Neotectonics in Southern Ryukyu arc by means of paleostress analysis
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概要
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Paleostress estimation is essential in understanding the tectonics of the Southern Ryukyu arc. We have used Multiple inverse method (Yamaji, 2000) and Ginkgo method (Yamaji, 2003) to detect paleostress from the fault-slip data collected in the Southern Ryukyu arc where 23 groups of stress (stresses A-W) are devided. We suggest that stress transition in the Southern Ryukyu arc is more complex than those from the previous studies. As the slickenside of faults in the Ryukyu Group can be measured, detailed stress transition is deduced. Stress field of the after Ryukyu Group in the Southern Ryukyu arc is divided into three zones; Miyako island and Yaeyama islands excluding for Hateruma island (MY zone), Hateruma island (HA zone) and Yonaguni island (YO zone).We have tried to re-create a complex stress transition after the deposition of the Ryukyu Group by the finite element method imposed on the convergent rate of the Philippine Sea Plate (present, 1 Ma ago and 2 Ma ago). We have re-created stress field in the Southern Ryukyu arc by comparison between the stress pattern detected by the fault-slip analysis and that simulated by the finite element method. From the result of the finite element method, we have proposed that the stress field is caused by the arc-parallel stretching which is responsible to the increasing curvature of the whole arc accompanying the still active back arc basin (Okinawa Trough) proposed by Fabbri and Fournier (1999) and Fabbri (2000). Before 1 Ma when the distance between the Yonaguni island and the collision zone at Taiwan is the nearest by the westward migration of the Philippine Sea Plate, strike-slip fault regime of after Ryukyu Group detected in the Yonaguni island is recognized. This is caused by the effect of collision at Taiwan. Progressive rifting of the Okinawa Trough before 1 Ma gave rise the stress transition of the after Ryukyu Group from NW-SE extension to NE-SW extension in the MY zone. Strike-slip fault regime detected in the Hateruma island is caused by the shear under the influence of oblique subduction because the Hateruma island is located near the Ryukyu Trench. These results give us the useful information to reconstruct the neotectonic evolution in the Southern Ryukyu arc after 1 Ma.
- 琉球大学理学部の論文
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