Plate tectonic history, basin development and petroleum source rock deposition onshore China

Abstract

China comprises a mosaic of distinct continental fragments separated by fold belts. These fold belts are suture zones resulting from the accretion of various fragments formerly separated by intervening areas of oceanic crust. The major sedimentary basins onshore China can be classified into four groups. Those in western China are flexural, developing as a result of north-south compression. In contrast, those in the east are extensional and related to development of the Pacific oceanic margin. In central China, basins have a more problematic origin. Those of north central China (Ordos, Sichuan) are flexural basins controlled by eastward directed thrusting along their western margin. In contrast, basins further south (Chuxiong, Shiwandashan) are predominantly extensional and related to major strike-slip movements. By synthesizing basin stratigraphies across China in tectonostratigraphic terms (and in particular comparing the nature and timing of unconformities), it is possible to formulate a coherent model for the palaeoreconstruction of China. We identify five major tectonostratigraphic breaks which equate with the collision of the following continental fragments: Tarim/North China (Carboniferous-Permian), South China Block (Permian-Triassic), Qiantang (Late Triassic-Early Jurassic), Lhasa Block (Late Jurassic-Early Cretaceous) and India (Early Tertiary). Prior to Permian times, the southern margin of Eurasia ran approximately along the northern border of modern China. The Late Carboniferous collision of Tarim/North China with Eurasia resulted in the development of a flexural basin (Junggar) and deposition of non-marine clastics. To the south of the suture, shallow marine deposition continued. In the Late Permian-Early Triassic, the progressive collision from east to west of the South China Block with the North China Block resulted in a change to fluvial/lacutrine sedimentation across the entire North China-Tarim block. Open marine carbonate deposition in the north of the South China Block passed southward into a deeper marine clastic sequence deposited in a backarc basin. Further south, a subduction zone existed along the southeastern margin of the South China Block. In western China, northward subduction throughout the Triassic resulted in the development of the Songban-Ganzi accretionary prism with retroarc thrusting resulting in flexure and the first development of the Tarim basin. Oblique collision of the Qiantang Block in the Late Triassic along the east of the South China Block resulted in east-west directed thrusting which initiated the Suchuan and Ordos basins. Continued strike-slip deformation along the south western margin of the South China Block resulted in the development of basins with a significant extensional component such as Chuxiong. The collision of the Qiantang Block with the southern edge of the Tarim Basin (Early Jurassic) resulted in a renewed clastic influx in both the Tarim and Junggar basins. Along the eastern (Pacific) margin a compressional arc and retroarc basin in the south passed northwards into an extensional arc system. Subduction rollback of the extensional arc initiated rifting in the Late Jurassic in the Eren and Songliao basins. The Late Jurassic-Early Cretaceous collision of the Lhasa Block in the west rejuvenated the thrust systems bordering the western basin and resulted in a renewed clastic influx. In the southeast, the compressional arc phase culminated in widespread thrusting and folding of Early Cretaceous age. In the northeast, extension continued with the progressive migration of the rift system southward with time. The arrival of the Indian Block in the Early Tertiary rejuvenated the bounding thrust belts of all the western basins. In the east, the change in convergence of the Pacific plate to a more westerly direction is marked by extension and widespread rifting along the entire length of Eastern China. Throughout most of China, Mesozoic and Cenozoic deposition occurred in predominantly non-marine environments. Source rocks in such settings comprise principally mudstones deposited in lakes (organic-rich mudstones). These can accumulate in both deep and shallow lakes. In order to accumulate substantial volumes, the lake must be significant in space and time. In China, lacustrine ORMs occur in both rift and flexural basins. Lacustrine ORMs deposited under humid climatic conditions are restricted to the period of maximum tectonic subsidence. In the flexural basins of western China, source rock deposition follows basin initiation by 20–30 Ma. In the extensional basins of eastern China, source rock deposition takes place 5–15 Ma after basin initiation. By contrast, semi-arid and arid climate lacustrine ORMs, whilst being best developed during the period of maximum tectonic subsidence, occur at all stages in the basin history.