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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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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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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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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
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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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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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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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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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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:
oceanic crust
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.
