|Figure 1: Taiwan topography and surrounding bathymetry. After Houvis et al. (1999).|
The Taiwan orogenic belt is in southeastern Asia, just off the coast of China. It is separated from the mainland by the South China Sea and is at the corner of two subduction zones: the Ryukyu and Manila trenches (figure 1). These two subduction zones separate the Eurasian tectonic plate to the west from the Philippine Sea plate to the east. The subduction polarity switches between these subduction zones, so the plate geometry is complex (figure 2). Maximum deformation is occurring off the southern shore of the island and decreases northward. This is due to the oblique nature of the collision. The northern part of the island is currently undergoing extension. This system has traditionally been called an arc-continent collision with the Luzon volcanic arc colliding into the Chinese continental margin. This calls up a picture of an arc encroaching on a continent and then colliding, creating a backstop against which to pile up a fold and thrust belt of continental sediments. This may not be the case in light of recent geophysical, structural, and geochemical data available about the island.
|Figure 2: Schematic diagram showing the present plate configuration of Taiwan (after Ernst et al. (1985)). The Eurasian plate is marked here as the "Asiatic" plate.|
Figure 3: Major geologic terranes of Taiwan and the faults that separate them. CP = Coastal Planes terrane; WF = Western Foothills terrane; HSR = Hsuehshan Range terrane; WCR = Western Central Range terrane; ECR = Eastern Central Range terrane; LV = Longitudinal Valley; CR = Coastal Range terrane; IP = Ilan Plane. Adapted from Chang et al. (2000), Ho (1988), Kao et al. (2000), and Lee et al. (1997).
|Many authors have different names for the same geologic and structural units. I will use the geologic terranes depicted in figure 3. I do not assign a specific geologic history to these terranes at this time because their interpretation has been reconsidered by recent structural papers (Chang et al., 2000; Lee et al., 1997). In general, the metamorphic grade of geological formations increases from west to east across the island (Ernst et al., 1985). The progression of terranes across the island from west to east is as follows; geologic descriptions are those of Ernst et al. (1985). The dividing faults are from Kao et al. (2000). On the west coast is the Coastal Plains terrane which is composed of recent, poorly consolidated alluvium, mostly terrace gravels . The Deformation Front divides the Coastal Plain from the Western Foothills terrane. The Western Foothills are composed of Miocene to Pleistocene shallow marine detrital materials and are bounded to the east by the Chukou fault. Next is the Hsuehshan Range terrane consisting of Eocene-Oligocene quartz and carbonate sandstone, argillites, and shales. Eastward of the Hsuehshan Range terrane is the Central Range terrane bounded on the west by the Lishan Fault and on the east by the Longitudinal Valley. The Central Range is divided into two parts: the Western Central Range and the Eastern Central Range. The Western Central Range is Paleocene, Eocene, and Miocene slatey and phyllitic metaclastites. The Eastern Central Range is composed of the Tananao complex, a belt of schist, greenschist, gneiss, carbonates, tuffs, and oceanic pelitic and mafic schists. It has been shown that the Tananao complex is composed of basement rocks of the Eurasian continental crust (Lan et al., 1996)-this is discussed in the geochemistry section. East of the Longitudinal Valley is the Coastal Range terrane, containing rocks of Luzon arc affinity, both volcanics and sedimentary rocks.|
|Figure 4: Bathymetric features off the south shore of Taiwan. KC = Kaoping Slope; HR = Hengchun Ridge; SLT = Southern Longitudinal Trough; HTR = Huatang Ridge; TT = Taitung Trough. After Houvis et al. (1999), terminology from Kao et al. (2000).|
Because Taiwan is growing to the south and most of the present-day convergence happens offshore south of Taiwan, it is important to point out large offshore topographic features (as described by Kao et al. (2000)) (figure 4). The Kaoping slope extends southwestward from the island into the South China Sea and the Manila trench bends westward around its front. It is characterized by westward-verging thrust faults clearly visible in figure 4. The Hengchun Ridge extends south from the Central Ranges. The Southern Longitudinal Trough, the Huatang Ridge, and the Taitung Trough occupy the space between the Hengchun Ridge and the Luzon volcanic arc. The interaction of these features with each other in the convergent tectonic setting are discussed below in the structural analysis.
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