The Manila Trench-West Taiwan Foldbelt: A Flipped Subduction Zone

The Manila Trench- West Taiwan Foldbelt is an anomalous west-facing island a rc segmen t of the western Pacific. It is postulated to be a young, east- d ipping su b­ duction zone which flipped in Pliocene time from a former west-dipping position on the east sid e of Luzon and Taiwan, where it probably formed a continuous subduction zone with the Ryukyu and Philippine Trenches. The eastward dip of the Manil a Trench is supported by the published work of Ludwig, Hays, and Ewing. Its continuation into wes tern Taiwan is inferred from topographic align­ ments, published cross sections, a nd the probable eastward dip of the seismic focal plane. The hypothes is of flipping is introduced to explain the geologic characteri stics of the structural belts of Taiwan and the apparent offset between the northern end of the Philippine T rench and the southern end of the Manila Trench. The time of flipping is inferred fr om tectonic events dated by surface geologica l means. The northern termination of the subd uction zone is a trench -to-trench transfor,m be­ tween northernmost Taiwan and Yonaguni Island of the southwest Ryukyu Islands. The southern end is a lso a trench-to-trench transfor m expressed at the su rface by left-l atera l wrench faulting west of Atimonan, at the northern end of the Bondoc Peninsula. The Central Taiwan Range is consider.to be a fo ss il Upper Cretaceous s ubduction zone containing ocean ic sed iments of Late Paleozoic to Cretaceo us? age. It is flanked on the west by a Paleogene continental rise fly sch prism, deposited when subduction was dormant or had shifted to the east, and on the east by a Neogene andesi tic island arc complex, which prior to flippin g overlo oked a west-dipping subduct ion zone on the Pacific side. '