Structural heterogeneities in the crust and upper mantle beneath Taiwan

High-resolution 3-D P-wave velocity structure at depths of 0-400 km was determined to investigate the structural heterogeneities and their implications for mountain building, plate collision and tectonics beneath Taiwan by simultaneously inverting a large number of arrival-time data from local and teleseismic earthquakes. In the joint seismic tomography, 227,258 absolute arrival times from 8982 local and regional earthquakes and 18,869 relative arrival-time data collected from 3179 teleseismic events were used. Our tomographic images provide direct geophysical evidence for a tectonic model proposed by previous studies and revealed some new features of structural heterogeneity related to the subducted Eurasian lithosphere. The seismic images show that the Philippine Sea slab descends northwestward from the Ryukyu Trench down to a depth of 200 km, showing agreement with the previous seismic, geochemical and geophysical studies. Low-velocity anomalies beneath the Kueishan volcano are revealed at depths of 0-65 km in North Taiwan, indicating its close relationship to the subducting process of the Philippine Sea slab. The Eurasian lithosphere is generally imaged as a high-velocity zone with a thickness of 65-80 km and it has subducted down to a depth of 300 km under South Taiwan, whilst it is not visible beneath North Taiwan. Low-velocity anomalies above the subducted continental lithosphere are observed at depths of 0-200 km, which might reflect, at least part of the subducted continental crust. Our tomographic results indicate that the plate convergence of the Eurasian plate varies from subduction beneath South Taiwan to collision with the Philippine Sea slab under North Taiwan. These features of the structural heterogeneities in the crust and upper mantle suggest that the mountain building process in the central part of Taiwan, arc magmatism in northeast offshore region and seismotectonics are mainly attributed to the subduction-collision configuration at the boundary between the Eurasian continental lithosphere and the subducting oceanic lithosphere of the Philippine Sea slab.