Large amount of fluid migration around shallow seismogenic depth preserved in tectonic melange: Yokonami melange, the Cretaceous Shimanto Belt, Kochi, Southwest Japan

Studying subduction zone fluid at shallow seismogenic depths is important to understand the nature of fault rocks at the updip limit of the seismogenic zone because fluidrock interactions affect heat and mass transfer, and fault strength. In this study, we conducted detailed analyses of distribution of shear veins, and estimation of pressuretemperature conditions for shear vein formation for the Yokonami melange, Shikoku, Southwest Japan, which is tectonic melange zone in an on-land accretionary complex. We found a seismogenic fault at the upper boundary of the Yokonami melange, indicating that the Yokonami melange was active at seismogenic depth. The field-transect distribution of shear veins was examined. The frequency, the total and mean thicknesses of the shear veins were about 3.7 per meter, about 10 mm per meter, and about 3 mm per shear vein, respectively. Quartz within the shear veins shows elongate-blocky textures, suggesting precipitation from advective flow. The pressure and temperature conditions for shear vein formation were examined by fluid inclusion analysis, ranging 175-225 degrees C and 143-215 MPa, respectively. The temperature is almost consistent with the paleotemperature determined from vitrinite reflectance, suggesting that the shear veins were formed at up to the maximum depth. The depth might be consistent with that where the seismogenic fault was formed. On the basis of the pressure and temperature conditions and the distribution of shear veins, we estimated the minimum volumetric ratio of fluid to host rocks, assuming that the shear veins had precipitated from advective flow. The estimated amount of fluid is about 106 m3 per cubic meter of host rocks. The results suggest that a large amount of fluid migrates through melange zones at shallow seismogenic depths. This fluid possibly originates from the dehydration of clay minerals from underthrusted sediments and an altered subducting slab.