Westward Extension of the Okinawa Trough at its Western End in the Northern Taiwan Area: Bathymetric and Seismological Evidence

Abstract

In this paper, we used detailed bathymetry, earthquake distribution and focal mechanisms to study the phenomenon of active westward exten sion of the Okinawa trough in the northern Taiwan area. We found a dis tinguishable subsiding and collapsing area on the continental shelf edge and the continental slope on the northern side of the Okinawa trough. This area extends westwards to at least 121.5°E and includes several morpho logical units related to the existence and formation of three major canyons. The canyons and the morphological units are still evolving through the sedi ment transport and through the subsidence and collapse of material due to the formation of the Okinawa trough. According to the degree of develop ment, we found that these morphological units have developed from the east to the west. There are two parallel E-W trending central graben at the westernmost part of the Okinawa trough, with each corresponding to a narrow shallow seismic belt. The widths of the central graben are 10-15 km. There is geophysical and geological evidence that the formation of these central graben has been extended westwards to the onland area of Taiwan. Focal mechanisms of earthquakes and the topographic features show that the formation of the Okinawa trough is associated with the down-dip ex tensional stress along the subducting slab of the Philippine Sea plate, and most of northern Taiwan and all the northeastern offshore area of Taiwan are under tensional stress. New portions of the Okinawa trough have been forming across the whole width at its western end through subsidence in the continental shelf, the continental slope and the traditionally recognized area of the Okinawa trough in the northeastern Taiwan area, to make the Okinawa trough develop gradually and extend westwards.

Hydrothermal Activity in the Okinawa Trough Back-Arc Basin: Geological Background and Hydrothermal Mineralization

“…Submarine channels and meanders at the trough floor , as well as limestone outcrops discovered from a water depth of 1,400 m , indicate the remarkable rate of the subsidence. At least in the southernmost area of Okinawa Trough, this stage is thought to be still in progress with the westward migration towards Taiwan (Wang et al 2000).…”

Spatial variations in the frequency-magnitude distribution of earthquakes in the southwestern Okinawa Trough

“…Normal faulting is a common tensional process evidenced by seismic reflection data (Sibuet et al, 1998) and focal mechanism analyses (Fournier et al, 2001;Kao and Jian, 2001;Kubo and Fukuyama, 2003). In the southern OT, the volcanic front of the Ryukyu subduction zone is located within the backarc basin (Sibuet et al, 1998;Wang et al, 2000) ( Fig. 1(a)).…”

Tectonic Implication of the 5th March 2005, Doublet Earthquake in Ilan, Taiwan

“…The Ilan Plain is a large, triangular basin that reflects the current foundering of the western tip of the westward propagating OT (Wang et al 2000). The plain is bounded on its northwestern and southern sides by two of the major mountain ranges of Taiwan.…”

“…Based on seismological observations, Kao et al (1998) suggests that this region is located where an interaction exists between the extension of the opening of OT and the compression of the PSP oblique collision. Generally, normal faulting in this region would be consistent with the opening and extension of the OT (e.g., Wang et al 2000;Shyu et al 2005b;Wu et al 2008a). A strike-slip focal mechanism may imply the lateral extrusion at the transition zone between the Taiwan mountain range and the OT (Liang et al 2005).…”