Crustal Structure of the High Magnetic Anomaly Belt, Western Taiwan, and its Implications for Continental Margin Deforma
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Springer 2005
Crustal structure of the high magnetic anomaly belt, Western Taiwan, and its implications for continental margin deformation Win-Bin Cheng Department of Environmental Management, Jin-Wen Institute of Technology, 99 Ann-Chung Road, Hsin-Tein, Taipei County, Taiwan (E-mail: [email protected]) Received 29 December 2003, accepted 26 August 2004
Key words: Seismic tomography, sequential inversion, South China Sea, Taiwan, magnetic anomaly belt, continental margin deformation
Abstract A joint analysis of gravity anomaly and seismic travel-time data has been used to construct a three-dimensional velocity structure for the northeastern extension of the northern South China Sea’s high-magnetic belt in the Taiwan region. The earthquake data used in this study was collected by the Central Weather Bureau Seismological Network from 1991 to 2002, while the gravity data around Taiwan was compiled by Hsu et al. (1998), Terr. Atmos. Oceanic. Sci., 9, 509–532, and Wang et al. (2002), Terr. Atmos. Oceanic. Sci., 13, 339–354. A modified velocity model obtained by local earthquake tomography was used to construct an initial three-dimensional gravity model, using a linear velocity–density relationship. To derive a crustal velocity–density model that accounts for both types of observations, this study performed a sequential inversion of travel-time and gravity data. The main features of our three-dimensional velocity model are: (1) an uplifted zone with velocity greater than 6.5 km/s being observed in the lower crust, (2) the width and the shape of the uplifted zone being found to be strongly correlated with the high-magnetic belt, (3) a trend by which the lower crustal high-velocity zone turns from northeast to north in central Taiwan, where the high-magnetic zone was truncated. A combination of seismic, gravity, and structural interpretations suggests that the crustal deformation relating to the magnetic truncation observed in northwestern Taiwan could be correlated closely with the collision between the Philippine Sea plate and the Asian continental margin.
Introduction A large positive magnetic anomaly belt (Xia et al., 1994; Yao et al., 1994), lying on the northern continental margin is a dramatic feature of the northern South China Sea (SCS). To the northeast it extends to western Taiwan, forming a very large basement high with high magnetic anomalies (Figure 1). The magnetic anomaly belt is about 450 km long and 50–70 km in width. The average of the magnetic anomalies is approximately 300 nT in the eastern and middle sections, while the anomaly in the western portion of the belt is lower and generally less than 200 nT (Xia et al., 1994). Although previous geophysical studies (e.g. Nissen et al., 1995; Kido et al., 2001; Yan et al., 2001) reported the existence of high-velocity material in the lower crust of the magnetic anomaly belt, not much is known about its three-dimensional (3-D) velocity structure and that of its northeastern extension
in the Taiwan region, and this deserves detailed investigation. It is wor
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