Continental Tectonics (1980) / Chapter Skim
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II. Plate-Boundary Tectonics
II. Plate-Boundary Tectonics
Pages 31-62
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From page 33... ...
Geological Survey Until the advent of plate-tectonic concepts in the late 1960's, geologists and geophysicists studying the components of ancient erogenic and magmatic terranes on the continents had no actualistic frarneworlc within which to comprehend the origins and interrelationships of those components. We now see that modern analogs for many features of the ancient terranes form primarily in continental-margin arcs and island arcs, above oceanic lithospheric plates sliding beneath continental plates or other oceanic plates.
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From page 34... ...
. The subducting plate dips very gently beneath the continental slope, landward of a trench with gently sloping sides, for a distance of 100 km or more; only beyond this distance does the plate roll toward the steeper dip of the inclined Benioff zone as deduced from mantle earthquakes.
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From page 35... ...
like that of Figure 3.1, den elops only where the leading edge ofthe continental plate consists of a strip of oceanic angst, either formed in contact w ith the continental crust by a previous spreading event or sutured to the continent following pre~ious subduction. The basin is formed by the raising of the leading edge of this strip, not by depression fit the center.
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From page 36... ...
Ophiolitic melange~pelagic sediments and mafic oceanic crustal rocks, in a matrix of serpentinite disrupted and hydrated from oceanic mantle rocks~re known in many erogenic assemblages that are now parts of continents, where they are associated with magmatic assemblages analogous to modern island arcs. These ophiolitic melanges may mostly represent the small accretionary wedges fanned along oceanic island arcs, and contest win melanges, dominated by te~Tigenous elastic sediments that fanned along active continental margins.
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From page 37... ...
The strata lap eastward across the oceanic crustal strip and in the subsurface butt against what was in Cretaceous time the moderately sloping edge of continental crust. The leading edge ofthe oceanic strip was rotated upward concurrent with Cretaceous subduction.
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From page 38... ...
38 WARREN H~ILTO! 120° l ~ ~ ~ ~ volcanic ~ coLuMl~la PLATeAi} .
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From page 39... ...
Here, the leading edge of the continental plate was ~~..'ed of continental cutest; the narrow strip of attached oceanic crust is missing, and no o~ter-arc basin developed. LATE CENOZO}C DISRUPTIO.N During Cretaceus and early Tertiary time, the Pacific margin ot Califomia was ~ relatively simple system of parallel tectonic and magmatic belts, controlled by the igoro~s s~bd~ction then occurring beneath all of Califiornia The subduction was more rapid than the spreading of the East Pacific Rise offsl?
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From page 40... ...
40 3so 30 WARREN H~!
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From page 41... ...
Cvmplexitie.s of Modern and Patient Subduct:<'n System.s the northwest states. Mom northwest Nevada and southen, Idaho northwestward' continental crust older than middle Tertiary was thinned by extension in some areas and completely rifled in others, the volumes has ing been filled subsequently mostly by middle and upper Cenozoic basalts (Figure 3.21.
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From page 42... ...
These are all ens ironments ~ ith dominantly extensional tectonics, and active and ancient rift occurrences are associated with a number of different extensional plate-tectonic regimes. The Wilson cycle is the concept that permits application of plate-tectonic principles to the record of historical geology within the continents.
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From page 43... ...
Such rids will hare failed to develop into ocean. Failed rifts of this kind stretching into continents at a variety of angles Tom Atlantic-t~pe ocean margins are numerous and are rapidly becoming the best known ol all h'ssi!
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From page 44... ...
the sediment fill of the failed rifts is oil-bearing in some places. Oil is produced from three major ens ironments in rifts ofthis class: nonmarine graben facies at the bottom of the rifts, marine limestones formed during rapid subsidence of the rifts a few tens of millions of years after continental rupture, and younger elastic material progracIing off the continent along or across the rift.
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From page 45... ...
For this reason, rifts of this kind are of less general interest in continental geological history than the four major rid classes: continental rupture rifts, failed rifts at Atlantic margins, aulacogens, and impactogens. A generalization of this type always has excel tions, and the Devonian rift basins of Nonvay (Horn, Solur~d, and Hornelen)
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From page 46... ...
For example, both the Rhine graben and the Polish trough strike from northern Europe into the Alpine fold belt. The start of the geological history of the former is contemporary with the mid-Eocene Nleso-Alpine collisional event (5engoret al., 1978)
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From page 47... ...
