Continental Tectonics (1980) / Chapter Skim
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V. Continental Crust
V. Continental Crust
Pages 159-197
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From page 161... ...
Thus, study of Cenozoic volcanism in He western United States may reveal structural and compositional constraints on the lithosphere of the American plate and on the geometry of past interactions with other plates. This chapter focuses on four features of continental tectonics: (1)
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From page 162... ...
Oligocene volcanic activity in the San Juan field, southwestem Colorado, is interpreted as recording the rise, differentiation, and consolidation of a composite batholi~, largely of intermediate composition and covering an area of roughly SO km by 100 km (Steven and Lipman, 1976; Lipman et al., 1978~. Early eruptions that fanned stratovolcanoes of intermediate composition in the San Juan field were accompanied by intrusion of small stocks into the cores of these volcanoes (Figure 14.1A)
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From page 163... ...
Lake City Creede Coct~etopa Park San Luis Bachelor La Garita Mount Hope S Iverton San Juan U ncompaNgre Lost Lake Ute Creek Summ.tv~lle Platoro Bonanza FIGURE 14.2 Calderas in the San Juan volcanic field in relation to Bouguer gravity field (Steven and Lipman, 1976)
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From page 164... ...
Recent Pb and Sr isotopic studies of continental volcanic rocks in the western United States indicate that the isotopic composition of volcanic rocks reflects the age and composition ofthe underlying lithosphere, demonstrating major compositional control by the lower crust or the li~osphenc upper mantle. For example, isotopic composidons of both Tertiary volcanic rocks of the Absaroka volcanic field and Quatemary volcanics of the Yellowstone Plateau in the same region (Petennan et al., 1970)
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From page 165... ...
STRUCTURAL DISCONTINUITIES IN THE CONTINENTAL LITHOSPHERE Structural discontinuities in the lithosphere also seem important in controlling the distribution and type of Cenozoic volcanism in the western United States. Especially conspicuous as controls for Cenozoic volcanism seem to be several no~east-trending zones of different ages and origins: the Snake River-Yellowstone zone, the Springerville-Raton zone, and the Colorado mineral belt (Figure 14.41.
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From page 166... ...
, referred to venously as the Spnngerville~aton zone, the Jemez zone, or the Raton volcanic chain, has also been proposed as marking the trace of a mantle plume (Suppe et al., 1975~. This major locus of late Cenozoic activity includes the Pinacate basalt field in northwest Mexico, the Spnngerville-White Mountain volcanic zone in eastern Arizona, the Mount Taylor and lemez volcanic fields in PETER W
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From page 167... ...
The recurrence and general confinement of Cenozoic volcanism to regions where previous episodes of magnmatism and tectonism may have healed, annealed, and weakened the lithosphere seems an important consideration in interpreting the distribution of Cenozoic volcanic activity in the westem United States. PLATE-TECTONIC INTERACTIONS Continental volcanism also provides evidence for the nature of plate interactions in the western United States during the past 100 m.y.
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From page 168... ...
Nevertheless, through late Mesozoic time, a longlived, relatively steeply dipping, subduction system apparently was maintained, along the western margin of the American plate, concurrently with consumption of an eastern Pacific plate. Starting at about 80 m.y.
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From page 169... ...
. Northeast-t~ending lines mark approximate traces of the Snake River-Yellowstone zone, the Colorado mineral belt, and the Springenrill~Raton zone.
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From page 170... ...
The confinement of early Tertiary igneous activity in the southern Rockies to the Colorado mineral belt, despite apparently continuous subduction at Me western margin of the plate, suggests the presence in this region of lithosphere that is too thick, cold, or rigid to permit penetration of subduction-related magmas, except along major structural flaws. Any igneous expression of passage of a Kula-Farallon-American plate triple junction, infected to have swept northward in the early Tertiary (Atwater, 1970)
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From page 171... ...
ago (Figure 14.7~) , andesitic volcanism in the western United States was restricted to a narrow belt near the margin of the American plate and was represented by a southem extension into California and Nevada of the Cascade volcanic arc.
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From page 172... ...
