In-situ stress and strength in the Nankai inner accretionary prism at Site C0002, IODP NanTroSEIZE

Abstract:

As a part of the International Ocean Discovery Program (IODP) Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), a deep riser borehole has been drilled into the Kumano forearc basin and the underlying inner accretionary wedge at Site C0002, located ~35 km landward from the trench. One of the primary objectives of drilling the riser site was to characterize in-situ stress and pore pressure in the hanging wall above the locked plate boundary. Here, we: (1) investigate the mechanical strength and deformation behavior of prism sediment via laboratory experiments on core samples; and (2) quantify in-situ stress (Sv, Sh, and SH), and pore pressure (Pp) in the Kumano basin and the inner prism. We conducted triaxial compression experiments on core samples recovered from ~ 2200 meters below sea floor (mbsf) during IODP Expedition 348, at effective pressures (Pe) ranging from 8 and 36 MPa, and at temperatures of either 25°C or 60°C.

Our preliminary results indicate that the prism (20 - 42% porosity) rocks deform brittlely at Pe < 22 MPa, but exhibit strain hardening at Pe = 36 MPa. This pressure-porosity condition for a brittle-ductile transition is consistent with previous work defining yield models for incoming sediments at the Nankai Trough (Kitajima and Saffer, 2012). Combining P-wave velocity logs and downhole measurements of leak-off pressure at Site C0002 with an empirical relationship between P-wave velocity, porosity, and effective stress, we show that the Kumano forearc basin is in a uniaxial-strain loading path, which defines a normal faulting stress regime (Sv>SH>=Sh), whereas the inner accretionary prism is in a triaxial-strain loading path that defines a strike-slip faulting regime (SH>Sv>Sh). We estimate excess pore pressure below ~2000 mbsf ranging from 0-12 MPa, corresponding to a pore pressure ratio λ* of 0 - 0.40.