| Summary: | 博士 === 國立中央大學 === 地球物理研究所 === 99 === The Eurasian plate (EP) and Philippine Sea plate (PSP) convergence in the region between Luzon and Taiwan islands gradually evolves from normal subduction to initial collision. In order to better understand the structural characteristics of the subduction-collision transition between Taiwan and Luzon islands, we have analyzed the multichannel reflection seismic data and multibeam bathymetry data collected by R/V Marcus G. Langseth in 2009 and by R/V Ocean Researcher I in 2003.
The results indicate a clear hemipelagic seismic sequence in the South China Sea (SCS) and Manila Trench. This sequence is dipping eastwards and is covered by thick trench-fill sediments in the south of the trench but has been eroded in the north. Structural analysis shows that the Manila Trench area can be distinguished by six structural zones: a normal fault zone (NFZ), a proto-thrust zone (PTZ), an imbricated thrust zone (ITZ), a frontal out-of sequence thrust zone (FOZ), an upper slope zone (USZ) and a back-thrust zone (BZ). Obvious subduction-related characteristics can be found in the south profiles between 18°45’N and 20°N, better than in the northern profiles between 20.5°N and 21°45’N. To the north, the décollement beneath the trench and frontal wedge is not clear and is disrupted by several buried seamounts or basement highs. Moreover, the width of PTZ becomes narrower, but ITZ and FOZ become wider. The subduction accretionary prism also gradually evolves into the foreland basin system receiving sediments from the Taiwan orogen.
Because of the strong convergence between PSP and EP, the uplift of stratigraphic strata has occurred at the west side of the North Luzon Trough (NLT). To the north of 19°15’N, pop-up, uplifted structures, and west-vergent thrusts can be observed in the west side of NLT from MCS and bathymetry data, which indicates both thrusting and left-lateral strike-slip faulting due to the oblique plate convergence. But the strata are relatively flat in the east side of NLT, suggesting that the plate deformation mostly happen to the west of Luzon volcanic arc. This transpressional convergence along the west side of NLT might extend to the southern Longitudinal Valley off SE Taiwan.
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