11:30 AM - 11:45 AM
[SSS02-09] In situ stress state within the inner accretionary prism in the Nankai Trough: Inferences from drilling observations during IODP Expedition 348
As an ensemble, these observations - for the first time - constrain stress state and pore pressure in the deep interior of an accretionary wedge. The LOTs show that the minimum principal stress as less than the vertical stress defined by the overburden (Shmin = S3), and define a nearly linear gradient in Shmin from the seafloor to the base of the hole. Several observations of mud loss, and the lack of observed gas shows even during pipe connections, indicate that formation pore pressure is not significantly (<~10 MPa) greater than hydrostatic. Our estimate of SHmax is close in magnitude to the vertical stress, and defines either a normal or strike-slip faulting regime. At 3002 mbsf we estimate that the effective stresses are as follows: Sv’ = 33 MPa; SHmax’ = 25-36 MPa; and Shmin’ = 18.5-21 MPa. A key implication of our analysis is that, at least to ~3 km depth in the hanging wall of the subduction thrust, differential stresses are low, on the order of 10 MPa or less. On this basis, we posit that: (1) the inner wedge is not critically stressed in horizontal compression, consistent with its flat surface slope and the development of a large forearc basin above; (2) basal traction along the megathrust must be low, in order to permit concurrent sliding along décollement and low differential stresses deep within the upper plate; and (3) although differential stresses may remain low all the way to the plate boundary at ~5.4 km bsf, the maximum horizontal stress SHmax must transition to become greater than the vertical stress below the base of the borehole in order to drive thrust motion along the décollement.