North American Continent-Ocean Transects Program (1989) / Chapter Skim
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5. Major Topical Problems
5. Major Topical Problems
Pages 21-33
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From page 21... ...
11. Deep continental crustal structure and origin.
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From page 22... ...
How much of the deformation in rocks of ancient orogenic belts was acquired during early accretionary phases? Tectonic attrition at active North American margins is poorly understood because the material removed presumably by strike slip, rifting, or subduction—is no longer present locally.
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From page 23... ...
Over what range of widths of continental plate boundary zones are horizontal displacements taken up relative to cratonal North America, and what parameters control the width: slab age and/or dip, lithospheric thickness, material strengths, inherited continental structure, obliquity of convergence? Further, how does contemporary active margin structure relate to obliquity and velocity of convergence?
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From page 24... ...
No magmatism accompanied this initial phase of deformation. Subsequently, Jurassic movements created swarmers of vertical diabase dikes oriented at high angles to the normal faults in the southeastern United States and fanning into near parallelism with the normal faults in the northeastern United States.
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From page 25... ...
5. Identification and Processes of Terrane Boundaries Orogenic belts of the North American continent contain displaced terranes that make up much if not all of zones 3 and 4 (Figure 1~.
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From page 26... ...
Last, the timing and ejects of superposed deformations must be clear at all positions. How much of Precambrian North America was removed between the times of Late Proterozoic passive margin formation and the present edge of sialic North America that is mainly below the erogenic belts?
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From page 27... ...
Metamorphism is attributed to the doubling of the continental crust by the overthrusting of two superterranes onto the North American continental margin. In the Klamath Mountains of northern California and southern Oregon, it has recently been proposed that four tectonostratigraphic terranes were imbricated and accreted to the continental margin, producing a tectonic stack ~18 km thick in which there is an upward increase in metamorphism through the prehnite-pumpellyite and greenschist facies into highly deformed garnet-bearing schists and gneisses.
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From page 28... ...
Good resolution of ages of an erogenic phase at a given place would include ages of onset of movements, of maximum depth penetration/metamorphism, of rapid uplift, of attachment to North America, and completion of deformation. It is also important to resolve the timing of superimposed deformations.
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From page 29... ...
Island arcs, with their subducted oceanic crust, accretionary sedimentary wedge, voIcanicIastic sedimentary deposits mixed with intrusives in the magmatic arc, and backarc basins, provide distinctive tectonic megaunits. Passive rifted continental margins have continental crust and oceanic crust separated by a rifted crust of faulted blocks and intrusives, overlain by evaporite, carbonate, and ciastic sedimentary rocks deposited in shelf, slope, and rise paleoenvironments .
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From page 30... ...
11. Deep Continental Crustal Structure and Origin Deep seismic reflection profiling shows that the continental crust contains reflectors at all depths above the Moho, but with varied characteristics and concentrations Such reflectors occur in the zone of deformed siaTic continent as well as In the cratonal crust, and moreover, in displaced terranes.
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From page 31... ...
The Late Proterozoic breakup of the supercontinent and growth of oceanic lithosphere led to the isolation of a protocontinent of Phanerozoic North America. A Phanerozoic tectonic history therefore must account for the changes in extent and configuration between the Late Proterozoic protocontinent and the part of today's North American continent cored by Precambrian rocks that have remained contiguous or nearly so (Figure 1, zones 1 and 2~.
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From page 32... ...
These provide a glimpse into the complexity of the development of the transitional region: the superpositions of processes, temporal variability of processes along the margin, large and rapid vertical motions of 30 or more km, and the enigmas of origins of terranes and their approach and attachment to North America. A well understood Phanerozoic evolution of the North American margin requires both more widespread and more highly resolved dating that will indicate ages of deposition, magmatism, maximum depth penetration, uplift times and rates, fluid passage, and the superpositions of these and other effects.
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From page 33... ...
33 margins of the North American plate, ancient global plate reconstructions, and paleobiological excursions. The detection of major strike-slip components has proved to be elusive thus far, and different techniques sometimes yield greatly conflicting results.
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