Geologic Evidence of Lithostatic Pore Fluid Pressures at the Base of the Subduction Seismogenic Zone

Abstract Geophysical and rheological work below the subduction seismogenic zone suggest that elevated pore fluid pressures (Pfs) may facilitate slow slip and tremor. However, quantification of Pf and its variation in this environment remain indirect and incomplete. We examine brittle‐frictional deformation within metasedimentary rocks from an exhumed subduction zone. Here, penetrative foliation formed under viscous deformation conditions by pressure‐solution creep at 0.9 GPa, 300–350°C, and differential stresses <35 MPa. Three sets of crack‐seal veins, precipitated at peak subduction temperatures, cross‐cut this foliation. Mechanical inversions of these veins indicate they formed within a single stress field orientation with foliation, but at different magnitudes of differential stress. These deformation structures reveal near‐lithostatic Pf that fluctuated in time by perhaps only ∼4 MPa, resulting in cyclic mode I and shear fracturing and viscous flow, and support slow slip and tremor mechanisms that require or are enhanced by high Pf and low differential stress.