Active continental subduction and crustal exhumation: the Taiwan orogeny

It is generally accepted that continental subduction and crustal exhumation play an important role in the evolution of many orogenic belts. A variety of geological evidence suggests that continental crust is occasionally subducted to depths of tens to perhaps 150 km (e.g. Chopin, 1984; Dewey et al., 1993; Matte et al., 1997). Recently, a series of simulations (Chemenda, 1993; Chemenda et al., 1995, 1996; 2001) has further suggested not only that continental crust can be subducted to depths of tens to 200 km, but also that the buoyancy of subducted continental crust eventually will lead to failure seaward of the subduction front and the subsequent return of the previously subducted crust to the surface. However, evidence of active crustal exhumation has heretofore been missing as most of the postulated examples of crustal exhumation are from longextinct systems such as the Urals and Oman (Chemenda et al., 1996). In addition, the significance of continental subduction is often dismissed because a number of simple observations argue for the persistence of continental crust at the Earth’s surface. These include the longevity of the continents, and the shortening (rather than disappearance) of continental crust during collision. Because the Taiwan orogeny is very active, dynamic processes are largely represented by significant crustal deformationandstrongseismicactivity. A lot of useful data can be directly extracted from surface geology as well as from deep crustal structures. Although Taiwan is frequently characterized as an archetypal steady-state accretionary wedge (e.g. Suppe, 1981; Platt, 1987; Dahlen and Barr, 1989), recent studies of seismic tomography (Lin et al., 1998) and thermal modelling constrained by heat flow and seismicity (Lin, 2000) suggest that continental subduction and crustal exhumation have occurred not only in the past, but also in the present in eastern Taiwan. Thus, detailed examination of the Taiwan orogeny might provide many valuable data for improving understanding of the dynamic processes of continental subduction and active crustal exhumation. Firstly, in this paper, general tectonics and geology will be briefly reviewed. Then, data including seismic tomography, earthquake locations and focal mechanisms, GPS geodesy, leveling, isotope, fission track, and illite crystallinity, will be discussed in the light of prevailing models for orogenic processes in Taiwan. Finally, these data will be used to argue that the best explanation for the geophysical and geological data from this area is the active exhumation of previously subducted continental crust.