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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
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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
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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
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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
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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
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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 OCR for page 40
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
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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
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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
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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
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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!
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Representative terms from entire chapter:
70 100° 36°~
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/'
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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