HR: 1340h
AN: S33A-1036    [Abstracts]
TI: New insight into structural heterogeneity beneath Taiwan
AU: * wang, z
EM: wangzhi@cdut.cn
AB: To know whether the Eurasian lithosphere subducts beneath Taiwan is an important issue for a better understanding of mountain building, arc magmatism and plate collision in the western Pacific region. High- resolution 3-D velocity images are estimated at depths of 0-400 km beneath Taiwan by inverting a large number of arrival times from local and teleseismic events simultaneously. We used 215,676 P-wave arrival time data from 6782 shallow and intermediate-depth earthquakes that are located in and around the Taiwan Island. We also used 12,078 P-wave arrival times that are collected from 3-componenet seismograms of 1108 teleseismic events recorded by the networks installed by Taiwan, Japan and China. Our tomographic images provide further direct geophysical evidence for the tectonic models proposed by previous studies and revealed some new features of structural heterogeneity related to the subducted Eurasian lithosphere and the subducting Philippine Sea slab. Low-velocity anomalies beneath the active volcanoes are visible in the subduction zone of Taiwan, which might caused by the collision between the subducted Eurasian plate and the subducting Philippine Sea slab. In the southern portion of Taiwan, the Eurasian lithosphere is clearly imaged as a high velocity zone with a thickness of 65-80 km and subducted down to a depth of 300 km, whilst it has not been observed beneath North Taiwan. Despite that the existence of subducted Eurasia slab beneath Taiwan has been documented by Lellamant et al. (2001), the present study is the first one to provide high-resolution image and indicate that the Eurasian lithosphere stops at the depth of 300 km beneath South Taiwan but not under North Taiwan. Meanwhile, the present tomographic results are also coherent well with the geology and with plate reconstructions in the region. The previous study proposed that the plate convergence rate is constant at about 7 cm/yr (Seno et al., 1993), it takes about 4-5 Ma for the subducted slab of the Eurasian plate to reach a depth of 300 km. This is remarkably consistent with geological inferences that the orogeny of Taiwan was initiated in the early Pliocene ( about 5 Ma) (e.g., Teng, 1990, Kao et al., 2000). The results of physical modeling of the arc-continent collision in Taiwan also require a slab remnant down to about 300 km beneath the island (Chemenda et al., 2001). Therefore, it is possible that the observed high-V anomalies are associated with the subduction system since the inception of the regional collision. Our results also indicate that the Philippine Sea slab is subducting northwestward from the Ryukyu Trench down to a depth of 200 km, showing good agreement with the previous seismic, geochemical and geophysical studies. The plate convergence of the Eurasian plate varies from subduction beneath South Taiwan to colliding with the Philippine Sea slab under North Taiwan. These characteristics of the structural heterogeneities in the crust and upper mantle suggest that mountain building process in the central region of Taiwan, magmatism and seismictectonics in the subduction zone are mainly attributed to the collision between the subducted Eurasian lithosphere and the subducting Philippine Sea slab.
DE: 7218 Lithosphere (1236)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 7270 Tomography (6982, 8180)
DE: 7280 Volcano seismology (8419)
SC: Seismology [S]
MN: 2007 Fall Meeting