SLIP BEHAVIOR OF A SUBDUCTION ZONE: PRELIMINARY INFERENCES FROM QUARTZ AND CALCITE FILLED VEINS (HYDROFRACTURES) IN AN EXHUMED ACCRETIONARY WEDGE, SHIMANTO BELT, JAPAN

Subduction zones are heterogeneous in their seismic characteristics along the plate boundary, and the range of observed behaviors has widened in the past 15 years to include (very) low frequency earthquakes, episodic tremor, and slow slip. Locking can also be spatially and temporally heterogeneous, so new models are required to account for the full range of fault behavior observed along the plate interface. The Lower Shimanto belt in Japan exposes several regional fault zones interpreted as either paleodécollements or out-of-sequence splays that accommodated seismic slip related to subduction. These rocks boast pervasive veins (e.g. hydrofractures formed and cemented at seismogenic depths within the plate interface), that are now part of sheared, stratally disrupted mélanges exposed at the surface. These mélanges offer a great opportunity to investigate the relationship between microstructures of mineralized hydrofractures and the slip behavior of the plate boundary at different depths, e.g. could seismic asperities be locations where the underthrusting sediments are dominated by dissolution and complete sealing of veins, whereas microseismicity triggers partial sealing of hydrofractures? Focusing on scaly fabrics and vein textures from assorted lithologies, we used various SEM techniques (CL, EDS, BSE, VPSE) to identify vein composition, potential cement-fill sources, and to reveal microstructures resulting from differences in trace element composition and mineral structure, including potential synkinematic crack-seal bands with associated fluid inclusion trails. Preliminary results reflect that rocks from the hotter, down-dip end of the seismogenic zone contain veins with increasingly complex textures and compositions, even though the host rocks investigated thus far are similar. For instance, veins are mainly quartz with some calcite or albite in the shallower, cooler mélanges, but become increasingly complicated with multiple stages of quartz, calcite, and albite fill and dissolution in the hottest mélange. Veins also transition from partial sealing with remnant porosity in the cooler samples to full sealing in veins formed towards the down-dip end, indicating faster cement accumulation rates as a result of increased temperature and/or pressure solution with depth.