Active continental subduction and crustal exhumation: the Taiwan orogeny Cheng-Horng Lin Institute of Earth Sciences, Academia Sinica, PO Box 1–55, Nankang, Taipei, Taiwan Introduction It is generally accepted that continen- tal subduction and crustal exhumation play an important role in the evolution of many orogenic belts. A variety of geological evidence suggests that con- tinental crust is occasionally subduct- ed 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. How- ever, evidence of active crustal exhu- mation has heretofore been missing as most of the postulated examples of crustal exhumation are from long- extinct systems such as the Urals and Oman (Chemenda et al., 1996). In addition, the significance of conti- nental subduction is often dismissed because a number of simple observa- tions argue for the persistence of con- tinental 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 char- acterized 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 model- ling 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 exam- ination of the Taiwan orogeny might provide many valuable data for improving understanding of the dynamic processes of continental sub- duction and active crustal exhumation. Firstly, in this paper, general tec- tonics and geology will be briefly reviewed. Then, data including seis- mic tomography, earthquake loca- tions 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 explana- tion for the geophysical and geolog- ical data from this area is the active exhumation of previously subducted continental crust. General background The island of Taiwan is located at a complex intersection between the Philippine Sea and Eurasian plates (Fig. 1). East of Taiwan, the Philippine Sea plate subducts northward beneath the Ryukyu arc, while south of the island Eurasian plate oceanic litho- sphere beneath the south China Sea subducts to the east beneath the Philippine Sea plate (Tsai et al., 1977). The major part of the island results from the strong convergence between the two plates and the con- vergent boundary is along the Longi- tudinal Valley, i.e. the suture (Ho, 1988). The main structures are parallel to the strike of the suture in a NNE– SSW direction. East of the suture, the Coastal Range is a manifestation in Taiwan of the Luzon volcanic arc of the Philippine Sea plate. West of the suture, the principal geological units are the eastern Central Range (Tana- nao schist), the western Central Range (Slate belt), the Western Foothills and the Coastal Plains. Among them, the eastern Central Range exposed on the surface comprises greenschist facies rocks. Recent observations In the past few years, a number of geophysical, geological and geochem- ical observations have been amassed ABSTRACT A tectonic model of active continental subduction followed by crustal exhumation is proposed to explain the orogeny in Taiwan. The subducted crust is represented by a low-velocity zone dipping eastwards beneath the major part of Taiwan, while the exhumed crust is marked by a high-velocity bulge, high heat flow and absence of seismicity beneath the eastern Central Range. The boundary between the subducted and exhumed crust has been identified from surface geology and analyses of thermal history across the Central Range. The dynamic force that has been driving the exhumed crust is identified by results from focal mechanisms, structural geology and geodetic survey in the eastern Central Range. Such a tectonic model may provide a good explanation for the evo- lution of the Taiwan Orogeny, as well as an active case for studying other long-extinct systems of continental subduction and exhumation. Terra Nova, 14, 281–287, 2002 Correspondence: Cheng-Horng Lin, Insti- tute of Earth Sciences, Academia Sinica, PO Box 1–55, Nankang, Taipei, Taiwan. Tel.: +886 2 278 39910 ext 521; fax: +886 2 278 39871; e-mail: lin@earth. sinica.edu.tw Ó 2002 Blackwell Science Ltd 281