Stephan A. Graham

Constraints on the early uplift history of the Tibetan Plateau

The surface uplift history of the Tibetan Plateau and Himalaya is among the most interesting topics in geosciences because of its effect on regional and global climate during Cenozoic time, its influence on monsoon intensity, and its reflection of the dynamics of continental plateaus. Models of plateau growth vary in time, from pre-India-Asia collision (e.g., ≈100 Ma ago) to gradual uplift after the India-Asia collision (e.g., ≈55 Ma ago) and to more recent abrupt uplift (<7 Ma ago), and vary in space, from northward stepwise growth of topography to simultaneous surface uplift across the plateau. Here, we improve that understanding by presenting geologic and geophysical data from north-central Tibet, including magnetostratigraphy, sedimentology, paleocurrent measurements, and 40Ar/39Ar and fission-track studies, to show that the central plateau was elevated by 40 Ma ago. Regions south and north of the central plateau gained elevation significantly later. During Eocene time, the northern boundary of the protoplateau was in the region of the Tanggula Shan. Elevation gain started in pre-Eocene time in the Lhasa and Qiangtang terranes and expanded throughout the Neogene toward its present southern and northern margins in the Himalaya and Qilian Shan.

Late Paleozoic tectonic amalgamation of northwestern China: Sedimentary record of the northern Tarim, northwestern Turpan, and southern Junggar Basins

Sedimentary rocks contained in basins adjacent to the Tian Shan provide a long and complex record of the late Paleozoic continental amalgamation of northwestern China, complementing that provided by rocks preserved within the range. This record, which comprises dramatic changes in sedimentary facies, sediment dispersal patterns, sandstone provenance, and basin subsidence rates, broadly supports previous interpretations of a two-part evolution of the Tian Shan: Late Devonian to Early Carboniferous collision of the Tarim continental block with the Central Tian Shan, followed by collision of this combined block with island arcs in the north Tian Shan and Bogda Shan in Late Carboniferous–Early Permian times. The first collision resulted in widespread angular unconformities within the Tarim basin. Continued convergence following the collision created a long-lived flexural foredeep along the northern margin of the Tarim block, which received at least 2000 m of Lower Carboniferous through Lower Permian fluvial and marine sediment derived from the interior of Tarim. Subsequent Early Permian continental extension of the northern Tarim basin resulted in the deposition of interbedded nonmarine siliciclastic sedimentary rocks and mafic to felsicvolcanic rocks. Sandstone within this interval was derived from the paleo–Tian Shan, and is composed predominantly of lithic volcanic grains similar to the rhyolite. In contrast to the Tarim basin, calc-alkaline volcanic rocks and volcanogenic sedimentary rocks dominated Carboniferous and Permian sedimentation in the northern Turpan and northwestern Junggar basins. Volcanic arcs remained active in the North Tian Shan and Bogda Shan through the early Late Carboniferous, depositing a kilometers-thick interval of deep marine sediment-gravity flows in the northwestern Junggar basin. Major arc magmatism ceased in the Late Carboniferous in response to closure of the oceanic basin between the combined Tarim/Central Tian Shan block and the North Tian Shan/Bogda Shan arcs. Upper Carboniferous through Lower Permian rocks in the northwestern Junggar basin compose the sedimentary fill of a bathymetric basin of oceanic depth (on the northern side of the volcanic arcs), culminating in a 1000-m-thick marine regressive sequence. Middle to Upper Permian sandstones were derived from the uplifted paleo–Tian Shan and bear the distinctive provenance imprint of granitic rocks presently exposed within the range. Late Permian subsidence of the Junggar basin accommodated >5 km of nonmarine sediments; however, the cause of this subsidence and its relationship to regional tectonic events remain controversial.

Late Oligocene-early Miocene unroofing in the Chinese Tian Shan: An early effect of the India-Asia collision

Apatite fission-track data indicate that Mesozoic strata exposed on the northern flank of the Chinese Tian Shan underwent ∼4-5 km of late Cenozoic unroofing, beginning at ∼24 Ma. This age apparently dates initial reactivation of the northern Tian Shan in response to the India-Asia collision, which continues to raise the mountain range today. Numerous studies of the Himalaya and Tibet suggest that a major shift from extrusion-dominated to crustal thickening-dominated tectonics occurred in latest Oligocene-early Miocene time, approximately coincident with the start of unroofing in the Tian Shan. This suggests that Tian Shan unroofing was a distant effect of that shift within the collision zone.

