect the initial stages of the Late Paleozoic Alleghanian orogen in the central Appalachians. The Alleghanian orogeny is characterized by amphibolite facies and penetrative fabrics in the eastern Piedmont. In the central and western Piedmont, it is charac- terized by local discrete zones of ductile deformation and mylonite zones. Regionally

65 the central and western Piedmont behaved as a semi-brittIe block during ADeghanian deformation. Dextral transpressional movements were the main theme during the Al- leghanian from the maritime Canadian crystalline Appalachians to the Piedmont of Georgia and Alabama. Translation parallel to the erogenic axis is distributed over the crystalline terrain in discrete zones and conservatively amounts to 200 km of right lateral movement. Therefore, the movement parallel to the erogenic axis ~ of the order of magnitude of the movement orthogonal to the axis and could be much larger. The Alleghanian suture originally lay to the east of the present middle Atlantic Coast and may now be on the African continent. A major cIastic sequence was deposited during the Alleghanian in the Valley and Ridge Province, and most structure of the Valley and Ridge was produced at this time. Middle Ordovician to Pennsylvanian plutons were generated during times of tec- tonic thickening of the orogenic belt. Several mechanisms may have been involved: (1) frictional heat of deformation, (2) depression of the crust to higher temperature zones, (3) decompressional melting in uplifted nappes, (4) introduction of mantIe-derived mafic melts into the crust, and (5) melting in the politic sequences overridden by hot nappes. The Mesozoic to Holocene rift and drift history related to the opening of the Atiantic Ocean is well documented by: (1) Mesozoic rift basins on both sides of the hinge zone, the Norfolk Fracture Zone, and Norfolk Basin; and (2) drift facies that are post-middle Jurassic prograding shelf and slope sequences. Problems 1. The tectonic history and crustal architecture in Transect E3 show major differences from interpretations in other E Corridor transects. To what extent will these new interpretations from E3 be found to apply more widely? 2. E3 Corridor shows the easternmost known North American Grenvillian crust. With additional seismic reflection data, how far east can the eastern margin of Pro- terozoic North America be traced? 3. Recent reprocessing of USGS I-64 seismic data (just north of the E3 crustal section) shows abundant east-dipping layers indicating deformation of the deep crust as far east as the Blue Ridge; two intervals of near horizontal layering also occur, one near the middle of the crust, and another several kilometers thick at the base of the crust. Most reflectors end abruptly along a near vertical zone near Richmond, and few reflections are seen in the line from Richmond to the coast. What is the origin of the lower crustal laminations, and how far east toward the suture will they persist? Is the termination of reflectors under the Coastal Plain real or a problem of data acquisition? Additional reflection work is needed to resolve the enigma. This should include wide angle seisrn~c surveying to determine the deep velocity (density/Ethology) structure and the geometry of the suture at depth. Structure in the Carolina plate east of the suture at Richmond should also be sought by reflection work. Deep velocity information will also allow better evaluation of the rise of the Moho under the Piedmont and its relation to the Piedmont Appalachian gravity anomaly. Present analysis of the crustal structure under the Piedmont suggests that the crust was thinned by more than 10 km during the early Mesozoic rifting that preceded the opening of the Atlantic. If this is the origin of the upwarping of the Moho under the Appalachian gravity anomaly, then the anomaly cannot be used to directly infer buried sutures as has been done in the past. Certainly the anomaly does not coincide with the Taconian suture in the

