Quartz microtextures of the Sambagawa schists and their implications in convergent margin processes

Abstract

Quartz c-axis fabrics of the Sambagawa schists produced along a late Mesozoic convergent plate margin were analysed so that their tectono-metamorphic history could be clarified. It has been noted by many authors that quartz fabrics produced by earlier phase deformation are easily modified by strain increment during later phase deformation. This paper attempts to elucidate the high-temperature phases of prograde metamorphism (Sim-Bim phase) and of retrograde metamorphism (Sbl phase and Sb2-1 phase) from quartz grains included in garnet and plagioclase porphyroblasts. Quartz c-axis fabrics for all these phases are explained in terms of a type I crossed girdle, without (only rarely with) higher concentration in the principal axis of strain Y (X>Y>Z), that must have been produced by the activity of a dominant slip system such as rhomb<a, and basalca,. As a result, the plastic deformation of quartz, which was responsible for the formation of the type I crossed girdle, occurred even under temperatures greater than 500°C and pressures a little greater than 10-1 1 kb, which correspond to the physical condition of the Sim-Bim phase. It has been assumed that a high strain rate (and/or low H 2 0 content) caused rhomb*a, and basal*a, to be active as dominant slip systems in the subduction zone related to the formation of the Sambagawa schists even under high temperatures (> 500°C).

Brittle normal faulting in the highest-grade Sambagawa metamorphic rocks of central Shikoku, southwest Japan: Indication of the exhumation into the upper crustal level

“…flow. In particular, well-defined c-axis fabrics formed by the E-W flow during the D 1 phase (Sakakibara et al, 1992;Wallis et al, 1992;Wallis, 1995;Tagami and Takeshita, 1998;Yagi and Takeshita, 2002) indicate that the amount of D 1 shortening is very large, at least 30-50% normal to the foliation (e.g. Takeshita and Wenk, 1988).…”

“…Turcotte and Schubert, 1982) in subduction channel (Cloos, 1982;Otsuki, 1992;Iwamori, 2003;Beaumont et al, 2004) could be most likely for exhumation of the Sambagawa metamorphic rocks. This is because these metamorphic rocks experience a huge amount of ductile shear deformation with a dominant top-to-the-west sense across the belt during exhumation (Wallis, 1990(Wallis, ,1995(Wallis, ,1998Sakakibara et al, 1992;Wallis et al, 1992;Abe et al, 2001;Yagi and Takeshita, 2002;Takeshita and Yagi, 2004). The fact could be most simply predicted by the model of return flow on the condition of strong coupling between subducting plate and overlying sedimentary rocks under the setting of left-lateral oblique subduction (Otsuki, 1992).…”

Inclusion‐trail geometry of albite porphyroblasts in a fold structure in the Sambagawa Belt, central Shikoku, Japan

“…The sense of shear from shape and lattice fabrics of recrystallized quartz grains in the quartz schists from the Suryo fold was determined in order to compare the kinematics of progressive deformation deduced by the inclusion trail geometry. Quartz schist textural features under the microscope in the Sambagawa Belt have already been described in detail (Faure 1985; Sakakibara et al 1992; Wallis et al 1992). The dynamically recrystallized elongate‐to‐equant quartz grains are aligned with the D 1 ‐foliation defined by the preferred alignment of phengite, chlorite, carbonaceous materials and opaque minerals.…”

Quartz Microstructures from the Sambagawa Metamorphic Rocks, Southwest Japan: Indicators of Deformation Conditions during Exhumation

“…Quartz c-axis fabrics in the quartz schist along the Asemi-River from all the peakmetamorphic zones were analyzed by [58], which they inferred to have formed at deformation stages under different temperature conditions. They concluded that some quartz c-axis fabrics formed in temperature conditions greater than 500 • C. Nevertheless, all the quartz c-axis fabrics that they analyzed are classified into type I crossed girdles (Figure 5a, see [59] for the original definition, and [40] for their occurrences in the Sambagawa metamorphic rocks).…”

“…Quartz c-axis fabrics in the quartz schist along the Asemi-River from all the peakmetamorphic zones were analyzed by [58], which they inferred to have formed at deformation stages under different temperature conditions. They concluded that some quartz c-axis fabrics formed in temperature conditions greater than 500 °C.…”