Abstract
Late 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.
Highlights
Late Quaternary tectonic deformation of coastal areas is usually examined using height distribution of paleoshorelines observed on marine terraces (Ota 1968; Nakata 1970)
The surface of T2 is relatively flat compared with modern coral reefs; it is characterized by topographic features, including small mounts distributed along the outer margin of the surface and a shallow depression along its inner edge
Late Pleistocene marine terraces are distributed throughout these islands (Koike and Machida 2001), the crustal deformation of the region has not previously been known, primarily because the islands are too small to detect such motions, and since they extend NE, i.e., perpendicular to the tilting direction
Summary
Late Quaternary tectonic deformation of coastal areas is usually examined using height distribution of paleoshorelines observed on marine terraces (Ota 1968; Nakata 1970). In the outer zone of Southeast Japan along the Nankai trough, the northward. It remains difficult to examine deformation along the subduction zone, where small, isolated islands, such as the Nanseishoto Islands, are distributed. Active onshore faults in the outer zone of Southeast Japan were revealed to extend to the northwestward seafloor according to interpretation of coastal topographical maps and images of the Kikukawa fault (Ito and Izumi 2009; Sugiyama et al 2010), the Nishiyama fault (Sato and Ito 2011), and the Tsujimiya fault on Yoronjima Island (Goto et al 2018). Using examples from Santa Catalina Island near the San Andreas fault, Schumann et al (2012) showed that the submerged terraces at depths greater than approximately − 90 to − 120 m can indicate crustal deformation
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
