Termal history of the NE Japan frontal are since the Late Miocene inferred from vitrinite reflectante

Abstract

Paleogeothermal gradients are estimated from the vitrinite reflectance-depth (Ro-z) relations of Middle to Late Miocene sediments in the Tanakura and Hirono áreas, Northeast Japan. Both areas have been situated in the frontal arc since 15 Ma and have been free from magmatism. Thus, the sediments would have undergone a limited amount of local, thermal disturbances, and are suitable for a study of regional-scale paleogeothermal conditions. Late Miocene to Early Pliocene uplift resulted in an angular unconformity between the Miocene and Pliocene rocks and caused cooling of the Miocene sediments that would have retarded and effectively stopped coalification. The Ro-z relations of the sediments therefore reveal the paleogeothermal gradient between deposition and unroofing. The estimated paleogeothermal gradient is 64Â11° C km-1 at Tanakura and 45Â10° C km-1 at Hirono. The present gradients are respectively 30° and 18° C km-1, indicating that subsurface temperature was higher in the Late Miocene than at present under the frontal arc. Cooling from the Pliocene through Quaternary appears to have been of regional scale across the arc, as is also suggested by the geochemical signature of volcanic rocks erupted at the volcanic front (Ban et al., 1992). Westward retreat of the volcanic front (Ohguchi et al., 1989) is also in accord with the regional cooling.