66 autochthonous crust in Virginia. If Mesozoic thinning of crust was so extensive, then the present thicknesses cannot be taken to have their origin in lapetus-related rifting or in compressional erogenic deformation as has been assumed in gravity modelling along the COCORP line in Georgia and elsewhere. 4. What is the extent and cause of the Alleghanian Central transpressional de- formation? 5. Did early Mesozoic rift basins offshore inherit their structural control from Paleozoic deformation zones as did those onshore? TRANSECT Ed CENTRAL EENTUC1lY TO THE CAROLINA TROUGH Synopsis and Findings Transect E4 (Figure 1) extends from cratonal North America at the Grenville front in central Kentucky, east across the southern Appalachian orogen through Cape Fear to oceanic lithosphere east of the Carolina Trough (Figure 8~. As in other Appalachian transects, the Phanerozoic history comprises three phases: (1) Late Proterozoic-Early Cambrian passive margin development, (2) Ordovician to Perrn~an evolution of the Appalachian orogen, and (3) Mesozoic clevelopment of the Atiantic passive margin. Distinctive onshore geological features from northwest to southeast include: Pa- leozoic sedimentary cover as thin as 0.7 km over the Jessamine Dome and as thick as 3.7 km under the Appalachian Plateau in the Early Cambrian Rome trough; the Al- leghanian forehands thrust belt (thrust faults root deep in the crystalline Appalachians); two large windows (Mountain City and Grandfather Mountain) exposing duplex struc- tures; Middle Proterozoic basement exposed in allochthonous external (Blue Ridge) and internal (Sauratown Mountains) massifs; Late Proterozoic rift-facies sedunentary and volcanic cover sequences; a zone of abundant ultramafic rocks (Blue Ridge); and a Late Proterozoic to early Paleozoic magmatic arc (Kings Mountain, Charlotte, and Carolina slate belts of the Carolina terrane) intruded by Devonian and Alleghanian plutons. In the Piedmont, E4 crosses the Appalachian gravity gradient at the latitude of its greatest amplitude (-100 mGal on the west +20 mGal on the east). Crustal thickness is poorly constrained by seismic refraction data but gravity modeling sug- gests that it is 40 to 50 km under the gravity low and 30 to 40 km east of the gravity gradient. On the modern margin, depth to pre-Mesozoic basement increases 2 km at 100 km offshore, where a narrow graben of inferred Triassic age and basement hinge zone, marked by a magnetic anomaly (Brunswick), form the seaward edge of the Carolina platform. Continental crust (35 to 40 km thick) occurs west of the mag- netic anomaly; transitional crust (20 to 25 km) occurs beneath the Carolina trough (which contains slightly more than 12 km of postrift sediments) between the Brunswick anomaly and East Coast Magnetic Anomalv fECMAl and n`~.~ani`~. ~r,,,;:t. if; km t.hi~k) extends landward to the ECMA. ~~ ~ _ · ~ ~ ~ ~ _ ~ ·—— ~ ~ ~~w ~ Transect E4 contains well-preserved evidence for the tectonics and timing of the first or Laurentian phase of passive margin development, perhaps better understood here than elsewhere in eastern North America. I,ate Proterozoic continental rifting is indicated by rift-facies sedimentary and igneous rocks and continental crust containing dike swarms. The Taconian orogeny (closing of lapetus) is indicated by the Middle to Late Ordovician (Llanvernian to Caradocian) flysch elastic wedges, abduction and

67 fragmentation of ophiolites and metamorphism. A thick sequence of nonfossiliferous gneiss, schist, and amphibolite (Ashe Formation) lies southeast of the Blue Ridge external massif. Parts of the base of the formation are tied stratigraphically to North America through a basal conglomerate. Elsewhere the lowest parts of the Ashe and most of the upper Ashe contain numerous bodies of ultramafic rock thought to be fragmented ophiolites. The leading edge of the Taconian lapetus suture Is probably the northwestern limit of ultramafic-bearing Ashe. The effects of the Acadian orogeny in E4 are uncertain. The Alleghanian orogeny (collision with Africa?) produced the foreland thrust belt, a Late Paleozoic cIastic wedge, and large crystalline thrust sheets. Possibly the Carolina terrane was accreted at this time. Palinspastic reconstruction suggests that autochthonous North American crust must extend in the subsurface well east of the Appalachian gravity gradient. The accreted magmatic arc of the Carolina terrane has no apparent prearc basement and may have originated elsewhere than on the eastern rifted margin of lapetus. The westernmost Mesozoic continental graben (Dan River-Davie County basin) probably formed along reactivated Paleozoic thrusts, which in turn may have been reactivated lapetan structures. The present width of the Iapetan extensional zone (area of Rome trough to lapetus suture) is about 400 km; the width of the Mesozoic extensional zone (Dan River-Davie County basin to the ECMA) is about 570 km. The original width of the lapetan extensional zone is not known, but 200 km of Paleozoic shortening, which includes westward thrusting of Laurentian crust, is reasonable. Sedimentation in the Carolina Trough (CT) was most rapid during the Jurassic; the paleoshelf edge lay just landward of the ECMA. A string of salt diapirs, presumed to originate from an evaporite unit near the base of the CT, was emplaced along the axis of the ECMA. Salt flow contributed to the configuration of the trough by causing growth faults to form along the CT. Problems 1. Amount of crustal extension during lapetan rifting. 2. Better knowledge of the absolute timing and sequencing of events such as Iapetan rifting, metamorphism across E4, khrusting, and accretion of the Carolina terrane. 3. Provenance and age of the ultramafic bearing parts of the Ashe Formation. Does it represent an accretionary wedge from the eastern side of lapetus or are the sediments derived from the Laurentian Margin and somehow mixed with ophiolitic material during underplating of an obducting oceanic slab? 4. Provenance of the Inner Piedmont terrane and how it related to the terranes around it. Tectonic history of the internal massif of the Sauratown Mount a~ns anticlino- 5. r~um. 6. Significance of the Acadian orogeny. 7. Origin of the Appalachian gravity gradient and the location of the subsurface Iapetus suture. 8. Geological history of the crystalline rocks beneath the emerged and submerged Atiantic Co~tal Plain.

