Edmund Z. Chang

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

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.

Detrital zircon provenance from three turbidite depocenters of the Middle–Upper Triassic Songpan-Ganzi complex, central China: Record of collisional tectonics, erosional exhumation, and sediment production

To test the idea that the voluminous upper Middle to Upper Triassic turbidite strata in the Songpan-Ganzi complex of central China archive a detrital record of Dabie ultrahigh-pressure (UHP) terrane unroofing, we report 2080 single detrital U-Pb zircon ages by sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) analysis from 29 eastern Songpan-Ganzi complex sandstone samples. Low (<0.07) Th/U zircons, consistent with crystallization under UHP conditions, are rare in eastern Songpan-Ganzi complex zircon, and U-Pb ages of low Th/U zircons are incompatible with a Dabie terrane source. An unweighted pair group method with arithmetic mean nearest-neighbor analysis of Kolmogorov-Smirnov two-sample test results reveals that the eastern Songpan-Ganzi complex is not a single contiguous turbidite system but is instead composed of three subsidiary depocenters, each associated with distinct sediment sources. The northeastern depocenter contains zircon ages characterized by Paleozoic and bimodally distributed Precambrian zircon populations, which, together with south- to southeast-directed paleocurrent data, indicate derivation from the retro-side of the Qinling-Dabie (Q-D) collisional orogen wedge. In the central depocenter, the dominantly Paleozoic detrital zircon signature and south- to southwest-oriented paleocurrent indicators reflect a profusion of Paleozoic zircon grains. These data are inter preted to reflect an influx of material derived from erosion of Paleozoic supra-UHP rocks of the Dabie terrane in the eastern Qinling-Dabie orogen, which we speculate may have been enhanced by development of a monsoonal climate. This suggests that erosional unroofing played a significant role in the initial phase of UHP exhumation and likely influenced the petrotectonic and structural evolution of the Qinling-Dabie orogen, as evidenced by compressed Triassic isotherms/grads reported in the Huwan shear zone that bounds the Dabie terrane to the north. The central depocenter deposits reflect a later influx of bimodally distributed Precambrian zircon, signifying either a decrease in the influx of Paleozoic zircon grains due to stalled UHP exhumation and/or dilution of the same influx of Paleozoic zircons by spilling of Precambrian zircon from the northeastern depocenter into the central depocenter basin, perhaps due to infilling and bypass of sediment from the northern depocenter or due to initial collapse and constriction of the eastern Songpan-Ganzi complex basin. The southeastern depocenter of the eastern Songpan-Ganzi complex bears significant Paleozoic, Neoproterozoic, and Paleoproterozoic zircon populations derived from the South China block and Yidun arc complex, likely recording nascent uplift of the Longmenshan deformation belt due to impingement of the Yidun arc complex upon the western margin of the South China block.

Collisional orogene between north and south China and its eastern extension in the Korean Peninsula