Sedimentary record and tectonic implications of Mesozoic rifting in southeast Mongolia

The East Gobi basin of Mongolia is a poorly described Late Jurassic–Early Cretaceous extensional province that holds great importance for reconstructions of Mesozoic tectonics and paleogeography of eastern Asia. Extension is especially well recorded in the structure and stratigraphy of the Unegt and Zuunbayan subbasins southwest of Saynshand, Mongolia, where outcrop and subsurface relationships permit recognition of prerift, synrift, and postrift Mesozoic stratigraphic megasequences. Within the synrift megasequence, three sequences developed in response to climatic and rift-related structural controls on sedimentation. Where best exposed along the northern margin of the Unegt subbasin, each of the synrift sequences is bounded by unconformities and generally fines upward from basal alluvial and fluvial conglomerate to fluvial and lacustrine sandstone and mudstone. Resedimented ashes and basalt flows punctuate the synrift megasequence. Rifting began in the Unegt subbasin prior to 155 Ma with coarse alluvial filling of local fault depressions. Subsidence generally outstripped sediment supply, and fresh to saline lacustrine environments, expanding southward with time, dominated the Unegt- Zuunbayan landscape for much of latest Jurassic–Early Cretaceous time. Episodic faulting and volcanism characterized the basin system for the balance of the Early Cretaceous. A brief period of compressional and/or transpressional basin inversion occurred at the end of the Early Cretaceous, prior to deposition of a widespread Upper Cretaceous overlap sequence. The driver(s) of Late Jurassic–Early Cretaceous extension remain uncertain because southeast Mongolia occupied an intraplate position by the beginning of the Cretaceous. Extension in the East Gobi basin was coeval with collapse and extension of early Mesozoic contractional orogenic belts along the northern and southern borders of Mongolia and probably was a linked phenomenon. Strike-slip faulting associated with collisions on the southern Asian and Mongol- Okhotsk margins likely also played a role in late Mesozoic deformation of the East Gobi region, perhaps partitioning the Gobi from apparently coeval large-magnitude contractional deformation in the Yinshan- Yanshan orogenic belt south of the study area in Inner Mongolia.

Himalayan-Bengal Model for Flysch Dispersal in the Appalachian-Ouachita System

The relation of the modern Bengal subsea fan to the Cenozoic Himalayan suture belt and the analogous relation of the Carboniferous Ouachita flysch to a presumed Paleozoic Appalachian suture belt suggest a guiding principle of synorogenic sedimentation. Most sediment shed from orogenic highlands formed by continental collisions pours longitudinally through deltaic complexes into remnant ocean basins as turbidites that are subsequently deformed and incorporated into the orogenic belts as collision sutures lengthen. India first encountered a southern Eurasian subduction zone near the end of Paleocene time. Northward movement of India since Oligocene time choked the subduction zone, stifled the associated magmatic arc, and created a suture complex of deformed Cretaceous flysch and younger Tertiary molasse. Strata derived from the resulting orogen include continental clastic wedges shed southward toward India and voluminous turbidites fed longitudinally through the Ganges-Brahmaputra Delta into the Bay of Bengal. The eastern flank of the Bengal subsea fan is being subducted now beneath the still-active eastern extension of the subduction zone. The sequential, north-to-south welding of Europe and Africa to North America formed the complex Appalachian-Caledonide-Mauritanide suture belt, from which Taconic, Acadian, and Alleghanian clastic wedges were shed toward the North American craton. Turbidites of the Carboniferous Ouachita flysch were fed longitudinally, as sediment supplied through the Alleghanian clastic wedge, into a remnant ocean basin lying south of North America. The Ouachita system was then thrust northward across the continental edge during arc-continent collision that progressed from east to west.

Detrital zircon provenance of the Late Triassic Songpan-Ganzi complex: Sedimentary record of collision of the North and South China blocks

The majority of the Songpan-Ganzi Triassic flysch sequence is believed to have derived from denudation of the Dabie and Sulu ultrahigh-pressure (UHP) metamorphic belt in eastern China (e.g., [Nie et al., 1994][1]; [Zhou and Graham, 1996][2]); however, intense debate still exists on the sources of

Junggar basin, northwest China: trapped Late Paleozoic ocean

Abstract The Junggar basin originated during the late Paleozoic as either a remnant lower to mid-Paleozoic ocean basin or a mid-Carboniferous back-arc (intra- or inter-arc) basin bounded by emergent volcanic arcs south of the Siberian Craton. The retreating Junggar Sea left behind a regressive sedimentary section comprising at least 3–4 km of marine volcaniclastics in the southern Junggar area. Sandstones deposited in the Junggar basin since the Devonian are exclusively volcaniclastic, demonstrating that Precambrian basement rocks have never been exposed in the basin, and supporting the hypothesis that the Junggar is underlain by oceanic crustal materials. The mid-Carboniferous Junggar Ocean may have been a remnant ocean basin of early to mid Paleozoic age, or alternatively may have been an extending intra- or inter-arc basin formed behind an emergent volcanic arc in the northern Turpan region. Subsequent strike-slip deformation and the general sparsity of geologic and geophysical data from the Junggar area make it difficult to distinguish between these models. Late Early Permian through Triassic sediments of the Junggar basin are exclusively non-marine, deposited in a flexurally subsiding foreland basin during initial uplift of the ancestral Tian Shan mountains. The sedimentary section deposited during the Late Permian-Early Triassic appears to be inconsistent with a proposed rifting episode during this period.

