In situ pressure–temperature conditions of a tectonic mélange: Constraints from fluid inclusion analysis of syn‐mélange veins

Abstract

Pressure and temperature (P–T) conditions of mélange formation are estimated from fluid inclusions within “syn‐mélange” veins developed in the necks of boudins of sandstone blocks in the mélange of the Shimanto accretionary complex, south‐west Japan. The mélange records décollement‐zone processes. P–T conditions are in the range of 81 (+15) to 235 (±18) MPa and 150 (±25) to 220 (±31)°C. Assuming a constant fluid‐pressure to lithostatic‐pressure ratio for each data set, we estimate a P–T gradient of between 10.0°C/km (+0.2/−1.5) (lithostatic pressure) and 4.2°C/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 mélange was formed within the seismogenic zone (hypothesized from thermal modeling), although the deformation mechanism of mélange (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‐mélange vein formation, a shale matrix of mélange has injected into the vein, which indicates a ductile deformation of shale. A possible explanation for this discrepancy is that the mélange was formed during the interseismic period.

Sources and physicochemical characteristics of fluids along a subduction‐zone megathrust: A geochemical approach using syn‐tectonic mineral veins in the Mugi mélange, Shimanto accretionary complex

“…This result suggests that the H 2 O‐CH 4 immiscible fluids might have coexisted at the time of vein precipitation. In the case of immiscible fluid, homogenization temperature is directly regarded as fluid trap temperature [ Alderton and Bevins , 1996; Lewis et al , 2000; Hashimoto et al , 2002, 2003; Matsumura et al , 2003]. Assuming that the modes observed on frequency histograms of homogenization temperatures represent the original homogenization temperatures and fluids were H 2 O‐CH 4 immiscible fluid, the temperatures of precipitation of the network and fault‐fill veins are therefore estimated to be ∼150–160°C and ∼160–170°C, respectively.…”

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

“…Vrolijk et al . 1988; Sakaguchi 1999; Hashimoto et al . 2002, 2003; Lewis & Byrne 2003; Matsumura et al .…”

Tectonolithification of sandstone prior to the onset of seismogenic subduction zone: Evidence from tectonic mélange of the Shimanto Belt, Japan

“…Geological studies of on-land tectonic mélange have shown that sandstones are finely broken [e.g., Cowan, 1982;Byrne, 1984;Agar, 1990]. Although the mechanical behavior of sandstone has been well studied by many experiments [e.g., Zhang et al, 1990aZhang et al, , 1990bMarone et al, 1990;Wong et al, 1997], quantitative evaluation of the deformation conditions are sparse, except for some inferences on the P-T conditions [Vrolijk, 1987;Hashimoto et al, 2002Hashimoto et al, , 2003Lewis and Byrne, 2003;Matsumura et al, 2003]. Constraining the deformation and lithification process of sandstone is therefore one of the keys to understanding the onset of the seismogenic zone.…”