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 ve­ locity structures of the Taiwan-Ryukyu junction area. The result shows a prominent low-velocity structure at about 30-40 km deep beneath the Tai­ wan-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 sub­ ducted portion of the Luzon arc or a subducted thick oceanic crust belong­ ing to the Huatung basin. A "A -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 A -shaped high-velocity structure upthrusts or exhumes rela­ tive to the low-velocity structure that leads the northwestward motion of the PSP. The locality of the A-shaped high-velocity structure coincides with the high density of earthquakes in this region. The PSP subducts north­ ward 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 northwest­ ern tip of the PSP. This tear fault probably develops and propagates south­ eastward along an existing crustal discontinuity, sub parallel to the conver­ gent 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. (

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