Detrital zircon provenance analysis of the Great Valley Group, California: Evolution of an arc-forearc system

The improved resolution of sediment provenance from detrital zircon analysis of Great Valley stratigraphy enables recognition of previously undocumented arc magmatism and the evolution of regional drainage systems within the Cretaceous arc-forearc system related to uplift, magmatism, and structure in the arc. Great Valley detrital zircon age data confirm previous studies that indicate that the locus of the sediment source in the southern Sierra Nevada arc migrated east with the active volcanic front and suggest rapid rates of uplift and unroofing of the southern arc. Sacramento Valley detrital zircon age data indicate a more complex history of drainage in the northern Klamath-Sierran arc than previously documented. Detrital zircon age distributions from the Cache Creek section of the Great Valley Group broaden through time from nearly unimodal age distributions to signatures with multiple age peaks. This transition to more broadly distributed detrital zircon age spectra likely results from a combination of (1) expanding subaerial drainage systems from highly localized to more broadly distributed catchments; (2) changing shelf and submarine-canyon morphology with rising sea level and/or basin subsidence; (3) increased degree of dissection of the Klamath-Sierran arc; and (4) potential drainage capture and redirection within the arc. Sacramento Valley detrital zircon age data also record a pulse of Late Jurassic to Early Cretaceous magmatism in the northwestern Sierra Nevada arc, an age of Cordilleran magmatism and deformation represented by limited exposure in the modern Sierra Nevada. These results offer significant new insights into the evolution of a well-studied arc-forearc system.

Paleozoic oil-source rock correlations in the Tarim basin, NW China

We studied a suite of 40 oils and extracts of purported source rocks from the Tarim basin in NW China. The main group of oils comes from Tazhong and Tabei wells, which sample the largest known petroleum accumulations in the basin. These oils can be statistically correlated with extracts of Ordovician rocks based upon high relative concentrations of 24-isopropylcholestanes and low relative concentrations of dinosteranes, triaromatic dinosteroids, and 24-norcholestanes. In contrast, extracts from Cambrian rocks have low relative concentrations of 24-isopropylcholestanes with high relative concentrations of dinosteranes, triaromatic dinosteroids, and 24-norcholestanes. Although some Tarim basin Cambrian rocks yield high total organic carbon contents, we see little evidence in the analyzed oil samples to suggest that they came from Cambrian source rocks.

Uplift, exhumation, and deformation in the Chinese Tian Shan

The terranes composing the basement of the Tian Shan were originally sutured together during two collisions in Late Devonian–Early Carboniferous and Late Carboniferous–Early Permian time. Since then, the range has repeatedly been uplifted and structurally reactivated, apparently as a result of the collision of island arcs and continental blocks with the southern margin of Asia far to the south of the range. Evidence for these deformational episodes is recorded in the sedimentary histories of the Junggar and Tarim foreland basins to the north and south of the range and by the cooling and exhumation histories of rocks in the interior of the range. Reconnaissance apatite fission-track cooling ages from the Chinese part of the range cluster in three general time periods, latest Paleozoic, late Mesozoic, and late Cenozoic. Latest Paleozoic cooling is recorded at Aksu (east of Kalpin) on the southern flank of the range, at two areas in the central Tian Shan block along the Dushanzi-Kuqa Highway, and by detrital apatites at Kuqa that retain fission-track ages of their sediment source areas. Available Ar/Ar cooling ages from the range also cluster within this time interval, with very few younger ages. These cooling ages may record exhumation and deformation caused by the second basement suturing collision between the Tarim–central Tian Shan composite block and the north Tian Shan. Apatite data from three areas record late Mesozoic cooling, at Kuqa on the southern flank of the range and at two areas in the central Tian Shan block. Sedimentary sections in the Junggar and Tarim foreland basins contain major unconformities, thick intervals of alluvial conglomerate, and increased subsidence rates between about 140 and 100 Ma. These data may reflect deformation and uplift induced by collision of the Lhasa block with the southern margin of Asia in latest Jurassic–Early Cretaceous time. Large Jurassic intermontane basins are preserved within the interior of the Tian Shan and in conjunction with the fission-track data suggest that the late Mesozoic Tian Shan was subdivided into a complex of generally east-west–trending, structurally controlled subranges and basins. Apatite data from five areas record major late Cenozoic cooling, at sites in the basin-vergent thrust belts on the northern and southern margins of the range, and along the north Tian Shan fault system in the interior of the range. The thrust belts *Now at ExxonMobile Exploration Company, P.O. Box 4778, Houston, Texas 77060, USA Dumitru, T.A., et al., 2001, Uplift, exhumation, and deformation in the Chinese Tian Shan, in Hendrix, M.S., and Davis, G.A., eds., Paleozoic and Mesozoic tectonic evolution of central Asia: From continental assembly to intracontinental deformation: Boulder, Colorado, Geological Society of America Memoir 194, p. 71–99. 72 T.A. Dumitru et al.