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

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.

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

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.

New species of Adeonellopsis (Bryozoa: Adeonidae) from southern Zealandia and the western Tasman Sea

Seven new species of Adeonellopsis MacGillivray, 1886 are described: Adeonellopsis macewindui, A. gracilis (endemic to New Zealand), A. gemina (New Zealand and Norfolk Island shelf), A. tasmanensis (Norfolk Island shelf and Gascoyne Seamount), A. periculosa Norfolk Island shelf) and A. wassi and A. minor (New South Wales shelf). All have flattened staghorn branches, which range in width from 0.8 to 5 mm, depending on species. Based on underwater photos, the largest species, A. macewindui n. sp. forms locally significant habitat on fiord walls and parts of the continental shelf in New Zealand, sometimes in association with A. gemina n. sp.. The latter can survive as isolated fragments that can regenerate from broken ends. Three species have a number of large gonozooids at selected locations on their branches and two of these species have vestigial ooecia in their gonozooids, recorded for the first time in Adeonidae. The remaining four species have among their autozooids only a few zooids that are a little larger, with larger compound spiramina. These are suggested to function as gonozooids, representing the larger end of a size spectrum for reproductive zooids, of which those at the lower end are the same size as autozooids. The encrusting Australian species known as Adeonellopsis baccata (Hutton, 1878) is transferred to Reptadeonella as Reptadeonella baccata n. comb..

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

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, we focus on the widespread shallow submarine terraces surrounding the Iheya-Izena islands in the middle part of the Nanseishoto islands, 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 Gard (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, with a tilting rate of 4×10^5/kyr, which formed by steep northward tilting against the Nankai subduction zone. We suggest 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.

Topographic Anaglyphs from Detailed Digital Elevation Models Covering Inland and Seafloor for the Tectonic Geomorphology Studies in and around Yoron Island, Ryukyu Arc, Japan

Anaglyphs produced using a digital elevation model (DEM) are effective to identify the characteristic tectono–geomorphic features. The objective of this study is to reinvestigate the tectonic geomorphology and to present novel tectonic maps of the late Quaternary in and around the Yoron island based on the interpretation of extensive topographical anaglyphs along the map areas that cover the inland and seafloor. Vintage aerial photographs are used to produce the 3-m mesh inland digital surface model (DSM); further, the 0.6-s to 2-s-mesh seafloor DEM is processed using the cloud point data generated through previous surveys. Thus, we identify anticlinal deformation on both the Pleistocene marine terrace and the seafloor to the north of the island. The deformation axis extends in a line and is parallel to the general trend of the island shelf. The Tsujimiya fault cuts the marine terraces, which extend to the Yoron basin’s seafloor. If we assume that the horizontal compressive stress axis is perpendicular to the island shelf, these properties can easily explain the distribution and style of the active faults and deformation. This study presents an effective methodology to understand the island arc tectonics, especially in case of small isolated islands.