North American Continent-Ocean Transects Program (1989) / Chapter Skim
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6. The 23 Transects: Synopses, Findings, and Problems
6. The 23 Transects: Synopses, Findings, and Problems
Pages 34-80
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From page 34... ...
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~.
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From page 35... ...
- 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.
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From page 36... ...
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.
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From page 37... ...
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.
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From page 38... ...
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~.
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From page 40... ...
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.
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From page 41... ...
These include 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.
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From page 42... ...
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?
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From page 43... ...
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 underlying oceanic crust. Examples in the northern corridor are as follows.
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From page 44... ...
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~.
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From page 45... ...
onset of active margin tectonism that has existed to the present and includes accretion of terranes, continental arc magmatism, foreland thrusting, and distributed strike-slip deformation with senses of slip and obliquity that have varied with time and place. East of the Cascade arc and its recently extinct southerly prolongation, active tectonism is manifested by a broad region of regionalized uplift as great as 1.5 km in the last 10 my or less.
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From page 46... ...
Both passive margin orogenies probably represent arc-continent collisions, although the magnitude of arc translation before collision is disputed. The active margin to western North America in C1 began in mid-lYiassic time and continues today.
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From page 47... ...
The Sierra Nevada and Klamath Mountains (Figure 6) witnessed copious continental arc magmatism from Late Triassic through Cretaceous times and record collisional tectonism.
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From page 48... ...
48 TRANSECT C2 CENTRAL CALIFORNIA OFFSHORE TO COLORADO PLATEAU Synopsis and Findings Voluminous batholithic activity of the continental arc in the Sierra Nevada that was related to Franciscan subduction began between the axial Great Valley and western Sierra metamorphic belt at about 130 Ma with the emplacement of mainly gabbroic to tonalitic plutons. Subsequent magmatism swept eastward and became more granitic with an ~85 Ma termination along the eastern Sierra.
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From page 49... ...
contains the following modern tectonic provinces, in eastward succession (Figure 6~: a region of the Pacific plate that comprises a seamount province on abyssal oceanic crust and displacing terranes of the continental borderland, including a batholithic belt in the Peninsular Ranges; a transform rift floored by nascent oceanic crust that is the Pacific-North American plate boundary; the greatly extended Basin and Range province, here in its widest zone in North America and developed mainly in former cratonal lithosphere; and the stable craton. Older terranes exist both east and west of the plate boundary.
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From page 50... ...
inferred from the geochern~stry of suprajacent rocks. Rifting and Truncation: Gradients in thickness and facies among Lower Paleozoic and latest Precambrian North American continental shelf sediments in Nevada, north of C3, imply Late Proterozoic rifting and the onset of drifting at about 600 mybp.
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From page 51... ...
Gren~rille Province, Quebec, to Newfoundland; (East sheet) Rifted margin offshore northeast Newfoundland Synopsis and Findings Transect D1 displays the full width of transitional lithosphere in eastern Canada between the craton composed of unmodified Grenvillian basement and an arm of Atlantic oceanic lithosphere in the southern Labrador Sea (Figures 1 and 7~.
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From page 52... ...
These were structurally overri(lden by a sequence of contrasting rock assemblages in separate slices. The structurally lowest slices consist of sedimentary rocks from the nearby continental margin, and the highest ophiolitic slices represent farther travelled oceanic crust and mantle.
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From page 53... ...
The vertical Dover fault penetrates the crust, and the Avalon terrane has deep lithospheric underpinnings. Subsurface data confirm that Grenvillian basement extends eastward about 70 km beneath the ophiolitic Dunnage terrane but also indicate the sialic crust of the Gander terrane extends westward to the subsurface edge of Precambrian North America.
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From page 54... ...
The Tail of the Bank at the Grand Banks transform margin is a modern promontory that paraBels the Paleozoic St. Lawrence Promontory of the Appalachian orogen.
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From page 55... ...
Transect D2: Transform margin south of Grand Banks: Offshore eastern Canada Synopsis and Findings Transect D2 is designed to illustrate the nature of the transitional region formed at a transform fault in a passive margin system. The edge of North America south of the Grand Banks (Figure 7)
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From page 56... ...
