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In-situ mechanical weakness of subducting sediments beneath a plate boundary décollement in the Nankai Trough

Hamada, Yohei and Metcalfe, Kyle (2018) In-situ mechanical weakness of subducting sediments beneath a plate boundary décollement in the Nankai Trough. Progress in Earth and Planetary Science, 5 . Art. No. 70. ISSN 2197-4284.

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The study investigates the in-situ strength of sediments across a plate boundary décollement using drilling parameters recorded when a 1180-m-deep borehole was established during International Ocean Discovery Program (IODP) Expedition 370, Temperature-Limit of the Deep Biosphere off Muroto (T-Limit). Information of the in-situ strength of the shallow portion in/around a plate boundary fault zone is critical for understanding the development of accretionary prisms and of the décollement itself. Studies using seismic reflection surveys and scientific ocean drillings have recently revealed the existence of high pore pressure zones around frontal accretionary prisms, which may reduce the effective strength of the sediments. A direct measurement of in-situ strength by experiments, however, has not been executed due to the difficulty in estimating in-situ stress conditions. In this study, we derived a depth profile for the in-situ strength of a frontal accretionary prism across a décollement from drilling parameters using the recently established equivalent strength (EST) method. At site C0023, the toe of the accretionary prism area off Cape Muroto, Japan, the EST gradually increases with depth but undergoes a sudden change at ~ 800 mbsf, corresponding to the top of the subducting sediment. At this depth, directly below the décollement zone, the EST decreases from ~ 10 to 2 MPa, with a change in the baseline. This mechanically weak zone in the subducting sediments extends over 250 m (~ 800–1050 mbsf), corresponding to the zone where the fluid influx was discovered, and high-fluid pressure was suggested by previous seismic imaging observations. Although the origin of the fluids or absolute values of the strength remain unclear, our investigations support previous studies suggesting that elevated pore pressure beneath the décollement weakens the subducting sediments.

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Additional Information:© 2018 The Author(s). This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (, which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Received: 6 April 2018; Accepted: 9 October 2018; Published: 3 November 2018. This research used data provided by the International Ocean Discovery Program (IODP). This manuscript was improved following discussions with Prof. G Kimura and Dr. Jun Kameda. We are grateful to Dr. W. Tanikawa for his repeated help with the numerical calculations. We also thank M Ikari and one anonymous reviewer for their constructive comments. The authors wish to thank Mantle Quest Japan for operating the vessel and drillings. We received generous supports from Technology Department in CDEX. We gratefully acknowledge the support provided by the operational staff of D/V Chikyu and the onboard laboratory technicians (Marine Works Japan). This study was supported by JSPS KAKENHI grant 16H04064, 15H05717. Availability of data and materials: Data sharing not applicable to this article as no datasets were generated during the current study. Please contact author for data requests. Authors’ contributions: YH, THi, and AI proposed the topic and supervised the study. LM, YK, VBH, FI, and YM took the lead of IODP Expedition 370 T-Limit project as the project managers or co-chief scientists. TH, NK, SB, MC, SH, KH, HI, MKa, LL, HM, HLM, KM, NO, DP, MJR, JS, FS, AS, ST, TT, MYT, BV, DTW, EW, YuY, KY, and MKi collected shipboard and shore-based data during IODP Expedition 370. TSa, NS, TY, and TSu acquired drilling data. YH and YS conducted data processing. SS and YaY collaborated with the corresponding author in development of the manuscript. All authors reviewed and approved the final manuscript. The authors declare that they have no competing interests.
Funding AgencyGrant Number
Japan Society for the Promotion of Science (JSPS)16H04064
Japan Society for the Promotion of Science (JSPS)15H05717
Subject Keywords:Plate boundary décollement; Nankai Trough; Excess fluid pressure; Equivalent strength; Drilling parameters; International Ocean Discovery Program (IODP); Temperature-Limit of the Deep Biosphere off Muroto (T-Limit); Site C0023; DV Chikyu
Record Number:CaltechAUTHORS:20181106-134435637
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:90674
Deposited By: Tony Diaz
Deposited On:06 Nov 2018 21:53
Last Modified:03 Oct 2019 20:27

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