TOMOGRAPHIC IMAGING OF LITHOSPHERIC STRUCTURES UNDER TAIWAN

Tomographic images of the crustal and mantle velocity structures under Taiwan are obtained by simultaneous inversion of local earthquake P-wave arrival times for hypocenters and P-wave velocity structures. In northern Taiwan, a high-velocity zone, coinciding with the Wadati-Benioff zone, can readily be identified as the subducted Philippine Sea plate. The imaged zone dips toward the north at an angle of 40 degrees from a depth range of 20-55 to 100-130 km. An upper-mantle low-velocity wedge, ranging from 40 to 80 km in depth, exists above the subducted slab. Above this wedge is the Ilan Plain of northern Taiwan which lies at the west end of the Okinawa Trough, a well recognized back-are basin; the crustal velocities under the Plain are also relatively low. The well-defined high-velocity zone and the low-velocity wedge provide some constraints on the thermal structures of the subduction system under northern Taiwan. In tomographic images across the central section of Taiwan, thickening of the crust and up-arching of the lower-crustal materials under the Central Range are commonly observed; the crust under the Western Foothills region is clearly thinner and the near-surface low-velocity layers are well developed. The structures under the Central Range show that although the Taiwan orogeny is quite young, a root, deeper in the north and shallowing to the south, has formed. The results of our tomography show that a significant portion of the lithosphere is involved in the Taiwan orogeny.