Uplifted structures due to the obliquely subducted Gagua Ridge below the Nanao Basin imaged from MCS/OBS data

Abstract

At the southernmost part of the Ryukyu subduction zone, data recorded by 28 ocean-bottom seismometers with active sources during the TAiwan Integrated GEodynamics Research (TAIGER) experiment in 2009 were re-analyzed. Additionally, six long-offset multi-channel seismic profiles across three forearc basins and the Yaeyama accretionary prism north of the southernmost Ryukyu Trench, obtained by the TAIGER experiment in 2009 and the TAICRUST project in 1995, were processed to generate pre-stack depth migration (PSDM) sections. Due to the NW convergence of the Philippine Sea Plate (PSP), the Gagua Ridge may have been obliquely subducting northwestward beneath the Yaeyama accretionary prism and below the Nanao Basin. The PSDM sections and the velocity-interface models show that the Yaeyama accretionary prism near the trench, the sedimentary basement and the lower crustal structure below the Nanao Basin were uplifted by the tip of the subducted Guagua Ridge. Furthermore, the sedimentary and crustal structures near ~ 122.5°E beneath the Nanao Basin were shown a north-trending uplift where shallow earthquakes had been occurred by the oblique subduction of the Gagua Ridge depicted in the isopach map of the crust. Moreover, a recent earthquake (Mw ~ 6.0) near the NE coast of Taiwan in 2018 at a depth of approximately 12 km below the Hsincheng Ridge was caused due to a thrust fault near the décollement that might have been formed by subduction of the PSP.