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Normal faults at same depth as thrust faults in an exhumed accretionary complex, Kayo Formation, Okinawa Islands, Japan

By
Yoshitaka Hashimoto
Yoshitaka Hashimoto
Department of Applied Science, Kochi University, Akebonocho 2-5-1, Kochi 780-8520, Japan
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Dai Ueda
Dai Ueda
Department of Applied Science, Kochi University, Akebonocho 2-5-1, Kochi 780-8520, Japan
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Yuhei Motomiya
Yuhei Motomiya
Department of Applied Science, Kochi University, Akebonocho 2-5-1, Kochi 780-8520, Japan
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Kota Tobe
Kota Tobe
Department of Applied Science, Kochi University, Akebonocho 2-5-1, Kochi 780-8520, Japan
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Ayaka Saiki
Ayaka Saiki
Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-0032, Japan
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Kiyohiko Morita
Kiyohiko Morita
Department of Applied Science, Kochi University, Akebonocho 2-5-1, Kochi 780-8520, Japan
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Kohtaro Ujiie
Kohtaro Ujiie
Graduate School of Life and Environmental Sciences, University of Tsukuba, Tennodai 1-1-1, Tsukuba 305-8572, Japan
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Publication history
25 September 201708 June 2018

ABSTRACT

We describe and interpret a system of well-preserved normal and reverse faults in the Kayo Formation of the Miocene Shimanto belt, an exhumed accretionary complex exposed on Okinawa Island. The normal and reverse fault systems both strike NE-SW, suggesting systematic horizontal stress variations between compression and extension. Temperature and pressure conditions for the normal and reverse fault systems were estimated from the densities of water in fluid inclusions in the veins along the faults, and previously reported maximum paleotemperature based on values of vitrinite reflectance and illite crystallinity. The fluid inclusion analyses yielded similar estimates for water density in both normal and reverse fault systems. The minimum geothermal gradient was constrained to a narrow range of 40–50 °C/km. These results suggest that the normal and reverse fault systems developed at a similar depth within the seismogenic zone. This can be interpreted as a change between horizontal compression and horizontal extension occurring at a maximum depth of 3.8–7.5 km below the seafloor, assuming lithostatic fluid pressure. This 90° rotation of the principal stress could be controlled by the seismic cycle, as exemplified by the rotation of stresses that occurred after the Tohoku-Oki earthquake.

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Contents

GSA Special Papers

Geology and Tectonics of Subduction Zones: A Tribute to Gaku Kimura

Geological Society of America
Volume
534
ISBN electronic:
9780813795348

GeoRef

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