Flow patterns during exhumation of the Sambagawa metamorphic rocks, SW Japan, caused by brittle-ductile, arc-parallel extension

Abstract

Mesoscopic and microscopic structural analyses of the high-pressure/temperature Sambagawa metamorphic rocks (accretion complexes), SW Japan, have been carried out. Deformation characterized by extreme layer-normal thinning and nearly arc-parallel stretching occurred during exhumation in the Late Cretaceous. Asymmetric quartz c-axis fabrics and orientation of shear band cleavages reveal a pervasive top-to-the-west sense of shear in the Sambagawa metamorphic rocks during the exhumation stage. The 3D strain geometries, inferred from quartz c-axis fabric patterns, vary from plane strain to flattening across the metamorphic belt. We hypothesize that the data are most reasonably explained by a model of counter-flow in the subduction channel. The counter-flow was induced by a left-lateral oblique subduction of the oceanic (Izanagi) plate, which was strongly coupled with the subducting sediments. The 3D strain geometries suggest that the counter-flow (i.e. simple shear in the model) must have been accompanied by some arcnormal 'press' component. The mode of deformation changed from ductile to brittle arc-parallel extension, when the rocks were elevated and cooled below the temperature condition for the brittle-ductile transition of quartz (c. 300 ~ The normal faulting (i.e. brittle extension) at subgreenschist conditions was often accompanied by the precipitation of actinolite. This change in deformation mechanism with decreasing temperature is recorded by a conjugate set of normal faults found in the oligoclase-biotite zone in the study area, for which the palaeostress directions conform to the ductile strain geometries.

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

“…The D 1 phase is characterized by ductile flow in an east-west direction, and during this phase of deformation the main foliation and lineation were formed. The lineation is defined by the shape-preferred orientation (SPO) of matrix amphiboles, which grew under retrograde conditions (Hara et al, 1990Wallis et al, 1992;Nakamura and Enami, 1994;Wintsch et al, 1999;Banno, 2000;Yagi and Takeshita, 2002;Toriumi, 2004, 2005), and by elongated quartz (Takeshita and Yagi, 2004), whereas the shape-preferred orientation of phengite and flattened quartz defines the foliation. D 2 phase in the chlorite zone is characterized by south-vergent overturned folds with crenulation cleavages (Hara et al, 1977;Faure, 1983).…”

“…Fig. 8c shows stereographic projections of fault-slip data collected in the Asemi river area (after Takeshita and Yagi, 2004). Although the fault plane orientations are scattered, the striations tend to cluster along the WNW-ESE directions.…”

“…faulting) are in fact very important for the final exhumation of metamorphic rocks toward the surface of the Earth. Takeshita and Yagi (2004) argued that a model of return flow (or counterflow, e.g. 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.…”

“…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).…”

“…However, Takeshita and Yagi (2004) have found that conjugate normal faults caused by WNW-ESE trending horizontal extension develop in the oligoclase-biotite zone (i.e. highest-grade metamorphic zone) along the Asemi river of central Shikoku, which is situated in an upper structural level.…”