68 TRANSECT E5 CUMBERLAND PLATEAU TO BLAKE PLATEAU Synopsis and Findings . Transect E5 (Figure 1) extends from the North American craton in the CumberIand Plateau of Tennessee across the southern Appalachian orogen to transitional crust of the Blake Plateau, ending in AtIantic oceanic lithosphere. The region includes a complete cycle of the opening and closing of an ocean (Iapetus) and the opening of a second ocean (Atlantic). It is therefore possible to compare the parallel evolution of two passive margins, one whose development began in the Late Proterozoic (Hadrynian) and the other which began in the Triassic-Jurassic. The accretionary and collisional history related to the destruction of the Late Precambrian-Early Paleozoic margin of North America and the formation of the Appalachian orogen throughout much of the Paleozoic may be observed in Transect E5. The Late Precambrian continental margin of eastern North America is clearly indicated by the rifted Grenvillian crust (1.0 to 1.2 Ga) overlain by suprajacent platform succession. The eastern edge of the Grenville crust is tectonically buried but has been located by restoration of surface structures, potential field, and seismic reflection data. Some of the Late Precambrian-Cambrian rifted margin sequence is visible in the frontal thrust zone of the Blue Ridge. The Cambro-Ordovician platform succession is clearly visible as a thin veneer on the basement. The accretionary/collisional phase of the history of the southeastern North Amer- ~can margin began in Middle Ordovician (~lanvirn/~landeilo) time, as indicated by the development of a flysch succession derived from the east that filled an earlier (latest Arenig or early LIanvirn) foredeep. This was also a time of metamorphism, deformation, and plutonism in rocks oceanward of the foredeep, and several pro- to synmetamorphic thrusts (for example, Greenbrier, Hayesville) occur in this region. The Hayesville thrust is considered an Early Paleozoic thrust that is rooted in the Ordovician continent-ocean transition zone, because it juxtaposes sedimentary and volcanic rocks deposited on oceanic or a rifted continental basement onto a contem- poraneous rifted margin sequence that was deposited on North American continental crust. Plutons emerged in this early phase and are confined to the Hayesville thrust sheet. The middle Paleozoic Acadian event is not reflected by deposition in E5 but is possibly indicated by unconformities in the foreland succession, indicating erosion, nondeposition, or both, and metamorphism in the hinterland. In the continental hinterland, however, plutons, several major faults, fabric relationships, and radiometric ages suggest that the Acadian was a major event. This may have been the time of accretion of the Avalon (Carolina) terrane along the central Piedmont suture. The Late Paleozoic Alleghanian deformation of the foreland, produced as a huge crystalline thrust sheet (Blue Ridge-Piedmont thrust sheet), was driven westward by the collision of Africa with the North American margin, and the lapetus and related Theic-Rheic oceans were finally closed. Structures produced at this time include both thrust and dextral strike-sTip faults. Thrusts attain displacements >250 km; Alleghanian strike-slip faults have much less movement. Alleghanian platonic activity was largely granitic; both S- and I-types are present. Alleghanian metamorphism is restricted to the southern and eastern exposed parts of the Appalachian orogen. The development of the most recent trailing margin/platform began in the latest

