Effects of Clay Mineral Provenance and Clay Diagenesis on the Hanging Wall of a Megasplay Fault: Results from Riser Drilling, Nankai Trough Seismogenic Zone Experiment

Abstract:

IODP Expedition 348 set a new record for sampling depth by scientific ocean drilling. Cores were recovered from the Nankai accretionary prism (Site C0002) at depths of 2163-2218 mbsf; cuttings were recovered continuously to 3058 mbsf. Shallower strata near the top of the accretionary prism are as young as 5.6 Ma, but the deeper interval has an apparent depositional age of 9.56-10.73 Ma. The structural context is within the hanging wall of a megasplay fault. Quantitative analyses of the clay mineral assemblages (using X-ray diffraction) show that the most abundant clay mineral is smectite, followed by illite, chlorite, and kaolinite. The mudstones at Site C0002 contain significantly lower percentages of smectite (~40% of clay-size fraction) as compared to coeval Miocene strata at Sites C0011 and C0012 in the nearby subduction inputs of Shikoku Basin (>50% of clay-size fraction). One likely reason for the difference is an overprint of detrital assemblages by smectite-to-illite diagenesis, which results in a steady down-hole increase in illite within the I/S mixed-layer phase. Another possible reason, however, is a spatial shift in depositional environments and detrital provenance. The mud-dominant facies of the accretionary prism is enigmatic (when compared to the frontal prism), and its original depositional setting remains uncertain. The accreted mudstones might have been deposited in a trench during a period in which supplies of sandy sediment were restricted. Northwestward migration of a triple junction that joins the Japan, Izu-Bonin, and Nankai plate boundaries also might have played a role, either by reducing the rate of plate convergence at ~7-12 Ma or by shifting pre-accretion depositional settings from the NE side of the triple junction (subducting Pacific plate) to the SW side (Shikoku Basin, subducting Philippine Sea plate). Regardless of the exact paleogeography, we predict that lower initial percentages of detrital smectite and gradual diagenetic loss of smectite with depth combine to progressively reduce the volumetric contribution of pore fluid from I/S dehydration toward the base of the hanging wall, in contrast to fluids sourced from smectite-rich sediments below the megasplay at potentially seismogenic depths.