In situ pressure-temperature conditions of a tectonic melange:: Constraints from fluid inclusion analysis of syn-melange veins

Pressure and temperature (P-T) conditions of melange formation are estimated from fluid inclusions within syn-melange veins developed in the necks of boudins of sandstone blocks in the melange of the Shimanto accretionary complex, south-west Japan. The melange records decollement-zone processes. P-T conditions are in the range of 81 (+15) to 235 (+/-18) MPa and 150 (+/-25) to 220 (+/-31)degreesC. Assuming a constant fluid-pressure to lithostatic-pressure ratio for each data set, we estimate a P-T gradient of between 10.0degreesC/km (+0.2/-1.5) (lithostatic pressure) and 4.2degreesC/km (+0.1/-0.9) (hydrostatic pressure) from these results. The estimated lithostatic P-T gradient is much lower than that calculated from the age of the subducting oceanic plate. The estimated P-T conditions suggest that the melange was formed within the seismogenic zone (hypothesized from thermal modeling), although the deformation mechanism of melange (i.e. dominant diffusive mass transfer mainly in shale matrix with minor brittle breakage mainly in sandstone blocks) does not show evidence of seismic deformation. In addition, at the time of syn-melange vein formation, a shale matrix of melange has injected into the vein, which indicates a ductile deformation of shale. A possible explanation for this discrepancy is that the melange was formed during the interseismic period.