Abstract The relative movement between the north China block (Sino-Korean craton) and the south China block (Yangtze-Cathaysian craton) had been right-lateral along a transform fault during the Late Paleozoic. Devonian deep-water deposits on oceanic crust existed only in the east Qinling and Tongbuo areas where subduction of ocean crust is evidenced by metamorphism, magmatism and deformation. Carboniferous and Permian clastics and minor carbonates of shallow marine and terrestrial facies were deposited in a series of narrow basins with general east-west trending, and were controlled by the transform fault. The biota of the basins were communicated with the contemporary sedimentary basins in south China, north China and west China. There has been no wide Mesozoic oceanic crust between the north and south China blocks. Tectonic framework of central China during the Late Paleozoic is quite similar to that of western North America during the Cenozoic. Amalgamation of Sino-Korean craton with Siberian craton at the end of Paleozoic changed the moving direction of the former to the southeast and contraction was prevailing between the already juxtaposed north and south China blocks and the intervening Dabie-Su-Lu microcontinent. The southeast corner of north China block was slipped into the concave of the microcontinent, and enabled the latter to be underthrusted to a depth of more than 80 km to form a ultra-high pressure metamorphic complex in Early and Middle Triassic time. Down-going oceanic crust is not a prerequisite of A-type subduction such as the case in Pamir. The Tan-Lu fault, primarily a hinge fault, took place at the climax of contraction between north and south China blocks in the Late Triassic. Differential uplifting and the consequent erosion of Dobie and Su-Lu terranes, which are constituted by piles of subhorizontal thrust sheets, led to an apparent left-lateral offset. Su-Lu was uplifted higher and unroofed deeper than Dabie. An ancient river system along the east-west trending suture had been drained off into Songpan-Ganzi ocean to the west. Collision of the Indian plate with the Eurasian plate shifted the relative movement between the north and south China blocks to the left-lateral and initiated the Fen-Wei graben since Paleogene. The Imjingang belt between North and South Korea connects to the suture zone in the Dabie-Su-Lu area. To the east, an active continental margin along the southern margin of the Hida belt of Japanese islands prevailed in the Late Paleozoic. The Ogcheon belt in South Korea is an Early Paleozoic rift zone which is the extension of the Huanan aulacogen in south China. The Gyeonggi and Ryeongnam massifs of South Korea recorrelated to Yangtze and Cathaysian massifs in the mainland of China.

Sinian through Permian tectonostratigraphic evolution of the northwestern Tarim basin, China

Sinian through Permian sedimentary rocks of the Kalpin and Bachu uplifts, northwest Tarim basin, record three major periods of basin evolution, as represented by stratigraphic megasequences divided by major unconformities. Each megasequence is marked by distinctive sedimentary facies, sediment dispersal patterns, sandstone provenance, subsidence history, and in two cases coeval magmatism. The same megasequences are recognized in both the surface Kalpin and largely subsurface Bachu uplifts, indicating that these areas shared an essentially identical history at least through the end of the Paleozoic. The Sinian–Ordovician megasequence overlies an angular basal unconformity with older metamorphic rocks. Siliciclastic facies directly above the unconformity are coarse grained and contain interbedded basalt flows. These facies grade upward into shallow-marine limestone and dolomite and interbedded deeper marine graptolitic shale, apparently as a result of thermal subsidence following a period of extension. Silurian and Devonian facies unconformably overlie Middle Ordovician strata, and are exclusively siliciclastic. They grade upward from green shelfal siltstone and sandstone into red fluvial sandstone and mudstone; paleocurrent indicators within the fluvial facies indicate derivation from the east. These deposits correspond in age with a proposed suture in the Altyn Tagh range adjacent to the eastern Tarim basin, suggesting that they may have been shed from a rising collisional orogen in that area. A pronounced angular unconformity separates Devonian strata from Carboniferous to Lower Permian fluvial and marine facies, which contain quartz-rich sandstone derived from recycling of underlying strata. Carboniferous–Permian rocks include relatively deep marine Carboniferous facies that are preserved in the most northwestern outcrop exposures of the Kalpin uplift. These progressively lap out to the southeast, where only thin Lower Permian fluvial and shallow-marine facies are preserved. These facies and thickness relationships suggest deposition in a flexural foreland basin, brought about by an ongoing collision between the Tarim and the central Tian Shan blocks. Lower Permian fluvial facies interbedded with basalt flows sharply overlie the marine facies in the Kalpin uplift. The basalts are closely tied in age with northwest-southeast–trending dikes, sills, and plutons in the Bachu uplift. The significance of this magmatism is unclear, but it may relate to limited extension normal to the collisional front. Carroll, A.R., et al., 2001, Sinian through Permian tectonostratigraphic evolution of the northwestern Tarim basin, China, 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. 47–69. 48 A.R. Carroll et al.