Except for the rift grabens and the northwestern segment of this margin, sediments are thin over much of the Grand Banks and a deep wide sedimentary basin, such as those along the rifted margins, did not develop along the transform margin, although a narrow basin occupies the continental slope. The contact between the Avalon and Meguma terranes appears to have been reactivated either by rifting or by plate reordering on a more global scale.
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From page 57... ...
Of these, the Orpheus sub-basin which lies on the Avalon-Meguma boundary is a prominent example (Figure 7~. Regionally, the rift basins are not uniformly distributed throughout the Nova Scotian and southern Grand Banks regions.
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From page 58... ...
This velocity, typical of basaltic rocks, is evidence for the intrusion of basaltic magma, which underplated or intruded the thinned continental crust during rifting. The transition zone is also associated with the East Coast Magnetic Anomaly, a prominent marker that can be traced southwards as far as the Blake Plateau.
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From page 59... ...
A prominent free air gravity anomaly exists across this rifted margin. A large positive anomaly lies over the outer continental shelf, and a much smaller negative anomaly lies over the continental slope or rise.
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From page 60... ...
10. How sharp is the break from continental to oceanic crustal thickness across a transform margin?
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From page 61... ...
basin buried Ma - (Jr) basin [~\~g rocks older 7.\~.\l than 1 billion years sedimentary -I hang basins east coast magnetic anomaly UK Hamburg klippe Iaconic Allochthon\ SG Baltimore Cneiss micra continent HI Hsrtland Terrsne RP Reading Prong SMA S~ur~town Mts.
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From page 62... ...
development of Late Proterozoic and Early Cambrian passive margins on the eastern edge of the North American craton and all margins of the Baltimore Gneiss microcontinent (Figure 8) subsequent to continental rifting; (2)
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From page 63... ...
Triassic and Jurassic rifting, in part along older faults, resulted in most marked thinning in the 100 km seaward of Atlantic City, forming the Baltimore Canyon Trough. Seafloor spreading began in the Middle Jurassic; the western edge of the oceanic crust is marked by the East Coast Magnetic Anomaly.
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From page 64... ...
The melange sequence contains several Cambrian to Early Ordovician plutons that may have been generated over oceanic crust during subduction of a spreading ridge. The Taconian cIastic wedge in Virginia was largely derived from cannibalized platform drift and deep water rift/drift sequences of the North American continental margin.
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From page 65... ...
Mesozoic rift basins on both sides of the hinge zone, the Norfolk Fracture Zone, and Norfolk Basin; and (2) drift facies that are post-middle Jurassic prograding shelf and slope sequences.
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From page 66... ...
Continental crust (35 to 40 km thick) occurs west of the magnetic anomaly; transitional crust (20 to 25 km)
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From page 67... ...
) produced the foreland thrust belt, a Late Paleozoic cIastic wedge, and large crystalline thrust sheets.
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From page 68... ...
The Hayesville thrust is considered an Early Paleozoic thrust that is rooted in the Ordovician continent-ocean transition zone, because it juxtaposes sedimentary and volcanic rocks deposited on oceanic or a rifted continental basement onto a contemporaneous rifted margin sequence that was deposited on North American continental crust. Plutons emerged in this early phase and are confined to the Hayesville thrust sheet.
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From page 69... ...
the Late Proterozoic through Middle Paleozoic evolution of a south-facing passive margin, (2) the Middle to Late Paleozoic closing of an ocean basin and collision of South America and associated Sabine/Yucatan terranes with North America, (3)
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From page 70... ...
North American passive margin. Flysch continued to fill a deepwater basin between the two colliding continents, but as collision continued, the basin shallowed and was finally filled with nonmarine molasse.
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From page 71... ...
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)
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From page 72... ...
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 BrunswickAltamaha Magnetic Anomaly (BAMA)
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From page 73... ...
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.
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From page 74... ...
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.
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From page 75... ...
75 1:/ an' ~ =c' i\ C ~ ~ 1~ ~ at: / \ :.: ~ ~ \\:; An ~ W;- ~ ~ =-~` Are ,- 13 ~ T: Of/ ~ :~ "o/' |
From page 76... ...
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
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From page 77... ...
77 ~ ,- ~ ~ ~d~ of ~ ,l \ Age !
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From page 78... ...
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 midJurassic 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)
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From page 79... ...
8. The tiering, nature, kinematics, and crustal evolution resulting upon truncation of the southern Pacific margin of Mexico.
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