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The Late Cretaceous to early Paleogene evolution of the Ryoke– Izumi belt is examined using apatite and zircon fission-track (FT) dating of the Sennan Rhyolites, the Takijiri Adamellite, and the Izumi Group in the Izumi Mountains, southwestern Japan, together with pre-existing geochronological data and interpretation of geologic structures. The results reveal that the geology of the area can be divided into three blocks on the basis of different cooling histories and east–west-trending boundary faults; namely, blocks A, B, and C from south to north. Block A consists mainly of the Izumi Group, which accumulated over a part of the Sennan Rhyolites and the Takijiri Adamellite at 70.9 ± 3.4 Ma (zircon FT weighted mean age of pyroclastic rocks of the Izumi Group). Previously determined Kfeldspar K–Ar ages of the Takijiri Adamellite of Block A (around 66 Ma) record the age of uplift of this block. Block B includes most of the Sennan Rhyolites, which erupted from 85.0 ± 5.2 Ma to 80.0 ± 11.2 Ma (zircon and apatite FT ages of a tuffaceous sample of the Sennan Rhyolites). The Sennan Rhyolites were also partly thermally metamorphosed by the Takijiri Adamellite and cooled before 73.9 ± 8.2 Ma (zircon FT age of a nontuffaceous sample of the Sennan Rhyolites). Block C includes the Takijiri Adamellite, which was exposed on the northern side of the faults after 50.0 ± 6.6 Ma (apatite FT age). The Sennan Rhyolites in Block B were not thermally affected by subsidence of the Izumi sedimentary basin (ISB), indicating that the ISB was formed at 71 Ma as a transtensional pull-apart basin by the subsidence of Block A. The subsequent rapid uplift of the Izumi Group and of the Takijiri Adamellite of Block A at 66 Ma was probably caused by reverse faulting under transpression.

Tectonics of the Ryoke-Izumi belt of the Izumi Mountains, Southwest Japan from thermochronological data