Fluid overpressure in subduction plate boundary caused by mantle-derived fluids

Subduction plate boundaries are commonly weak owing to fluid overpressure. The generation of fluid overpressure has been invoked for compaction disequilibrium and mineral dehydration in subducting sediments and altered oceanic crust. The subduction melange in the Shimanto accretionary complex, southwest Japan, preserves the quartz-calcite-filled shear veins and sigmoidal extension veins formed under fluid overpressure near the downdip limit of the thermally-controlled seismogenic zone. The shear veins record the repeated low-angle brittle thrusting under fluid overpressure, while the extension veins constitute the Riedel shear zones developed during subduction under vertical maximum principle stress. To examine the origin of overpressured fluids, we conducted fluid inclusion and noble gas analyses on the shear and extension veins. The result of fluid inclusion analysis shows various fluid pressures between lithostatic to hydrostatic pressures during the vein formation, which were associated with warm fluid infiltration. He-3/He-4 values of shear veins and extension veins range 1.6-2.5 Ra and 1.6-2.3 Ra, respectively, indicating the presence of mantle helium. Ar-40/Ar-36 values and Kr-84/Ar-36 and Xe-130/Ar-36 ratios of the veins are similar to those of serpentinized mantle, suggesting that mantle helium originated from serpentinized mantle. Because the mantle-derived rocks such as peridotite and serpentinite are absent in the melange, the results of noble gas analyses represent the infiltration of fluids from the serpentinized mantle into the melange shear zone. Our results show that an infiltration of fluids derived from serpentinized mantle contributed the fluid overpressure in the melange shear zone near the downdip limit of the seismogenic zone. (C) 2020 Elsevier B.V. All rights reserved.