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 interacting with the southwestern Ryukyu arc-trench system, as evidenced by intense seismicity. To better understand the complex tectonics of this area, we have used available P-wave seismic arrival times to study the velocity structures of the Taiwan-Ryukyu junction area. The result shows a prominent low-velocity structure at about 30-40 km deep beneath the Taiwan-Ryukyu region. A portion of the low-velocity structure beneath the southern Ryukyu arc west of 123.5°E might be interpreted as either a subducted portion of the Luzon arc or a subducted thick oceanic crust belonging to the Huatung basin. A "Λ -shaped" high-velocity structure is observed at about 10-20 km deep in the Taiwan-Ryukyu arc junction area. Below the high-velocity structure are the plate interfaces dipping to the north and to the west. The Λ -shaped high-velocity structure upthrusts or exhumes relative to the low-velocity structure that leads the northwestward motion of the PSP. The locality of the Λ -shaped high-velocity structure coincides with the high density of earthquakes in this region. The PSP subducts northward beneath the Ryukyu arc and underthrusts westward beneath eastern Taiwan north of 23.6°N. To accommodate the space problem in the corner of the subducting PSP, a tear fault is probably occurring at the northwestern tip of the PSP. This tear fault probably develops and propagates southeastward along an existing crustal discontinuity, subparallel to the convergent direction of the PSP relative to the Eurasian plate. Near 23.6°N, a second tear fault within the PSP is proposed to separate the subducting and non-subducted portions of the Luzon arc. North of this fault, northeastern Taiwan rotates clockwise, which may be linked to the extensional regime of the southwestern Okinawa trough.