Hualien Ridge: A tectonic ridge transitioning from plate collision to subduction

Abstract

The Longitudinal Valley in eastern Taiwan is generally considered as a collisional suture between the Philippine Sea and the Eurasian plates. Offshore to the northeast of the valley, the Philippine Sea Plate is subducting beneath the Ryukyu Arc. The corner between eastern Taiwan and the Ryukyu Arc system is therefore the transition from plate collision to plate subduction. In consequence, the area has complicated tectonics and frequent earthquakes. In this study, we used marine geophysical data to study the submarine Hualien Ridge that is situated in this plate collision/subduction transition zone. Our results show that the Hualien Ridge is tectonically divided into the active southern part and the inactive northern part. In the southern Hualien Ridge, we find several ~N30°E trending active faults and some could be linked to the active faults in the onshore Milun Tableland. The structures in the southern Hualien Ridge and the Milun Tableland display a pop-up structure that is subject to the oblique compression from the northwestward motion of the Philippine Sea Plate. The ~N30°E trending faults are the results of the transpressional system. However, the Milun Fault, the western boundary of the fault system, probably terminates near 24°03′N, where a pronounced bathymetric depression trending N300° cuts across the whole Hualien Ridge. In fact, all the active faults in the southern Hualien Ridge only appear to the south of the bathymetric depression. In contrast, in the northern Hualien Ridge we only find blind normal faults covered by ~100 m thick sediments. The distinct variation of tectonic activity in the Hualien Ridge underlines the transition from the active collision to inactive collision or partial subduction of the Philippine Sea Plate relative to the Eurasian Plate.