Geology and Tectonics of the Songpan-Ganzi Fold Belt, Southwestern China

In the Late Paleozoic, the Sino-Korean (North China) and Yangtze-Cathaysian (South China) cratons collided. The Carboniferous and Permian foreland basin to the north of the Tongbo-Dabie Mountains, and elongate intermontane basins in East Qinling, were filled by marine to terrestrial sediments, in which the fauna and flora communicated from North China, South China, and West China. In Triassic time, the Dabie-Sulu Mountains became a Himalaya-type mountain range as a result of continent-continent collision and doubling of the crust. Marked exhumation of this mountain range shed huge amounts of detritus to the west. First filled were the remnant ocean basins in Qinling. As the remnant basins filled, submarine fan deposition shifted to the west to gradually fill the Songpan-Ganzi area. Songpan-Ganzi is surrounded by continents with pre-Sinian basement. The Sinian and Paleozoic strata and their fauna and flora are of Yangtzean affinity. Beginning in the Permian, a midocean-ridge triple junction was developed in Songpan-Ganzi, and the new oceanic crust provided more space for submarine fans. Later, a Triassic subduction zone was developed along the western margin of Songpan-Ganzi, and the rising island arc provided a smaller amount of detritus to its backarc basin in the east, which became part of Songpan-Ganzi. During the Early and Middle Triassic, the Dabie-Sulu high mountain ranges blocked the monsoon from blowing to the north, and, therefore, typical redbeds were deposited in North China for at least 15 million years, whereas the deposits of the same age in South China are still shallow-marine and littoral facies with coal measures. In the Late Triassic and Jurassic, the Dabie-Sulu mountain range was leveled to low hilly country. The monsoon blew to the north very easily, and coal measures were deposited all over North China. In Songpan-Ganzi, the Triassic submarine fan deposits were folded and metamorphosed during latest Triassic time, and the Songpan-Ganzi fold belt was formed. The Cenozoic Himalaya and its relationship with submarine fans in the Indian Ocean is similar to the Triassic Dabie-Sulu mountain range and its relationship with the Songpan-Ganzi submarine fans. Huge submarine fans and ultrahigh-pressure metamorphism are consequences of continent-continent collision, but the involved continents should have considerable sizes.

POTENTIAL OIL- AND GAS-BEARING BASINS OF THE QINGHAI-TIBETAN PLATEAU, CHINA

A number of composite sedimentary basins are present on the Qinghai-Tibet (Tibetan) Plateau. Some of these basins are filled with marine sediments, whereas others are fault-bounded continental depressions. Mixed source-rock types are present, of variable organic matter content; source rocks are mainly carbonates and secondarily mudstones. Vitrinite reflectance and biomarker analyses show that Mesozoic and Cenozoic source rocks have reached the peak stages of oil generation; older source rocks may have generated natural gas. The hydrocarbon reserves of these basins are believed to total ∼37.2 billion barrels. The Qiangtang-Qamdo basin probably has the greatest potential for oil exploration.

The Jiangnan Complex—A Middle-Late Proterozoic Accretionary Fold Belt in South China

The Jiangnan accretionary fold belt in South China consists of an Archean and Early Proterozoic continental core and a Middle and Upper Proterozoic accretionary fold belt. Dismembered ophiolite and ophiolite melange in the accretionary fold belt are 700 to 1000 Ma in age. The Middle Proterozoic island-arc volcanics are tholeiitic and calc-alkaline types, and those of the lower part of the Upper Proterozoic are calc-alkaline to high-K calc-alkaline. Komatiitic basalt associated with the volcanics, especially in the western segment, implies that the island arc is a primitive one. High P/T metamorphism marked by the mineralogic association aragonite-jadeite-glaucophane and lawsonite is dated as 1024 and 799 Ma. Granitic intrusions evolved from I-type to S-type through time, reflecting maturation of the island arc. Docking of small pieces of continental terrane might have helped to produce a significant island arc. An associated back-arc basin is characterized by turbidite deposits. Subduction ended as the Ca...