Taiwan has been an area of much research over the past thirty years. Though many elegant, simple ideas about fold and thrust belts originated in Taiwan (Ernst et al., 1985; Suppe, 1981), more recent literature suggests a very different history than the simple classical model. Many of the differences center on the question of weather or not the lithosphere is involved in the island's deformation or if it is just a passive "belt" off which sediments are scraped and deformed into the island's ranges.
|Figure 5: Schematic diagram of the evolution of the accretionary prism in the Hengchun Peninsula. After Huang et al. (1997). DF = Deformation front; HF = Hengchun Fault; HP = Hengchun Peninsula; SLT = Southern Longitudinal Trough; HTR = Huatang Ridge; NLT = Northern Longitudinal Trough.|
In the classic model of Taiwan orogenesis the sediments of the island are interpreted as a continental shelf slope and rise progression (Ernst et al., 1985). The Tananao complex is a part of the Eurasian basement rocks caught up in the dynamic of the collision (Huang et al., 1997). Mechanically, the Eurasian lithosphere acts as a giant conveyer belt dragging sediments into the subduction zone. The sediments are scraped off the lithosphere, which continues to descend into the mantle. The volcanic arc acts as a backstop to all of this deformation (figure 5). The configuration of present-day Taiwan is then a "critically tapered wedge" which keeps deforming along its front as new sediment from the continental margin is dragged into the orogeny (Suppe, 1981). The critical wedge geometry is shown in figure 6.
|Figure 6: Critical wedge model of Suppe (1981). The lithosphere acts as a passive feature that drags sediment into the subduction zone. The sediments are scraped off when they run into the "backstop" of the Luzon volcanic arc. After Suppe (1981).|
|Figure 7: Lu and Hsu's (1992) version of the Taiwan orogeny. Note that in the Late Cretaceous to Early Miocene a block of the Chinese continental margin has rifted away from the mainland during the opening of the South China Sea. As collision ensues, two subduction zones are created to account for the subduction of oceanic crust between the mainland and proto-Taiwan (Gutaiwan) and between Gutaiwan and the arc. After Lu and Hsu (1992).|
This model becomes more complex when the origin of the basement (Tananao) complex is considered. Some authors consider the Tananao complex as a part of the Eurasian margin that was rifted away from the rest of China during the opening of the South China Sea (figure 7). In this version, it was subsequently caught up in the arc collision as two subduction zones formed, one west of the continental block, and one west of the Luzon arc (Lu and Hsu, 1992). The other interpretation of the Tananao complex is that it was always a part of the continental margin and that it was just separated from the lower crust when the margin was subducted. The crustal complex moved up toward the surface from the bottom of the wedge because of the dynamics of erosion and wedge stability (Willett et al., 1993) (figure 8).
|Figure 8: The effect of steady state erosion off the front of the wedge and denudation (after Willett et al. (1993)). The lines represent material trajectories and the dots are progressive equal-time positions that were initially aligned vertically. With this model, material at the bottom of the wedge (i.e. the Eurasian basement rocks) can dynamically rise to the top of the center of the wedge, which is consistent with the position of the Tananao complex in Taiwan.|
Recent structural analysis of crustal scale faults, geochemical analysis of the thermal histories of geologic units, and geophysical investigations of the lower crust and mantle serve to test the classic model. While some of the structural interpretations and the undoubted existence of some sort of wedge-shaped fold and thrust belt still hold, new models involve significant amounts of lithospheric involvement in uplift. This version relies, like the critical wedge classic model, on only one subduction zone, the Manila trench. Instead of an arc collision, however, the model calls on the continental subduction as the driving mechanism of uplift. As the continent is dragged into the Manila trench by the attached down-going South China Sea oceanic crust, it resists the down-going force and acts as a backstop for the collapse of the forearc basin and eventually the collision of the arc (figure 9). This model involves partial coupling of the continental lithosphere with the upper crust. As subduction of the continental margin progresses, the continental lithosphere disconnects from the oceanic lithosphere and this late "unzipping" of the oceanic crust allows an extreme flexural rebound and uplift of the crust. This uplift is what exposes the basement Tananao complex at the surface, and is the reason for the striking change in seismicity and stress regime from north to south and east to west along the island.
|Figure 9: Taiwan orogenic model involving the dynamic interaction of the lithosphere in orogenesis. First, the continental margin is subducted, then detachment and flexural adjustment generates uplift of the Taiwan island. After Lin (2000).|
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