Exhumation of the Sanbagawa blueschist belt, SW Japan, by lateral flow and extrusion: evidence from structural kinematics and retrograde P-T-t paths

Abstract

Abstract The correlation in time of a pervasive, nearly strike-parallel, retrograde stretching lineation in the Sanbagawa blueschists with a nearly down-dip, prograde lineation in the structurally underlying Shimanto belt suggests that the oblique accretion and underplating of this belt was at least in part responsible for the longitudinal exhumation of the blueschists. A series of sub-horizontal collinear overgrowths of alkali amphiboles on cores of riebeckite in radiolarian cherts of the Sanbagawa belt, western Shikoku, Japan, indicate essentially continuous retrograde crystallization and extension during nearly strike-parallel exhumation. Needles of magnesio-riebeckite are progressively overgrown by beards or infillings of magnesio-riebeckite, winchite, actinolite, and finally quartz. The Na/(Na + Ca) compositions of overgrowths over many grains show a continuous range from >0.95 to <0.2. We interpret this to reflect continuously decreasing metamorphic conditions from an inferred c. 10 to c. 4 kbar ( c. 35-15 km), and from c. 550°C to <300°C during continuous deformation of these metacherts. Published thermochronological data in related and correlative rocks show that this cooling occurred from mid-Cretaceous to Early Eocene time. Structural and geochronological studies of rocks of the northern Shimanto Belt show that they were accreted to southwest Japan by north-directed underthrusting contemporaneously with exhumation of the Sanbagawa belt. We propose that the retrograde metamorphic fabrics of the Sanbagawa belt developed as these initially high-pressure rocks were exhumed and laterally extruded to the east (present coordinates) during oblique, north-directed underthrusting of the Izanagi (or an equivalent) plate and its thick sedimentary cover (i.e. the Shimanto belt).