69 Triassic (Norian?) to early Jurassic (Sinemurian to Pliensbachian?) time with the formation of the rift basins in the eroding Appalachians which filled with terrestrial locally derived sediments. Mafic dikes were intruded as late as the Middle Jurassic (Bathonian), indicating the continental crust was still being extended. However, by Late Jurassic time a carbonate platform had developed on the Paleozoic/Precambrian crust and the Atiantic was open. The Cretaceous and Tertiary sediments deposited in the platform record progres- sive inundation of the eroded Appalachians to a maximum transgression during the latest Cretaceous to earliest Tertiary followed by general regression until the present. Problems 1. Timing of metamorphism is uncertain in the central part of the southern Ap- palachians, due to the lack of younger sedimentary rocks and detailed geochronological studies. 2. Eastern extent of the Blue Ridge-Piedmont detachment. 3. Origin of the Piedmont gravity gradient. 4. Distribution of North American and non-North American terranes beneath the Coastal Plain. 5. Nature of the Brunswick terrane-East Coast Magnetic Anomaly boundary in the offshore. 6. Crustal controls of Cretaceous-Tertiary depositional platform tal shelf-Blake Plateau area. TRANSECTS F1 AND F2 GULF OF MEXICO BASIN . in the continen- The F1 and F2 transects (Figure 1) are unique because they cross the southern edge of the North American craton and also cross entirely a small ocean basin, the Gulf of Mexico. A problems section is given for F1 and F2 together. Transect F1: Ouachita Orogen to Yucatan Synopsis and Findings Transect F1 (Figure 9) illustrates four major phases in the tectonic evolution of this area: (~) the Late Proterozoic through Middle Paleozoic evolution of a south-facing passive margin, (2) the Middle to Late Paleozoic closing of an ocean basin and colli- sion of South America and associated Sabine/Yucatan terranes with North America, (3) the Mesozoic (bate Triassic through mid-Cretaceous) development of the modern Gulf of Mexico basin, and (4) the post-mid-Cretaceous partial filling of the basin mainly by huge prograding silicicIastic wedges, first from the west and then from the north. Emphasis here will be only on the second and third phases. During Middle Paleozoic time, the Early Paleozoic ocean basin that flanked the south-facing passive margin of North America began closing along a south dipping subduction zone, as South America and associated terranes moved relatively north. An associated volcanic arc system, either isolated or within a continental block, provi(led flysch and voIcanics to adjacent forearc and trench areas. By Pennsylvanian time, a combined Sabine/Yucatan/South America terrane had collided with and overrode the

70 100° 36°~ <INTO | + + O + + + 4' | jOrol~en'~\+ + + +/+ Nort I +~; ~ + A\ &> '/ < 80 I + + rim ~~G~ Bean Uplift \,~<AmeriC~; + ~/ ~ \~< l ll ,' ( I \) , '1' + + + + + +~Po~ ~~ + + + + 7 1 ~ ~ ~ + ~ . -. 32°~+ + + + + + + /`a . 1+ + + + .... 1 + + + 1-+++++++/ 1+ + + / :++++ ~ tar | buried erogenic 2 8 ~— ~( 1/, ~ A ol front so A Is /~1 /. ~ : _ ... '~,'f~ ~ ~ _; (em — G A MZ salt ~ .~( '~ ~ \ salt basins / /\ \ Yucatan J ~ : ~> ~ ' a 4~/ _~'J ~ I ~,,annee \ ala,. | terrane l - F\~, O mi 200 ~ . O km 200 of' FIGURE 9 Map of southeastern United States, northeastern Mexico, and Gulf of Mexico, showing positions of corridors F1 and F2. North American passive margin. Flysch continued to fill a deepwater basin between the two colliding continents, but as collision continued, the basin shallowed and was finally filled with nonmarine molasse. Enormous tectonic thickening of the sedimentary section took place through northward-verging folding and thrusting that formed the Ouachita orogen (Figure 9~. Late stage detachment and northward displacement of a large block of North American crust formed the Benton Uplift and other basement cored uplifts (Figure 9~. The tectonic activity associated with this collision was accompanied by widespread metamorphism. Most tectonic activity had ceased by mid- Pennsylvanian (Desmoinesian) time and Sabine/Yucatan/South America was sutured to North America to form one welded continent. During Late Pennsylvanian through mid-Permian time, widespread postorogenic episutural or successor basins formed across the entire northern Gulf region, characterized by deposition of shallow marine carbonates and elastics. Late Permian through Middle Triassic was a time of widespread uplift and erosion

