The Taiwan orogeny is young and presently very active. It provides an excellent environment for studying ongoing orogenic processes, especially since the region is monitored intensively with dense seismological and geodetic networks, and new studies aiming at deciphering shallow and deep structures in and around Taiwan have been recently conducted or are being planned. The available data can be used continually to test critically hypotheses of the Taiwan orogeny. Hypotheses dealing with the mechanics of mountain building are basic to the understanding of Taiwan orogeny and are particularly amenable to testing. The widely cited 'thin-skinned tectonics' hypothesis was formulated to explain mainly the geologic and relatively shallow (< 10 km) seismic data. In various forms of this hypothesis, the mountain building involves the deformation of ready-to-fail (Tertiary) sediments in a thin (<20 km at the deepest point) wedge deformed by the advancing Philippine Sea plate; the Eurasian plate is assumed to subduct the Philippine Sea plate with the Taiwan orogenic belt on top as an accretionary wedge. We tested this hypothesis against newly acquired seismological and geophysical data and found it to be largely inadequate as a model for Taiwan orogeny, because the evidence for the participation of the lower crust and even the upper mantle in the orogeny is very strong. Rather than the result of deforming a thin wedge, the formation of the Central Range is shown to include the thickening of crust as well as the extrusion of mid-to lower crustal high-velocity materials to shallow depth. Seismicity and focal mechanisms demonstrate that significant deformation is taking place at depths far below what the thin-skinned tectonics hypothesis predicts. As an alternative, the lithospheric collision hypothesis is proposed. In this model the Eurasian and the Philippine Sea plates are colliding at least down to a depth of 60 km. This hypothesis involves not only greater depth but also greater lateral extent. It accounts for the formation of the deep-rooted Central Range on the Eurasian side, as well as the shortening and thickening of the margin of the Philippine Sea plate near Taiwan. It also asserts that the Central Range was built mainly under ductile conditions, while in the Western Foothills area, the deformation involves the whole brittle-ductile-brittle-ductile sandwiched crust and upper mantle. Furthermore, it is asserted that the collision effect is transmitted to the Taiwan Strait resulting in normal faulting striking perpendicular to the trend of Taiwan. Among the implications of this hypothesis some can be readily subjected to falsification. By critically evaluating the components and accepting or rejecting them, our understanding of the Taiwan orogeny in particular and mountain building in general can be improved.
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