Subduction/collision complexities in the Taiwan-Ryukyu junction area: Tectonics of the northwestern corner of the Philippine Sea Plate

The northwestern corner of the Philippine Sea plate (PSP) is actively inter acting with the southwestern Ryukyu are-trench system, as evidenced by inte nse seismicity. To better understand the complex tectonics of this area, we have used available P-wave seismic arrival times to study the velocity str uctures of the Taiwan-Ryukyu junction area. The result shows a prominent lo w-velocity structure at about 30-40 km deep beneath the Taiwan-Ryukyu regio n. A portion of the low-velocity structure beneath the southern Ryukyu are west of 123.5 degreesE might be interpreted as either a subducted portion o f the Luzon are or a subducted thick oceanic crust belonging to the Huatung basin. A "Lambda -shaped" high-velocity structure is observed at about 10- 20 km deep in the Taiwan-Ryukyu are junction area. Below the high-velocity structure are the plate interfaces dipping to the north and to the west. Th e Lambda -shaped high-velocity structure upthrusts or exhumes relative to t he low-velocity structure that leads the northwestward motion of the PSP. T he locality of the Lambda -shaped high-velocity structure coincides with th e high density of earthquakes in this region. The PSP subducts northward be neath the Ryukyu are and underthrusts westward beneath eastern Taiwan north of 23.6 degreesN. To accommodate the space problem in the corner of the su bducting PSP, a tear fault is probably occurring at the northwestern tip of the PSP. This tear fault probably develops and propagates southeastward al ong an existing crustal discontinuity, subparallel to the convergent direct ion of the PSP relative to the Eurasian plate. Near 23.6 degreesN, a second tear fault within the PSP is proposed to separate the subducting and non-s ubducted portions of the Luzon are. North of this fault, northeastern Taiwa n rotates clockwise, which may be linked to the extensional regime of the s outhwestern Okinawa trough.