71 throughout the region, but by Late Triassic, widespread continental rifting accom- panied by volcanism and deposition of nonmarine sediments in broad rift basins had begun. The rifting, which continued through Early Jurassic, was due to stresses created by the initial relative movements between Africa/South America and North America and concentrated along Paleozoic crustal weaknesses on sutures (collisional ant] extensional). The Sabine/Yucatan terranes were still in the northern Gulf region. During Middle Jurassic time, the entire Gulf area became the site of broad mantle upwelling and extreme attenuation of the continental crust, resulting in a large region of stretched or transitional crust. The crust along the outer rim of the region remained relatively thick (20 to 35 km), forming broad basement highs and lows with wavelengths of 100 to 500 km. In the central Gulf area, however, stretching was more concentrated and the crust thinned considerably (8 to 20 km). Thick salt was deposited in large basins throughout the entire area of transitional crust, as marine waters entered the Gulf controlled by sills in central Mexico. This Middle Jurassic period of rifting and stretching of the crust created much of the basic architecture of the Gulf basin as we see it today (primary basement highs and lows). This influenced subsequent deposition of the overlying Upper Jurassic through Lower Cretaceous sediments. Overall crustal stretching was as much as 400 to 500 km. During the Late Jurassic, rifting and mantle upwelling became even more concen- trated in the central Gulf, creating an elongate east-west basin floored by oceanic crust. The central Gulf basin is over 300 km wide in the west but considerably narrower to the east. Its distribution suggests that the Yucatan terrane moved out of the northern Gulf away from the Sabine terrane with counterclockwise rotation, separating the salt basins on either side of the ocean crust, similar to the Red Sea today. This was a time of general marine transgression or relative rise in sea level across the basm due mainly to basin subsidence as the crust cooled. Broad carbonate ramps and deep shelves extended across areas of transitional crust, while deep-marine sediments were deposited in the newly formed oceanic trough. By Early Cretaceous time, subsidence continued but the central Gulf area was locked in its present configuration, and spreading had jumped to the Caribbean region south of Yucatan. Broad carbonate platforms rimmed the entire deep basin with prominent margins (including Campeche and Florida escarpments, Figure 9) developed along regional tectonic hinge zones that marled the boundary between thick and thin transitional crust. Cyclic deep-marine carbonates were deposited in the adjacent deep basin seaward of the margins. Finally, by mid-Cretaceous time (Middle Cenomanian) the outer platform margins were terminally drowned, possibly by a rapid drop followed by a rapid rise in sea level, and the margins retreated to more landward positions. At this time a prominent, Gul£wide sequence boundary was formed, characterized by both subaerial erosion on the banks and submarine erosion along the submerged parts of the margins. This mid-Cretaceous sequence boundary marks a major turning point ~ the history of the Gulf, which set the scene for the later Cenozoic filling of the basin. Transect F2: Mississippi to Cuba Synopsis and Findings Transect F2 (Figures 1, 9) extends from northeastern Mississippi across the deep eastern Gulf of Mexico to western Cuba. The southern edge of continental North

