Neotectonics of the volcanic Kuei-Shan Tao island, and geodynamic implications (NE Taiwan - SW Okinawa Trough)

Abstract

What are the geodynamic processes during the transition from the onshore NE Taiwan collision to the offshore southwest Okinawa back-arc basin opening associated to the Ryu-Kyu subduction? What is the local neotectonic scheme of this transition (e.g.: structural sketch map of the outcropping volcanic edifice highlighting major faults and their associated earthquakes)? These are some of the fundamental questions adressed to the Kuei-Shan Tao volcanic island (KST hereafter) which is the unique emerged volcanic outcrops situated within this geodynamic transition area.Several incompatible KST geological mappings had been published without any faults, nor dykes, nor feeders (Ichikawa, 1934; Hsu, 1963; and Chiu et al., 2010) that needed to be updated and completed on the structural point of view. In order to do so, we acquired a new high resolution UAS-drone topography (Digital Surface Model) through photogrammetric processing, with a ground resolution <10 cm. We analyse and interprete it in detail using morphostructural photo-interpretation methods. Field works on KST is restricted due to technical and administrative reasons, so we compare our morphostructural map to the shoreline outcrops observed from a boat survey. Then, we have updated the Kuei-Shan Tao geological mapping (lava flows and pyroclastic falls), and the structural scheme as well as the major erosionnal landslide processes. Taking into account the re-interpretation of surroundings offshore bathymetry, old and new seismic profiles, the different drillings done in the Kuei-Chia northern flank, previous geophysical works, the existing massive andesite datings), as well as the inferred stress regimes deduced from the earthquake's focal mechanisms, we propose a KST neotectonic map. We propose also a new scenario for the recent KST volcanic evolution. Kuei-Shan Tao geology and geodynamics may have a so great importance for the 0.5 millions citizens of the so close flat lying Ilan Plain in terms of natural hazards (eruptions, tsunamis, earthquakes…).