Slope Analysis of the Taiwan Accretionary Prism and its Adjacent South China Sea Continental Slope

Off southwest Taiwan the northern continuation of the Manila accretionary prism has encroached obliquely on top of the northern margin of the South China Sea (SCS), resulting in a frontal accretionary wedge sitting on top of a continental-margin slope. We here applied a slope analysis method to submarine channels of the frontal accretionary wedge and continental slope to understand the tectonic and sedimentary controls on the morphology of submarine channels. These submarine channels are a series of tributaries, forming the upper reach of the Penghu submarine canyon and developing on top of the accretionary wedge as well as on the SCS continental slope, respectively. Using a dense coverage of seismic and bathymetric data provided by a recent marine survey for gas-hydrate related studies in this region, we are able to show that the SCS slope has been dominated by gravity-driven normal faulting, which has resulted in the rotation and truncation of strata on the upper slope and deposition in the lower slope. By contrast, seismic data show recent compressional tectonic movement for the most frontal five to six folds and thrusts in the accretionary wedge. Slope analysis of the longitudinal profiles of submarine channels indicate that, on SCS continental slope, the channel profile resembles that of an exponential curve. The gradient of individual channel profile becomes greater as the channel approaches the deformation front in a down-current direction. By contrast, channel profiles on the accretionary wedge show several step-like knickpoints at where channels cutting across tectonic ridges formed by folding or thrusting. The step-like knickpoints exhibit increasing height of steps toward the most frontal thrust from east to west, suggesting that the rate of substrate uplift for individual folds/thrusts is higher toward the frontal thrust. Our results indicate that systematic variations in channel gradient highlight variations in uplift rate and reflect the distribution and kinematics of active structures. Nevertheless, our slope analysis is subject to complications brought about by variations on transient erosional conditions and sediment flux.