72 America, which includes the stable interior, Appalachian thrust belt and Piedmont terranes, is in southwestern Alabama along a Late Paleozoic suture coincident with the western end of the Brunswick-Altamaha Magnetic Anomaly. North of the suture with the Piedmont terrane, North American Precambrian basement is overlain by Lower Paleozoic platform carbonates and Upper Paleozoic foreland basin cIastic rocks in the stable interior and Appalachian thrust belt. The Talladega belt contains marbles ancI slates; the Piedmont terrane, gneisses. Beneath the present coastal Alabama and Mississippi, accreted continental crust of the Wiggins terrane (Figure 9) lies south of the Alleghanian suture. Subsurface data along the proposed trace of the suture define a magmatic arc complex of serpentinites, basalts, and late-stage granites. The southern part of the Wiggins terrane was thinned during the Triassic-Jurassic opening of the Gulf. Oceanic crust underlies the abyssal regions of the eastern Gulf at depths of 8.5 to 10.5 km; thickness ranges from 6.0 to 9.0 km. Strata of earliest Cretaceous age overlie the oceanic basement. Rifted and injected transitional crust of South American or African affinity flanks the oceanic terrane in the southeastern Gulf, and thickens to continental dimensions beneath western Cuba. Basinal cIastic rocks of Jurassic age and Cretaceous platform carbonates overlie rifted basement in western Cuba and are, in turn, tectonically shingled with oceanic crust and sedunentary rocks abducted from the Caribbean during Late Cretaceous-Early Tertiary transpressional episodes related to Cuba-Bahama collision. North American crust adjoins the pre-Mesozoic Wiggins terrane along a zone generally coincident with the westernmost trace of the Brunswick- Altamaha Magnetic Anomaly (BAMA) in southwestern Alabama. The relationship between the Wiggins terrane and the Suwannee terrane (Figure 9) to the east and of the Wiggins and the Sabine to the west are not known, but commonality is possible. North of the BAMA, wells that penetrate pre-Mesozoic rocks define subsurface belts of metamorphic rocks laterally correlative to exposed suites in the Appalachian Piedmont. Major thrust faults juxtapose belts of northwestward decreasing metamorphic grade against one another and against unmetamorphosed sedimentary rocks continuous with the Appalachian thrust belt. The few wells penetrating pre-Mesozoic rocks south of the BAMA find basalt rubble (extrusive arc?), chlorite schist (back-arc basin?), and granites (magmatic-arc core and late-stage plutons?~; one well on the south flank of the anomaly penetrated serpentinite. Gravity modelling defines a relatively smooth mantle surface at depths of approximately 32 km beneath the BAMA, and about 36 km to the north and south. A southward-dipping suture zone is proposed because marbles, slates, and schists of the buried Piedmont terrane represent platform carbonates and slope and rise sediments of a southward-facing early Paleozoic continental margin that were later thrust northwestward during the Alleghanian continental collision. Our interpretation of a suture zone and accreted terrane in southwestern Alabama is consistent with recent interpretations of COCORP data in southern Georgia and Florida. Transects F1 and F2: Problems 1. Better geophysical definition of the Early Paleozoic south-facing passive mar- gin, its boundary with a Paleozoic oceanic crust, and the remnants of the inferred south-dipping subduction zone is needed. 2. The nature, timing, and distribution of Late Paleozoic collision events are poorly constrained, as is the associated regional metamorphism.

73 3. What are the depositional settings for the various thick flysch deposits- forearc, trench, or collisional trough? 4. Is the core of the Benton Uplift a displaced piece of the North American craton? When was it displaced? 5. What are the nature and age of the Yucatan and Sabine terranes? Are they rifted parts of South America or independent rn~croplates with different origins? 6. The origin and distribution of the post-orogenic, undeformed, successor basins are poorly known. 7. Several major linear potential field anomalies within the Sabine terrane need to be modeled (i.e., Houston magnetic anomaly). Are they major crust al boundaries or sutures? 8. More geophysical and geological evidence for a volcanic arc system within the Sabine terrane is needed. 9. Better definition of crustal types, crustal thickness, depth to basement, and sediment thickness are needed to better model total tectonic subsidence, amounts and mechanisms of extension, and accurate reconstructions of transitional crust. 10. Were the Late Triassic-Early Jurassic nonmarine sediments and voIcanics in the northern Gulf really deposited in large rift basins, as along the East Coast? If so, what is the nature of the boundary faults? Are they low-angle listric faults? Was the Late Triassic-Early Jurassic rifting concentrated along older Paleozoic weaknesses and sutures? 11. Was there a later, separate phase of mid-Jurassic rifting that formed the wide area of attenuated or transitional crust in the Gulf and created the prominent basement highs and lows that make up the basic architecture of the basin? How do these basement features reflect older Paleozoic elements? 12. What is the origin and timing of deposition of the thick salt in the Gulf? Was it deposited quickly in already existing deep rift basins? Was deposition controlled by sills in Mexico? Is the salt younger on the flanks of the basin? What was its original distribution and how has it been later mobilized? 13. What is the nature, origin, and timing of the tectonic hinge zone that forms the boundary between thick and thin transitional crust? Why did the Lower Cretaceous carbonate margins become established along this hinge zone? 14. An accurate distribution of oceanic crust and its associated boundaries, ocean ridges, and fracture zones is needed to constrain opening directions and allow for accurate reconstructions. Was Yucatan ori~inalIv in the northern Gulf? 15. The surveys are eastern Gulf have a different origin, and form later, than the crust of the central Gulf? ~ i' exact age of the oceanic crust is poorly constrained. Detailed magnetic needed to identify Jurassic magnetic anomalies. Did the crust in the ~ ~ - , . . . TRANSECT G SOMERSET ISLAND TO CANADA BASIN (Arctic Ocean Region) Synopsis and Findings Transect G (Figure 1) focuses on the polar continent-ocean transition in Canada, its geophysical character and geological structure, the time and manner of its origin, and the history of tectonic events in the region during the Phanerozoic.

