Structural characteristics of the décollement zone and underthrust sediments in the Nankai accretionary prism; geologic architectures in the Site C0023, IODP Expedition 370

Author(s): Yamamoto, Yuzuru; Yamamoto, Yuhji; Okutsu, Natsumi; Yamada, Yasuhiro; Bowden, Stephen A.; Tonai, Satoshi; Yang, Kiho; Tsang, Man-Ying; Hirose, Takehiro; Kamiya, Nana
Author Affiliation(s): Primary:
Japan Agency for Marine-Earth Science and Technology, Kanagawa, Japan
Other:
Geological Survey of Japan, Japan
Institute University of Tokyo, Japan
University of Aberdeen, United Kingdom
Kochi University, Japan
Yonsei University, South Korea
University of Toronto, Canada
Nihon University, Japan
Volume Title: AGU 2017 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2017; American Geophysical Union 2017 fall meeting, New Orleans, LA, Dec. 11-15, 2017. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: Expedition 370 penetrated the accretionary prism, plate boundary décollement zone, and underthrust sediment and touched the basement basalt on the Philippine Sea Plate. The drilling site (C0023) is located ∼4 km NE from the legacy sites, Sites 808 and 1174. Compared to the legacy sites, the décollement zone is characterized by weak and intermittent negative reflectors in the seismic profile. Onboard physical properties, e.g. porosity and P-wave velocity data, indeed show the smaller gaps at the top of the décollement zone. The nature of the deformation along the décollement zone represented ∼40 m thick phacoidal deformation zone composed of fragmented mudstone with slickenlines on the surfaces in the Sites 808 and 1174. Compare with this, décollement zone in Site C0023 represented the weaker and non-localized deformation zone comprised of alternating zone of ∼1 m thick phacoidal deformation zones and ∼a few 10 m of intact intervals in the Site C0023. Many normal faults striking parallel to the trench were identified just below the décollement zone, which is indicative of non-localized deformations along the décollement zone. Many of these faults were accompanied with calcite and sulphate mineral veins (anhydrite and barite), indicative of high-temperature fluid migration just above the ridge-spreading center. Based on the paleomagnetic restoration of structure to the geologic coordinate, attitudes of the bedding and fault planes in the Site C0023 are controlled by two factors: 1) subduction/accretion producing the trench-parallel bedding strikes and trench-perpendicular principal stress and 2) ridge spreading that produces ridge-parallel bedding and vein strikes. The former developed in the accretionary prism and the upper part of the underthrust sediment (<900 mbsf), whereas the latter occurs in the lower part (>900 mbsf). These tectonic variations might affect fluid migration pathways.
Year of Publication: 2017
Research Program: IODP Integrated Ocean Drilling Program
IODP2 International Ocean Discovery Program
Key Words: 18 Geophysics, Solid-Earth; Decollement; Expedition 370; Faults; IODP Site C0023; International Ocean Discovery Program; Nankai Trough; North Pacific; Northwest Pacific; Pacific Ocean; Seismotectonics; Tectonics; Thrust faults; West Pacific
Coordinates: N322200 N322200 E1345759 E1345759
Record ID: 2019016413
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