Early Tertiary fluid flow and pressure–temperature conditions in the Shimanto accretionary complex of south‐west Japan: constraints from fluid inclusions

Abstract

The pressure and temperature history of the Tertiary Shimanto belt of south‐west Japan has been elucidated by analysing fluids trapped in quartz crystals which grew syn‐kinematically along late‐stage brittle faults. The samples come from three areas that span the Paleogene exposures on the Muroto Peninsula of Shikoku Island. Applying microthermometric and laser Raman microsampling techniques to coeval water‐rich and carbonic fluid inclusions, we have constrained the pressure and temperature conditions that accompanied a widespread and kinematically distinct phase of deformation. The results suggest elevated geothermal gradients during late‐stage deformation, conditions that are in disaccord with previous plate reconstructions that have depicted old, thermally mature Pacific crust subducting beneath Eurasia during the early to middle Tertiary. These conditions can most easily be accounted for by including an additional plate boundary in the western Pacific during Paleogene time. Plate reconstructions that include the Kula plate in this region are therefore consistent with our findings. In addition, our results provide clues to the conditions that likely accompany seismogenic deformation at active convergent plate boundaries.

Field and Isotopic Evidence for Fluid Mobility in the Franciscan Complex: Forearc Paleohydrogeology to Depths of 30 Kilometers

“…Ultimately, some of this C is returned to shallower levels when it is released by devolatilization, to mix with fluid C generated by shallow-level biogenic and lower-T abiotic processes (and may again be precipitated in veins). It is worth noting that, despite the production of significant amounts of CH 4 in sediment-rich forearcs (Mashburn, 1985;Vrolijk, 1987;Lewis et al, 2000; this study), examination of the concentrations of reduced C (the organic C fraction) in high-P/T metasedimentary suites indicates very little effect of this loss to diagenetic and metamorphic fluids on whole-rock reduced C contents. Much of this C fraction is probably subducted to subarc depths and beyond into the deeper mantle (Bebout, 1995;Sadofsky and Bebout, 2003).…”

“…A striking resemblance in structural character and lithology is observed between the low-grade parts (lawsonite-albite facies; Grove and Bebout, 1995) of the Catalina Schist and large parts of the Franciscan Complex and the Western Baja Terrane (Bailey et al, 1964;Ernst et al, 1970;Magaritz and Taylor, 1976;Sedlock, 1988aSedlock, , 1988bBebout and Barton, 1993;comparison with Sanbagawa Complex by Ernst et al, 1970; also see Lewis et al, 2000). However, veining and fluid-rock interaction and chemical/isotopic composition have not previously been studied in sufficient detail in most of the other areas to make direct comparisons with the Catalina results.…”

“…Many of the fluid inclusions they examined were CH 4 -bearing, and the fluid inclusions were used to infer paleo-P-T and an elevated geothermal gradient inferred to represent upward advection of heat in rising fluids. Lewis et al (2000) and Lewis and Byrne (2003) likewise identified CH 4 -bearing fluid inclusions, and they attempted to reconstruct intraprism fluid flow during prism thickening based on the fluid inclusions and other veining textures. These authors estimated paleoconditions, and inferred relatively high geothermal gradients affected by permeability constrasts within the Shimanto accretionary prism, and the possible proximity of an active ridge segment.…”

K–Ar age and geochemistry of the SW Japan Paleogene cauldron cluster: Implications for Eocene–Oligocene thermo-tectonic reactivation

“…Other data also suggest a high paleogeothermal gradient in the forearc during the Paleogene. DiTullio et al (1993), Sakaguchi (1996) and Lewis et al (2000) suggested elevated Paleogene geothermal gradients in the Shimanto Belt in Shikoku, based on analyses of vitrinite reflectance, illite crystallinity and fluids trapped in quartz crystals.…”

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

“…Several fluid inclusion studies have been conducted in the Shimanto Belt. The majority of these studies have focussed on the latest stage veins (Lewis et al . 2000) or several generations of “syn‐tectonic veins” (Sakaguchi 1996).…”