74 Thick Cenozoic sediment wedges appear to be the main source of gravity anomaly highs and magnetic anomaly lows along the polar continental margin. Low level seismicity along the margin may be produced by the release of stresses built up by this load of prograding sediments in combination with a tensional regime across much of the continental boundary. The cIastic wedge began to form no later than latest Cretaceous time and it unconformably overlies the Early Carboniferous to Late Cretaceous Sverdrup Basin sequence which is superposed on the highly deformed Early Paleozoic Eranklinian Basin sequence (Figure 10~. The present continent-ocean transition was produced by the formation of the oceanic Canada Basin beginning in Early Cretaceous time. Although the kinematics of ocean formation and the crustal masses involved are constrained poorly, the favored mechanism is the anticlockw~se rotation of continental crust away from Arctic Canada about a pivot near the Mackenzie Delta (Figure 10~. The rotated blockers) is (are) thought to have included all of Alaska north from the Brooks Range. Two other major tectonic events affected what is now polar Canada during Phanerozoic time. The Late Devonian to Early Carboniferous Ellesmerian orogeny produced local intrusion and metamorphism plus regional folding and faulting through- out the Franklinian Basin (Figure 10~. Associated uplift created a thick southward- tapering prism of ciastic sediments that paleocurrent studies suggest were transported mainly from northeast to southwest across the Canadian arctic islands. Existing pale- omagnetic constraints are few, but the data allow that the Ellesmerian event may have been caused by collision and left-lateral shearing between Siberia and North America. The Maastrichtian to Miocene Eurekan orogeny terminated subsidence in the Sverdrup Basin and produced folding and uplift of broad northwest-trending arches ~ . . . . ... ~ . .. . . . . . ... . . . .... .. and syntectonlc deposition ot elastics in intervening deposltlonal basins wltllln the central and eastern Arctic Islands. This was followed by regional compression that produced at least 60 km of northwest-directed crustal shortening. CIastic detritus from uplifts produced by Eurekan tectonism migrated mainly northwestward from Campanian time onward and formed the thick deposits of the Arctic Terrace Wedge that cover the present continent-ocean transition. The lithospheric plate motions that produced the Eurekan event remain unclear but post-Ellesmerian geological continuity across the arctic archipelago precludes sig- nificant lateral displacement of rocks within the regions after mid-PaTeozoic time. Problems 1. Nature and degree of crustal thinning from Canadian Shield to Canada Basin. Franklinian Basin/Sverdrup Basin structural geometry in relation to crustal thickness. 2. Nature and age of the crust below northern Canada Basin and comparisons with southern Canada Basin. Correspondence between magnetic field variations and seafloor basement relief throughout the Canada Basin. 3. Nature and degree of changes in crustal properties along and across the polar continent-ocean transition in Canada. Multiparameter traverses of the margin at about 400 km intervals are envisaged along with deep drilling of the shelf where technically feasible.

75 1:/ an' ~ =c' i\ C ~ ~ 1~ ~ at: / \ :.: ~ ~ \\:; An ~ W;- ~ ~ =-~` Are ,- 13 ~ T: Of/ ~ :~ "o/'<O .' ~9 ~~ / =- ~ \ \ ~ ~ rat) ail, ~ ~ ~ ~ - C / --_ c~ as or 0, · ¢, /m , ~ ¢ in m ' Z t1,J —z A: _ me 5= om O: ~ o W /= / ~ ~ ._ C) :t _4 / ('-- - / a. ~ -. CI: oom o~ CRJ ma An. m or LL— m~ == < ~:E Z~ =0 00 om Z ~ ~ O _ _ J ~ _~ O ~ Z C) Z_ ~ <m 0 =< mm O a: :~ ~ 0 a 3 a, ~ ~n V v CS ~ cr: O ~ P4 ot:5 ~ 3 o

