Submarine terraces reveal Late Quaternary tectonic deformation in the intermediate zone between the island shelf and rift zone of the middle part of the Nanseishoto Islands, southwest Japan

Abstract

AbstractLate Quaternary tectonic deformation of coastal areas is usually examined based on the height distribution of paleo-shorelines observed on marine terraces. However, it is difficult to examine deformation along the subduction zone, in which small, isolated islands are distributed. In this paper, the author focuses on the widespread shallow submarine terraces surrounding the Iheya–Izena islands in the middle part of the Nanseishoto Islands, Southwest Japan, where crustal deformation is not known. The islands are located in the intermediate zone between island shelf uplifted during the Late Quaternary and the rift zone occurred to the northwest, along the Okinawa trough. Detailed topographic anaglyph images and maps of the islands were produced using a digital elevation model (DEM) of the seafloor, which is stored by the Japan Coast Guard (JCG) and the Advanced Institute of Science and Technology (AIST). Topographic anaglyph images enabled us to identify the widespread distribution and deformation of the shallow seafloor above − 200 m using red–cyan glasses. Four terrace-like features divided by small steps were found on the shallow seafloor, which are named T1, T2, T3, and T4, in descending order. Topographic expressions of paleo-shoreline depths are preserved on submarine terraces formed during the last glacial period. The paleo-shoreline depths of terraces T2 and T3 are − 60 m and − 70 m on the west side and − 70 m and − 80 m, respectively, on the east side of Iheyajima Island; this indicates southeastward tilting. The tilting ratio of T2 and T3 was calculated to approximately 1‰. The tilting rate is approximately 1 × 10–4/kyr, assuming that the T2 was formed in 10–11 kyr. This is much more rapid than that of the last inter-glacial marine terraces in the Muroto peninsula of Shikoku, Japan, with a tilting rate of 4 × 10–5/kyr, which formed by steep northward tilting against the Nankai subduction zone. The author suggests that this phenomenon is not related to mega-thrusting along the subduction zone, but rather to local deformation, probably caused by the reverse faulting of nearby active submarine faults along the west side of the islands.