Okino, K. and K. Fujioka

Geomorphology and geology of the Kinan Escarpment in the Shikoku Basin - Results of Dive #176 and #177-

JAMSTEC Deep Sea Res., 10, 111-122, 1994. (Japanese with English abstract)

Abstract.

Two dives of “Shinkai 6500” were carried out at the Kinan Escarpment, which marks the boundary between the central part (19-15 ma) and the eastern part (23-19 Ma) of the Shikoku Basin. The Kinan Escarpment is a long fault scarp extending from north to south and consists of several segments trending N18-30°W, which show right-stepping en-echelon structure, The maximum relative height exceeds 800 m. During two dives, the almost whole surface of the scarp is covered with manganese crust, so we were not able to observe he real outcrop or the succession of the sediment. The scarp is composed of the very steep cliff and the gentle slope. We observed that the pavement type manganese crust which looks like pillow lava covers the steel cliff. On part of the flat seafloor, the small round manganese granules are amalgamated and make thin sheet. Various sizes of manganese encrusted basalt breccias and mudstones were found in debris flow deposits or talus. We found ropy lava on the middle of the scarp, which is considered as the result of low viscosity lava flow in the last stage of the Shikoku Basin opening. The pavement type manganese crust we found is similar to the manganese crust observed at the seaward wall of the Ryukyu Trench (#118). This type manganese crust is important as well as a new type of manganese crust found at the seaward wall of the Yap Trench (#195). It is considered that the type of manganese crust is related to the slope angle and the depth. The ropy lava which indicates the low viscosity lava flow is commonly observed on the Mid-Atlantic Ridge as well as the spreading center of backarc basins, however, it is first time that they are observed in the Shikoku Basin, These samples are of particular importance to understanding the change of chemical components of basalts forming the Shikoku Basin.

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