RECENT TECTONICS OF TAIWAN

Abstract

Taiwan represents a very young arc-continental margin collision zone in a long subduction boundary. The collision started in Late Pliocene and is still vigorously taking place. Off coast of NE Taiwan a northward subducting slab, extending west at depth to northern Taiwan, can clearly be defined; although most parts of Taiwan have been rising steadily at about 5mm/year for the last 8, 500 years, northern Taiwan has had periods with no uplift. The intensity of the collision decreases toward the south off the island, and an east-dipping subduction zone can be delineated there. Thus Taiwan can be viewed as a transform zone in between two subduction zones with quite different geometries.Seismically Taiwan is much more active than its neighbors, the Ryukyus and Luzon. Large earthquakes reveal the nature of the intense on-going intra-plate deformation; on land, EW compression or left-lateral shear occur along NNE faults and right-lateral shear occur along nearly EW faults; offshore to the southeast of Taiwan, left-lateral shear along NWW or NW faults and thrusts in several directions coexist; to the northeast, the focal mechanisms agree well with other subduction zones. The Ryukyus are terminated at about 123° E by a number of NNE striking right-lateral faults. Focal mechanisms to the southeast of Taiwan are consistent with a tectonic stress direction of S46° E to S76° E and plunging at -2°to 15°.