Exhumation of an accretionary prism - results from fission track thermochronology of the Shimanto Belt, southwest Japan

摘要

The along-are variation of the thermal history of the Shimanto accretionary prism was studied with fission track thermochronology to investigate tectonic factors that control the exhumation of an accretionary prism. Twelve apatite and 35 zircon ages were analyzed on sandstone and granite samples from the Kii and Kyushu regions. southwest Japan, and the samples are interpreted in combination with the previously reported data from the Shikoku region. Zircon samples show various degrees of age resetting after deposition that are dependent on their position within the prism. Some of the zircon samples constrain the time of maximum heating within or cooling through the zircon partial annealing zone. Two lines of observation, i.e. simultaneous exhumation revealed along across-are transect with relatively high maximum burial at the rear part of the prism, and along-are coeval rebound from burial to exhumation, indicate that the exhumation of the entire wedge was probably caused by a progressive accretionary process such as underplating at depth. Because the driving force for this type of exhumation is the growth of the wedge, this exhumation process is probably a general exhumation process that occurs in all accretionary complexes. Apatite samples record cooling at similar to 10 Ma except for samples from sites in the north of the Kii region whose ages are similar to 35 Ma, suggesting the effect of the southwest japan block obduction onto the subducting young Philippine Sea Plate. This implies, along with the distribution of locally reset zircon ages, that the exhumation of the prism was also forced by local episodic events, for example, collision of blocks during differential rotational movement, change of the subduction parameters, and possible accelerated underplating beneath a narrow region probably caused by the increased material influx from the igneous activities. (C) 2001 Elsevier Science B.V. All rights reserved. [References: 84]