Deep-sea submarine erosion by the Kuroshio Current in the Manila accretionary prism, offshore Southern Taiwan

Abstract

The Kenting Plateau is characterized by unusual low relief surfaces that straddle the topographic crest of the northern Manila accretionary prism off southern Taiwan at 400–700 m water depth. Multibeam bathymetric data, reflection seismic data, Acoustic Doppler Current Profiler (ADCP) data, surface grab samples, and sediment cores were collected in and around the Plateau to identify evidence of erosion in the Kenting Plateau and understand how the morphological evolution has been influenced by submarine erosion over geological time scales. The most distinctive feature on the Kenting Plateau is a 3 km × 7 km bean-shaped flat elevated platform (Kuroshio Knoll) revealed by multibeam bathymetry. Seismic data show almost no reflections beneath the seafloor and erosional truncations at the seafloor, especially in the Plateau's eastern half, evidencing widespread erosion. The P-wave velocity of the gravels recovered from the top of the Plateau ranges from 2.2 to 4 km/s. After comparing the velocity with the borehole data from nearby basin the burial depth of the parent rocks was found to be around 2 to 4 km below the seafloor, indicating that the parent rocks have been uplifted and gravels were formed due to erosion of the Plateau. The truncation of the seafloor shown on seismic sections suggests significant erosion on the Plateau. Sand content of the sediment cores decreases away from the Plateau, suggesting that sediment transport is effective in this area with high energy deposition, thereby accumulating coarse sediments on the Plateau and removing fine particles away from it. The presence of a dune field migrating northward of the Plateau, parallel to the Kuroshio Current also evidences active sediment transport in the area. Flow velocity of the Kuroshio Current observed from the ADCP data is very high, reaching up to 1.8 m/s on top of the Kuroshio Knoll (SE domain). We thus interpret that the observed intense erosion is caused by the Kuroshio Current, while the uplift of the Kenting Plateau is partially due to isostatic rebound caused by sediment removal through erosion and compression of the accretionary wedge. The higher sedimentation rate and coarser in grain size during sea level lowstand (20,000–12,000 yrs. BP) suggests that the erosion was more intense during the glacial period compared to that of deglacial period (< 12,000 yrs. BP) as seen from the MD97–2145 core. Submarine erosion is predominant throughout the Plateau, and it controls the geomorphology of the Plateau, especially the Kuroshio Knoll.