PERSPECTIVE S Complex space~me~omposition patterns of Cenozoic volcanism in the western United States offer infonnation on the composition and structure of Me continental lithosphere, as well as provide some of the best available constraints on geometry of past interactions between Me American plate and various Pacific plates. These volcanic rocks are most readily interpreted in terms of two broad assemblages: an earlier Cenozoic, predominantly andesitic assemblage inferred to result from geometrically complex processes of plate convergence and a later, fundamentally basaltic assemblage associated with exten signal tectonic settings both in regions behind continuing subduction systems especially where slab dip has steed ened and in regions where subduction has te,~'inated.
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From page 173... ...
. Cenozoic volcanism and plate-tectonic evolution of the western United States: 1.
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From page 174... ...
. Calderas of the San Juan volcanic field, southwestern Colorado, U.S.
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From page 175... ...
Continental accretion of North America was supported by the early isotopic studies of Precambrian rocks and the apparent progressive younging of Phanerozoic erogenic belts toward the continental margins. Continuing work showed the presence of older rocks in the cores of hinterlands of erogenic belts and thus led to the recognition that continental crust, which had been stabilized in some earlier event, could be reactivated, have extensive sedimental belts (geosynclines)
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From page 176... ...
Three divisions of geological time are commonly used in discussing plate tectonics and its applicability to tectonic features: Archean, Proterozoic, and Phanerozoic (Anhaeusser, 1975; Burke et al., 1977; Sutton, 1973; Windley, 1977~. These are diachronous in space and time but roughly correspond to the standard geological time scale.
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From page 177... ...
, however, have shown that the volcanic rocks of the Abitibi greenstone belt were erupted in about 25 m.y., whereas the Uchi Lake greenstone belt spanned 220 m.y. The cooling and stabilization of the Superior province occurred near 9.5 b.y.
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From page 178... ...
.^ slgul~lcance. Figure 15.2 also shows the Proterozoic fold belts that transect the Archean of the Canadian Shield and that are now part of the Churchill province that was consolidated with the Archean elements about 1.7 b.y.
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From page 179... ...
appears to be as shown in Figure 15.3, although the steps in its evolution are only now being deciphered. Isotopic data in Colorado and southern Wyoming indicate that no Archean crust exists beneath the exposed Precambrian rocks (Hills and Armstrong, 1974~; thus crust evolved since the Archean.
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From page 180... ...
The suture has been proposed as the closing of an aulacogen or small ocean (Garrison and Rarnirez-Ramirez, 1978; Sengor and Budder, 1977~. This proposed suture is part of the zone that several investigators include in the Texas lineament, a major zone of recurrent tectonic activity that extends across the sou~western United States.
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From page 181... ...
and widespread rhyolitic volcanism across the central midcontinent region; late Proterozoic to Cambrian (1.~0.6 b.y.) with ricing and collision events of Phanerozoic style; and the Phanerozoic- the recognized realm of plate tectonics.
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From page 182... ...
. Earliest Precambrian ultramafic-mafic volcanic rocks: ancient oceanic crust or relic terrestrial maria?
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From page 183... ...
. Late Precambrian evolution of North America: plate tectonics implications, Geology 4, 11-15.
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From page 184... ...
analyzed all available geological data and attempted to elucidate systematically the tectonic characteristics of China in his book, On Major Tectonic Forms of China. Recently, greater attention has been paid by Chinese geologists to the theories of seafloor spreading and plate tectonics.
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From page 186... ...
E HUANG Subdivision of Orogenic Cycles and Important Events of the Tectonic Development of China .
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From page 187... ...
tillites and stromatolites are found, showing that the Tarim platform took shape near the end of the Proterozoic just as the Yangtze paraplatforrn did. TIENSHAN—EHINGAN GEOSYNCLINAL FOLD SYSTEM AND ARGON FOLD SYSTEM These two systems are components of the great central Asiatic-Mongolian arcuate fold region, which extends between the Sibenan platform, on the north, and the SinoKorean paraplatfonn and the Tarim platform, on the south (Figure 16.1~.
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From page 188... ...