76 TRANSECTS H1 TO H3 PACIFIC BASIN LITHOSPHERES TO OR ACROSS MAINLAND MEXICO Transect H1: La Paz to Saltillo, Northwestern and Northern Mexico Transect H2: Acapulco to Tuxpan across the Central Mexican Plateau Transect H3: Acapulco Trench to Gulf of Mexico across Southern Mexico Synopsis and Findings

77 ~ ,- ~ ~ ~d~ of ~ ,l \ Age ! : -. -. \ go, x ~ o ~ \, '.,-, ~1 O o, ,~ ,,, '. ·. --: \ LU — O of: LL O — ~ ~ ~ S

78 Jurassic. This was probably accompanied by the closing of small ocean basins in t'ne Pacific region and with collision of intervening arc terranes against the active Pacific continental margin. The Gulf of Mexico waters transgressed progressively to the west until, by the mid-Cretaceous, the passive continental margin created by the Gulf opening was depressed under the mixed Atlantic and Pacific waters. Continental rifts of mid- Jurassic age related to the expansion of the Gulf of Mexico, in one case, probably evolved into a small ocean basin that closed by Late Cretaceous time, creating the inboard intracontinental terrane (Cuicateco). In the second half of the Cretaceous (Laramide tectonic regime) accretion of ter- ranes into the mosaic of western Mexico was complete, but the long-lived Pacific subduction regime continued to exist at the edge of the tectonically prograding conti- nental margin. Superposition of magmatic arcs occurred during this outward building of the continental margin. The record of subduction-related ma~mati~m wan mare r.ont.inilo ~ _ __ O ~ 1 ~ ~ · 1 . · ~ · # · ~ ~ ~ · ~ ~ in southern Mexico, where an apparent migration of Island arc volcanism advanced inland from the Cretaceous to its present position well within the continent. In contrast, the magmatic activity of NW Mexico swept the continental crust forth and back creating, during its Oligocene return, the Sierra Madre Occidental Ignimbritic Province. By Miocene time Pacific ronver~f~nr.f' r.am~ t.n ~ halt. in t.hi~ Russian hilt. continued in southern Mexico. O O South of the Trans-Mexican VoIcanic Belt (TMVB), however, the Pacific margin of Mexico was truncated in Cenozoic time so that the history of older arcs there is mainly lost. The truncation was due either to subduction-erosion or strike-slip faulting. The modern configuration of the Pacific continental margin of Mexico developed from the fragmentation of the East Pacific Rise as it approached and finally collider! with the western margin of North America. The truncated margin of southern Mex- ico and the associated disappearance of a continental fragment by lateral sliding or frontal underthrust permitted the emplacement of oceanic crust closer to the present continental margin, and current arc magmatism jumped northward to the present site of the TMVB. At the same time, the opening of the Gulf of California marked the Neogene tectonic history of the NW Pacific margin of Mexico, and in the last part of geological history of the region, a new attempt by the East Pacific Rise to take another piece of continental crust off Mexico is revealed in the western part of the TMVB where coexistent calcalkalic and alkalic magmas are extruding through a tensionally fractured crust. Problems 1. The origin of terranes with Precambrian and Paleozoic basements in Mexico and their alliance to North America or another continent, and their history of accretion. 2. The tectonic kindred of Paleozoic blocks. 3. The pre-Jurassic extent of cratonal North America and the Appalachian oro- gen into Mexico. 4. Timing of accretion and provenance of the terranes of mainly Mesozoic base- ment that constitute the western margin of Mexico. 5. Thickness and structures of crusts and depths of sutures of the various ter- ranes.

79 6. Origin of the obliquity of the TOMB with respect to the trench and of some other anomalous characteristics in this intracontinental calcalkalic arc, such as (a) the low linear density of stratovoicanoes, (b) low Sr isotopic ratios of volcanic products with common petrographic evidence of crustal contamination, and (c) the aseismic nature of the underlying (is it one?) Wadati-Benioff zone. 7. The tectonic history of the Gulf of California region and the lithospheric structure of the rift and margins. 8. The tiering, nature, kinematics, and crustal evolution resulting upon trunca- tion of the southern Pacific margin of Mexico.

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This book discusses the results of the transects program, which involves compilations of maps and cross sections showing geological, geophysical, and geochemical data and interpretations along 23 transects from the continental craton to the oceanic lithosphere around North America. The book contains two sections, an overview and a set of recommendations for the program and synopses, findings, and problems associated with the 23 transects.

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