ABSTRACT: The Existence of Manila Trench Off Southwestern Taiwan: Geophysical Investigation and Interpretation

CHEN, ALLEN T., and WEN-ZUEY HO, Institute of Oceanography, National Taiwan Ocean University, Keelung, Taiwan, Republic of China

Magnetic, gravity, and seismic data were used to investigate the nature and extension of previously proposed Manila Trench near southwestern Taiwan. We fail to extract positive evidences for its existence and characteristics of an active trench: (1) topographic low coincides with the location of submarine canyon and is not sufficient to identify a trench, (2) submarine canyons with normal faulting appear in the "accretionary prism" and conflict to the concept of thrusting and convergent regime if a trench is present, (3) gravity low of free air anomaly is not found near the site of the proposed trench, and (4) landward earthquakes are relatively scarce and their focal mechanisms are predominantly normal faulting with various orientations of tensional axis at possible shallow depths.

The submarine canyons may represent the submerged channels of ancient drainage systems of Taiwan strait shelf and were formed when sea level rose during the most recent period of eustatic change. Our data show at least three unnamed submarine canyons found at shot points 200 and 500 along seismic line MCS265-3 and shot point 3150 along MCS265-1. Their origin and relation with ancient drainage systems during lowstand of sea level need further study.

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)