Use of fracture filling mineral assemblages for characterizing water-rock interactions during exhumation of an accretionary complex; An Example from the Shimanto Belt, southern Kyushu Japan

Ono, Takuya ; Yoshida, Hidekazu*; Metcalfe, R.*

Fracture filling minerals were formed during the development of an accretionary complex. Each mineral formed under favourable geological conditions and can be used to estimate the conditions of accretion. Geological observations and analyses were made on fracture fillings from boreholes of ca. 140 m depth, in the Shimanto Belt of Kyushu. Results revealed that the minerals were formed in 5 stages distinguished by the textural relationships of the minerals. Filling minerals show that the studied rock formation has been subducted to several km depth and the temperature reached was ca. 200 - 300C. After the subduction, the rock formation uplifted and surface acidic water penetrated up to 80 m beneath the present ground surface. The acid water dissolved calcite fillings to form the present groundwater flow-paths. The results shown here imply that filling minerals can be an effective tool to evaluate the environmental changes during accretionary complex formation.