Tectono-metamorphic evolution of the Cretaceous Shimanto accretionary complex, central Japan: Constraints from a fluid inclusion analysis of syn-tectonic veins

摘要

Micro-thermometry of water-rich fluid inclusions from two syn-tectonic veins sets (D1 and D2 veins) in the Otaki Group, part of the Cretaceous Shimanto accretionary complex of the Kanto Mountains, central Japan reveals the following tectono-metamorphic evolution. Combining the results of microthermometric analyses of fluid inclusions from D1 veins with an assumed geothermal gradient of 20-50 degrees C/km indicates that the temperature and fluid pressure conditions during D1 were 270-300 degrees C and 140-190 MPa, respectively. Peak metamorphic conditions during the development of D2 slaty cleavage involved temperatures in excess of 300 degrees C and fluid pressures greater than 270 MPa, based on analyses of microthermometry of water-rich fluid inclusions from the D2 vein and illite crystallinity. The estimated fluid pressure increased by approximately 80 MPa from D1 accretionary processes to metamorphism and slaty cleavage development during D2. Assuming that fluid pressure reached lithostatic pressure, the observed increase in fluid pressure can be accounted for by thrusting of the Jurassic Chichibu accretionary complex over the Cretaceous Shimanto accretionary complex. Following thrusting, both accretionary complexes were subjected to metamorphism during the latest Cretaceous.