Among the continents of the world, Asia has been subjected to the most significant Cenozoic deformation. The Cretaceous-Paleocene topography of China was generally tilted to the west, with the coastal areas of the Tethys in the west, a trans-Himalayan volcanic arc in the southwest, and relatively high land and endoreic basins in the east. These conditions lasted until the late Eocene when India collided with Asia, thereby bringing the maritime climate in western China to an end. Although some argue for a Cretaceous plateau with as much as 3 km elevation (Harrison et al., 1998), other studies indicate that uplift of the Tibetan Plateau began ~21-20 Ma (Copeland et al., 1987; Harrison et al., 1991, 1998) and was accompanied by a general subsidence of eastern China. Still other studies (Molnar et al., 1993) argue for a late Miocene (10-8 Ma) rapid uplift of the Tibetan Plateau from a low or intermediate level to its maximum elevation (>5 km) and subsidence to its current elevation (~4 km). These surface uplift changes reversed the topographic gradient in China from west tilting to east tilting. Subsequent tectonics have maintained or increased the west-east gradient, and some geologists and geomorphologists believe that the Tibetan Plateau was uplifted only 2-3 m.y. ago (e.g., Li, 1991).

Coincident with the large-scale deformation of Asia, many of the marginal seas in the western Pacific were formed during the early Miocene. One model links the opening of the SCS basin with the Red River fault zone, which has at least 500 to 600 km of left-lateral displacement created during the Oligocene and Miocene (Schärer et al., 1990; Briais et al., 1993). Alternative models relate the opening of the SCS to subduction under Borneo (Taylor and Hayes, 1983) and the influence of subduction beneath the Philippines, driving a backarc type of extension in the over-riding plate (Taylor and Hayes, 1980). The deep-water, rhomboid-shaped Central Basin is underlain by oceanic crust (Fig. 3A), which contains a sequence of seafloor-spreading magnetic anomalies ranging from 32 (magnetic Anomaly 11) to 16 Ma (Anomaly 5c), with a southward ridge jump at ~27 Ma (Anomaly 7/6b) (Briais et al., 1993). The slopes of the SCS contain numerous coral reef terrains, including the Nansha Terrain (Reed Bank and Dangerous Grounds) and the Xisha-Zhongsha Terrain (Macclesfield Bank and Paracel Island), which may have rifted southward during the opening of the SCS (Jin, 1992).

The structure of the northern SCS margin has been extensively studied through oil exploration and geophysical studies to determine the amount of crustal extension during formation of the SCS (e.g., Hayes et al., 1995a, 1995b). The shelf basins on the SCS contain >4 km of Cenozoic deposits that have been drilled by petroleum companies (Ru et al., 1994). The present shape of the SCS is also closely related to the rotation history of the Philippine Sea Plate to the east and the ongoing collision with the Australian Plate to the south (Packham, 1996). The counterclockwise rotation of the Philippine Sea Plate led to the arc-continent collision of the Luzon Arc with the underthrusting Eurasian Plate since 6.5 Ma (Huang et al., 1997), giving rise to the formation of Taiwan Island and the Bashi Strait. Both the collision processes have enhanced the enclosed nature of the SCS.

Evolution of Asian Monsoonal Climates

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