Circum-Pacific faulting in the Philippines-Taiwan Region

Abstract

Conflicting views of circum-Pacific tectonics have focused on the Philippines-Taiwan region, where there has been neither convincing documentation nor general agreement on the importance of transcurrent (strike-slip) faulting or the possible sense of regional horizontal displacement. Structural and physiographic features of the 1200-km-long Philippine fault zone are fully as spectacular as those of the better-known San Andreas and Alpine faults, and current activity is indicated by many localities in which scarps cut Recent gravels. Predominance of horizontal over vertical displacements is indicated by linearity of the fault trace, failure of one side to be consistently higher than the other, disregard for gross physiography, and scissoring of individual scarps within the zone. Consistent stream offsets on Luzon, Masbate, and Leyte demonstrate unequivocally that the sense of Recent displacement has been uniformly sinistral (left-handed). The Philippine fault has no obvious geologic relationship to active volcanoes, but the parallelism and proximity of the fault to the Mindanao trench suggest a close causal relationship. The remarkable Longitudinal Valley of eastern Taiwan represents another great transcurrent fault parallel to the western Pacific rim, and ground displacements during historic earthquakes indicate a sinistral sense of displacement here as well as in the adjacent Philippines. This study does not support the hypothesis of counterclockwise rotation of the Pacific basin, but more important is the further documentation of the predominance of transcurrent faulting in active circum-Pacific orogenic areas. These results reinforce earlier field studies in Alaska, California, Chile, and New Zealand, as well as emphasizing the geological reasonability of the results of seismic fault-plane solutions indicating the world-wide predominance of transcurrent movements.