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Abstract: The characteristics, evolutionary history, and triggering mechanisms of successive siliciclastic mass-transport deposits (MTDs) of late Cenozoic age on the northwestern South China Sea margin were studied using borehole and 2D/3D reflection seismic data. Multiple mass-transport deposits of various scales and morphologies formed from Pliocene to Holocene time in high-slope-gradient and high-sedimentation-rate parts of the Qiongdongnan and Yinggehai basins. In plan view, MTDs documented by 3D seismic data, deposited between 3 and 2 Ma, are 1 to 11 km wide and 4 to 29 km long. Two seismic geomorphologic characteristics of a typical MTD comprise a basal surface and displaced masses of sediments. Internal seismic facies of the displaced mass consist of extensional wedge facies in upslope areas, thrusted facies in intermediate areas, and chaotic or mounded facies in distal downslope areas. These MTDs likely were triggered by a combination of mechanisms. Seafloor oversteepening, rapid accumulation of thick sedimentary deposits, overpressure, and a tectonically active basin setting provide a background favoring formation of MTDs. Additionally, seismicity, abrupt increase of sedimentation rates, rapid slope progradation, and release of gas contributed to triggering mass-transport deposition in the study area.

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