Because the latter was strongly transfonned by Mesozoic tectonism and widely covered with Jurassic and Cretaceous continental volcanics, its tectonic character has long been misjudged. NATANHATA EUGEOSYNCLINAL FOLD BELT AND UPPER HEILUNGliCIANG MIOGEOSYNCLINAL FOLD BELT These are components of the Mesozoic geosynclinal fold systems located to the east of the Siberian platform (Figure 16.1)
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From page 189... ...
O It) o Con .,of coo s`3oA'o1 .9 Cohn _~ Talien ~ lij Kunlun-Altyn-North Nanshan essentially coincides with the depth fracture zones between the Tarim platforms and the Alashan Massif on the north and the geosynclinal fold belts on the south.
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From page 190... ...
190 ~ ~— En -~ 'my ~ ~~W-o ~ Ale-'- - N :-? I: brow ~ ~.
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From page 191... ...
These complicated fracture zones controlled the origination and development of the Kunlun-Tsinling geosynclinal fold system and constitute a geological dividing line between northern and southern China. This dividing line shifted from the line of North Nanshan-North Tsinling-North Huaiyang during the Caledonian cycle to the line of the northern margin of Tsaid~n~outh Kokonor Range-North Tsinling-North Huaiyang during the early Variscan cycle, and finally to the line of East Kunlun-North Tsinling-North Huaiyang since the late Variscan cycle.
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From page 192... ...
lc. HUANG Number Oepth Fracture Zone Depth Character Age of Activity Magmatism and Metamorphism 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 23 24 25 26 27 28 29 30 31 32 Darbut Irtish Karameili North Margin of Central Tienshan Derbugan Silamulun Cherchen No~ern Margin of Inner Mongolian Axis Altyn North Nanshem-Northem Margin of North Tsingling~orth Huaiyang Northen~ Margin of Tsaidam~outhem Margin of No~ TsinglingNorth Huaiyang East Kunlun Kantze-Litang Chinshakiang-Red River Lantsangkiang Nukiang Anningho Hsiaokiang Lungmenshan Lingshan Wuchuan-Szehwei Heynan Lishui-Haifeng Changle-Amoy Taihangshan East Tsangchow Liaocheng-Lankao Tancheng-Lukiang Yilan-Yitung Fushun-Mishan Tsang+Indus Longitudinal Valley of Taiwan aNumbers correspond to those circled on Figure 16.4 T
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From page 193... ...
, the Chinese proto-platform was disintegrated and partially transformed to form the Tienshan, the Nanshan, the Tsinling, and other geosynclines, all of which were again folded, transfol~ed, and consolidated at the end of the Variscan cycle. As a result, these newly consolidated geosynclinal folds joined and "cemented together" the pre-existing four platforms, the Siberian platform, the Tarim platform, the Sino-Korean paraplatforrn, and the Yangtze paraplatform, to form a gigantic craton called the Pal-Asia.
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From page 194... ...
also advocated a polycyclic development for the Indosinian geosynclines. Recent geological mapping in the Tienshan and Nanshan again confirms the view of polycyclic development of geosynclinal fold belts, which is briefly summarized below.
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From page 195... ...
From the above discussion, the author arrives at the preliminary conclusion that the Nanshan geosyncline is characterized by poIycyclic basic extrusives (tholeiites and spilites) , which, together with ultrabasic rocks, form polycyclic ophiolitic suites.
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From page 196... ...
E HUANG PRELIMINARY OBSERVATIONS ON PLATE TECTONICS IN CHINA TSANGPO DEPTH FRACTURE ZONE Geological and geophysical data indicating the applicability of plate tectonics along the Tibetan part of the Himalayan geosyncline were collected and analyzed by Chinese geologists (Huang et al., 1974; Chang and Cheng, 1973~.
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From page 197... ...
South of Batang, in the western side of the upper Chinshakiang fracture zone, occur typical ophiolitic melanges characterized by serpentinites, spilites with pillow structure, and various kinds of basic to ultrabasic rocks intercalated with radiolarian cherts, while exotic blocks of omphaciteeclogite; dillage~innamon stones; and particularly Devonian, Carboniferous, and Permian limestones are found in the matrix. In the eastern side of the same zone, wildflyschlilce deposits characterized by a matrix of argillaceous and arenaceous rocks with various exotic blocks yield many fossils ranging in age from Silurian to Permian, which are, without exception, older than those in the matrix.
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