Magnetic Surveying of the Chaochou Fault of Southern Taiwan: Culmination of Basement-Involved Surface Thrusting in Arc-Continent Collision

Abstract

Taiwan's southernmost, best-defined onshore surface geologic contact, resulting from the collision of the Philippine plate against the Eurasia plate, is the NNE-trending, 80 km long, oblique-slip Chaochou fault. This major crustal break in the Eurasian plate has an extremely conspicuous surface expression noted from satellite imagery and topographic maps as well as distinct subsurface magnetic and gravity signatures, up to 40 nT and 50 mgal, respectively. From a regional total magnetic intensity survey that included seven east-west profiles, integrated with Bouguer gravity and seismic reflection data, we suggest that the nearly vertical dip of the Chaochou fault in the shallow subsurface results from basement-involved thrusting that changes to high-angle reverse faulting near the surface. The horizontal offset on this basement thrust probably involves shallow left-lateral strike slip motion, whereas the major thrust geometry is controlled by deeper seated normal faults in the pre-Miocene basement rocks. The net effect of all this motion produces the present-day surface escarpment over a distance of 80 km between the villages of Liouguei and Fangliao in southern Taiwan. The overriding east-dipping Chaochou fault and associated Wanlong splay are upthrusts that bifurcate from a basement-involved decollement at approximately 6 km depth. As such, these two faults represent the westernmost advance of basement-involved thrusting exposed at the surface in southern Taiwan. Linear regression analysis applied to results from detailed field surveying of the total intensity of the magnetic field is useful in locating both extensional and compressional fault regimes in basement rocks. This technique should be considered not only by plate tectonophysicists, but by those researchers involved in the mapping of active faults where seismic risk is high.