THE EVIDENCE OF ALONG-ARC DIFFERENTIAL UPLIFT OF THE SHIMANTO ACCRETIONARY COMPLEX - FISSION-TRACK THERMOCHRONOLOGY OF THE KUMANO ACIDIC ROCKS, SOUTHWEST JAPAN

The Kumano Acidic Rocks (KARs) are the most extensive of the Tertiary granitic bodies in the southwest Japanese fore-arc. They intrude the Cretaceous-lower Miocene Shimanto accretionary complex, perturbing its initial thermal records related to accretion. To assess the timing of igneous activity and subsequent cooling history of the KARs, fission-track (FT) ages were measured on seventeen zircon and three apatite samples, along with four zircon confined track-lengths. Samples were collected mainly from the Granite Porphyry of the KARs along two traverses intersecting expected isotherms. Zircon and apatite ages show good agreement at approximately 15 Ma and, in conjunction with the observed track-lengths, suggest rapid uniform cooling of the KARs in the temperature range approximately 100- approximately 300-degrees-C. In addition, based on the petrographic features of the KARs, the inferred thermal history is best explained by rapid cooling subsequent to the eruption of felsic magma. The rapid cooling of the KARs at approximately 15 Ma indicates that the basement Shimanto accretionary complex in the area had already cooled at that time. FT apatite ages of approximately 10 Ma were reported in the Muroto Peninsula, which is about 100 km southwest of the Kii Peninsula. These contrasting results suggest differential uplift of the Shimanto accretionary complex along its length, presumably caused by some features characterizing the subducting Shikoku Basin. Furthermore, the cooling age of the KARs places constraints on the ages of the Orbulina Datum and the clockwise rotation of Southwest Japan.