Abstract
We have compiled new free-air gravity anomaly (FAA) and magnetic anomaly maps, shedding light on the tectonics in the Taiwan-Luzon region. To have a suitable datum level for both the available gravity and magnetic anomaly data, the set of data from an ACT cruise, conducted during May 27 to June 21, 1996, was chosen as a reference. Based on the cross-over error analysis, all the other data were adjusted accordingly. Some satellite-derived, airborne or land data were also added to the compilation to obtain better coverage. Several major new insights into the Taiwan-Luzon region are revealed by the new maps. (1) A prominent NE-SW trending belt of gravity and magnetic anomalies is present in the onshore and offshore areas of southwestern Taiwan. The Peikang High is located on this belt. (2) Located in the offshore region west of Taiwan and to the north of the belt described above, the Taishi Basin, in contrast, occupies a relatively low FAA area. It could be regarded as a flexural basin on account of the loading of a thrust-and-fold belt in western Taiwan. (3) A probable NW-SE trending old transform fault is well imaged off southwestern Taiwan, which separates the lithosphere (plate) of the South China Sea from a trapped piece of the Philippine Sea plate. (4) Located east of the Luzon Are, the Huatung Basin contains several E-W trending magnetic reversals and two N-S trending old fracture zones. The Huatung Basin is separated from the West Philippine Basin by the 123E Fracture Zone . Accordingly, the Gagua Ridge corresponds to a transverse ridge bounding the 123E Fracture Zone. (5) The Luzon Are is abnormally concave toward the Manila Trench and becomes wider toward the south. The internal deformation of the Luzon Are in terms of several NE-SW discontinuities could be related to its collision with eastern Taiwan, (6) Three previously proposed NW-SE trending strike-slip faults in the southern Okinawa Trough, west of 123...
Highlights
Gravity and magnetic anomaly maps generally provide fundamental reconnaissance of a geological province
Global satellite-based marine gravity anomaly maps have been deduced on the basis of the calculated geoid heights (e.g. Hwang and Parsons, 1996; Sandwell and Smith, 1997)
The accuracy and resolution of the gravity field recovered from satellite altimetry is limited due to the short-wavelength noise from ocean surface waves, especially from ocean tides on the continental shelves (Sandwell and Smith, 1997)
Summary
Gravity and magnetic anomaly maps generally provide fundamental reconnaissance of a geological province. A fairly complete gravity survey on Taiwan was done by Yen et al (1990) Their free-air gravity anomaly (FAA) map updated several previous studies (e.g. Chang and Hu, 1981; Hu and Chen, 1986). The two most complete cruises covering most of the offshore area around Taiwan are the EW9509 cruise in 1995 and the ACT cruise in 1996 (Figure 1) With these significant additional data, we have compiled a new regional FAA map with a view to better understanding the tectonic problems in the Taiwan region. A magnetic anomaly map is certainly important to investigations of regional tectonics by providing information in geological boundaries and the volcanic nature of underlying sources. We present a general description of tectonics around Taiwan in the light of these newly compiled potential maps
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