Depth of oceanic-crust underplating in a subduction zone: Inferences from fluid-inclusion analyses of crack-seal veins

Abstract

Fluid inclusions in crack-seal veins are analyzed in accreted mélange now on land. The vein I inclusions are observed in the necked parts of sandstone blocks in mélange, and the vein II inclusions developed in shale below the thrust fault, which cuts the mélanges. The pressure and temperature estimations from the inclusions within vein I show that the cracks were sealed at ϳ125-195 ؇C under ϳ92-144 MPa of fluid pressure. Vein II formed in cracks that might have opened in a damage zone caused by the thrust fault as it broke through oceanic crust and into the mélange; these veins contain fluid inclusions trapped at ϳ135-245 ؇C under ϳ107-149 MPa of fluid pressure. The depth (ϳ4-6 km below the seafloor) and temperature estimates are consistent with the conditions where an aseismic décollement within sedimentary rocks steps down into the oceanic basement, so that a thin section of oceanic crust underplates in the hanging wall. Such a step-down site is the updip limit of the seismogenic zone in the modern Nankai Trough.

Alteration and dehydration of subducting oceanic crust within subduction zones: implications for décollement step-down and plate-boundary seismogenesis

“…As mentioned above, Matsumura et al (2003) suggested that the site for the initiation of décollement step-down is almost coincident with the location of seismogenesis. This means that the seismogenic fracturing of oceanic basalt near the up-dip limit of the seismogenic zone could nucleate seismic ruptures to propagate along dynamically weakened plate-boundary faults.…”

“…Radiolarian biostratigraphy indicates that the complex is composed of Cretaceous to late Miocene deposits (Taira et al 1988). The maximum paleotemperatures in the complex range from ~150 to 350 °C, as inferred from vitrinite reflectance (e.g., Mori and Taguchi 1988;Laughland and Underwood 1993;Sakaguchi 1996;Ohmori et al 1997;Ohmori 1998;Sakaguchi 1999), illite crystallinity (e.g., Kameda et al 2011b), and fluid inclusion geothermometry (e.g., Matsumura et al 2003).…”

“…This eventuality can be explained by duplex underplating due to "décollement step-down" into the ocean-plate basalts, as exemplified by several ancient accretionary prisms (Silver et al 1985;Kimura and Mukai 1991;Kimura and Ludden 1995). Paleothermal analyses (Matsumura et al 2003;Ikesawa et al 2005) and seismic reflection surveys of the modern accretionary margin in the Nankai Trough, southwest Japan (Park et al 2002), suggest that the site where décollement step-down is initiated almost coincides with the up-dip boundary of the seismogenic zone where great earthquakes repeatedly occur (normally defined as having a temperature range of ~100-150 to ~350 °C; Hyndman 2007). Once such an emplacement of the décollement occurs at a certain depth, the downwards extension of the plate interface is thought to lie close to or within the oceanplate basalt over a wide range of the seismogenic zone.…”

Generation Depth of the Pseudotachylyte from an Out-of-Sequence Thrust in Accretionary Prism – Geothermobarometric Evidence

“…It is assumed that the densest methane fluid inclusions methane fluid inclusions fluid inclusions were trapped under nearly lithostatic conditions, because the veins are interpreted to be deposited soon after fracturing under the near-lithostatic fluid pressures Matsumura et al, 2003;Vrolijk, 1987). Densities are assumed to be 2700 kg m m m -3 and 1000 kg m m m -3 , respectively for solid and fluid.…”

Deformations and pressure-temperature history of tectonic mélange: Miyama assemblage, the Cretaceous Shimanto Belt, Kii Peninsula, Southwest Japan

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