Failed riRs of Late Triassic age around the Gulf of Mexico are deeply buried under the salt deposited soon after ocean opening in that area and few research results have, as yet, been published on these relatively inaccessible features. AULACO GENS The early development of aulacogens associated with the opening ofthe lapetus Ocean along the site ofthe Appalachians in late Precambrian times has been considered by Rankin (1976)
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From page 48... ...
, which is now being recognized to show strong sibs of convergence and is perhaps a Proterozoic Benue trough. Less well known are rifts striking into the circum-Ungava collisional fold belt near Richmond Gulf and Cambrien KEVIN BURKE Lake, but limited evidence indicates that these are aulacogens nether than impactogens.
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From page 49... ...
. Rifts at high 49 angles to erogenic belts: tests for their origin and the tipper Rhine graben as an example.
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From page 50... ...
Seismic profiles across divergent continental margins and regional geological studies indicate the occurrence of highs near the boundary that probably fortned during the late rift phase of divergent margin evolution. Our seismic and regional geological studies have provided us with sufficient data to develop a model for the evolution of these outer highs.
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From page 51... ...
Observations of cross sections across divergent margins indicate that outer highs are variable in size, elevation. and amount of faulting.
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From page 52... ...
For example, the Rhine Graben has two large volcanic centers, Vogelsberg and Kaisers~hl, separated by areas of limited volcanic activity (Rhine Graben Research Group for Explosion Seismology, 1974~. In general, divergent margins have a characteristic progression of sediment types that vary as an area progressively undergoes rifting and then driRing.
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From page 53... ...
o C, . 4 {: en · - ~ ~ · a: ca · ~ .,4 Q ~ ~ ._ ~ En ~ Cal ._ ~ m ~ ~ _ _ _ ._ O m~ _ ~ _ was 3wOSdO 9NIN30lM 1~101 ~ _ _ _ J z cr 1 ~ 1 ~ 1 L L: 1 a I 2 <_ ___ - Ct: Z ~ ~ ~ ~ ·` C)
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From page 54... ...
SUMMARY OF EVOLUTION OF DIVERGENT (PAS S IVE ) CONTINENTAL MARGIN S If the formation of outer highs during the late rid stage is as prevalent as we believe, it would add another phase to the evolution of divergent (passive)
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From page 55... ...
up- on ~ YoL~ ~ 1 .;1r,.~ ~ ~ ~~ \:: -.
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From page 56... ...
The initial grabening phase may be caused by crustal thinning over a zone several times wider than the graben itseIf (Rhine Graben Research Group for Explosion Seismoiogy, 1974~. Figure 5.11, reproduced £rom the Rhine Graben Research Group report, shows a cross section of the Rhine Graben.
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From page 58... ...
Cooling ofthe mantle below the thinned continental crust and the newly formed oceanic crust and sedimentary C ~ ~ LEO PROGRADHG SWAMPS OF DREG PHASE i, ~ MARE ~M~ 2- ~ / _ ', -- , / `'' \ _ I,' BROW ~ - Rem OF MOW ~ _' ~ ~~ 1__ _\ _ ~U RR ~ I -.-1 SEDIMENTS OF EARLY '' ' -- -"aid-,-' ~ PRERH~M~ FIGURE 5.10 Tectonic sedimentary model showing the evolution of outer lights on divergent margins.
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From page 59... ...
Subbasins thus formed can restrict the circulation within the late-rift- and early-driR-phase basins, thus causing conditions favorable to the deposition of black shales and/or evapontes. PROBLEMS AND RECOMMENDATIONS The combination of regional geological studies and multichannel seismic profiles used in conjunction with other geophysical data has provided the information for the definition of some key problems related to the evolution of outer highs on divergent continental margins.
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From page 60... ...
is verified, it will better explain many of the geological problems associated win divergent continental margins, including a better understanding of the nature of the oceanic~ontinental crusted transition, associatec3 deposition including salt and organic-nch shales, and Me relation of volcanism to the onset of seafloor spreading. SUMMARY Observation of a large number of geological and seismic profiles across divergent continental margins has demonstrated the widespread occurrence of structural highs that were formed during the late-nflc phase of divergent margin evolution and are now the boundary between continental and oceanic crust.
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From page 61... ...
;olution of Passive Continental Margins in the Light of Recent Drilling Results, Philosophical Transactions of the Royal Society, London, England. 61 Veevers, J
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