Evolution of mechanical properties and microstructures in the inner accretionary prism of the Nankai subduction zone

Author(s): Kuo, Szu-Ting; Kitamura, Manami; Kitajima, Hiroko
Author Affiliation(s): Primary:
Texas A&M University College Station, College Station, TX, United States
Hiroshima University, Japan
Volume Title: AGU 2016 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2016; American Geophysical Union 2016 fall meeting, San Francisco, CA, Dec. 12-16, 2016. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: Mechanical properties and microstructural characteristics of accretionary prism sediments can provide detailed deformation history and processes in subduction zones. The IODP Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) Expedition 348 has extended the deep riser hole down to 3058.5 meters below sea floor (mbsf) to the inner accretionary wedge at Site C0002 located ∼35 km landward from the trench. Here, we conducted deformation experiments on the core samples recovered from ∼2185 msbf at Site C0002 to understand mechanical behaviors and deformation of inner prism sediments. We deformed the siltstone samples with a porosity of ∼20% at 25°C or 60°C under isotropic loading path (S1=S2=S3) and triaxial compression (S1>S2=S3). In the isotropic loading test, we step-wisely increased confining pressure (Pc) from 11.5 to 194 MPa and kept pore pressure (Pp) at 10 MPa. In a series of triaxial compression loading tests, we first increased Pc to the targeting 42-78 MPa and Pp to 20 MPa, and then applied the differential load at a constant displacement rate of 0.005 µm/s while keeping Pc and Pp constant. We will analyze the microstructures of the experimentally deformed samples to understand deformation mechanism. We define yield points based on slope changes in relationships between volumetric strain and effective mean stress (p') for isotropic loading and those between differential stress (q) and axial strain for triaxial loading. The sample yields at p' of 100 MPa (q=0 MPa) in isotropic loading test. In triaxial loading, the samples at effective pressure (Pe) of 22, 28, and 58 MPa yield at q=30 MPa (p'=32 MPa), q=30 MPa (p'=38 MPa) and q=45 MPa (p'=73 MPa), respectively. Upon yield, the samples deformed at Pe of 22 MPa and 28 MPa show brittle behavior with a peak q of 50 MPa and 55 MPa followed by strain weakening to reach q of 36 and 46 MPa at steady state. Both samples show single fracture planes with angles of ∼30° to S1. On the other hand, the sample at Pe of 58 MPa shows strain hardening after the yield and exhibits barreling. In triaxial loading experiments, all samples show an increase in volumetric strain with increasing Pe. Our experiment results at different Pe are consistent with a critical state soil mechanics theory. We will further correlate the microstructural features of the deformed samples with the mechanical data.
Year of Publication: 2016
Research Program: IODP Integrated Ocean Drilling Program
Key Words: 07 Marine Geology and Oceanography; Cores; Expedition 348; IODP Site C0002; Integrated Ocean Drilling Program; Kumano Basin; Marine sediments; NanTroSEIZE; North Pacific; Northwest Pacific; Pacific Ocean; Sediments; West Pacific
Coordinates: N331800 N331801 E1363801 E1363800
Record ID: 2017041781
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