PALEOTHERMAL STRUCTURE OF THE SHIMANTO ACCRETIONARY PRISM, SHIKOKU JAPAN - ROLE OF AN OUT-OF-SEQUENCE THRUST

Vitrinite reflectance study on the Cretaceous Shimanto accretionary pr ism of southwest Japan shows that the vitrinite reflectance varies fro m 0.6 % to 3.0 %, corresponding to the estimated maximum paleotemperat ure variation of 90-210 degrees C. The thermal structure shows a sharp discontinuity across an out-of-sequence thrust separating it into two thermal blocks. In each block, vitrinite reflectance values increase systematically toward the prism younging direction, and the thermal st ructure is clearly overprinted on the tectonic imbrication of turbidit e and melange units. This provided us with a set of data to calculate the geometry and displacement of the out-of-sequence thrust, The fissi on-track zircon partial-annealing ages indicate that this thrust was a ctive about 43 Ma, probably in response to the accretion of Eocene roc ks, This study shows that initial thickening of the accretionary prism , in an order of several kilometres, was mainly controlled by the out- of-sequence thrust.