This paper reports fission track FT ages on apatite from Miocene igneous rocks of the Kii Peninsula. A total of twelve ages was determined, of which nine data passed a statistical test. Ages of around Ma were obtained for the Muro Pyroclastic Flow Deposit, the Sekibutsu Tuff, and the granite porphyry of the Kumano Acidic Rocks, and were indistinguishable from the zircon FT ages we reported previously Iwano et al., , Jour. Geol. Soc. Japan, ,-, indicating instantaneous cooling. On the other hand, two plutonic bodies of the Omine Granitic Rocks yielded apatite FT ages of about Ma that were significantly younger than the zircon FT ages approximately Ma , suggesting relatively slow cooling.
Collision between the eastern part of southwestern Japan (SWJ) and the Izu-Bonin Arc caused lateral bending of pre-Miocene geologic belts and boundary faults in central Honshu, including the Median Tectonic Line (MTL), producing a prominent cuspate structure. Paleomagnetic studies suggest that these geologic belts and the MTL were linear prior to that collision; however, a number of previously determined paleomagnetic directions have significant uncertainties and thus provide only poor constraints on the timing and amount of rotation that occurred during collision. Here, we present the results of a paleomagnetic study of sediments from the Hokusetsu Subgroup in Shitara, on the western limb of the cuspate structure, with the aim of obtaining a precise paleodirection. Samples were collected from sites within the uppermost Kadoya Formation, with principal component analysis of the results of stepwise demagnetization providing reliable site-mean directions for of these sites. Rock magnetic experiments determined that magnetite is the main remanence carrier, and these directions pass a fold test, indicating that magnetization predated tilting at Ma and is probably primary. The resultant overall mean direction has a NE declination: D (declination) = . °, I (inclination) = . °, α ( % confidence limit) = . °. The most likely cause of paleomagnetic deflection from the north is the clockwise rotation of SWJ relative to the Asian continent during the Early to Middle Miocene. The angle of deflection measured in these sediments is smaller than within the main part of SWJ, suggesting counterclockwise rotation in the western limb of the cuspate structure relative to the main part during or after clockwise rotation of SWJ. The Shitara paleodirection is deflected ° counterclockwise relative to the strike of the nearby MTL; this is also the case for the Chichibu area of the eastern limb, indicating that the MTL had the same strike direction in these two areas in the late Early Miocene.
We have investigated the geology of the Kumano Acidic Rocks KARs , a major Miocene volcano-plutonic complex in Japan, and in this paper we present a geologic map, geologic profiles, and lithologic and structural descriptions of the KARs in the Owase-Kumano area. The KARs in the mapped area consist of rhyolite lavas the Konogi Rhyolite , rhyolitic pyroclastics occurring as both ash-flow deposits the Owase-Shirahama Pyroclastic Rocks and pyroclastic vent fills, granite porphyry intrusions the Kumano Granite Porphyry penetrating the rhyolite and the pyroclastics, and a stock of fine-grained granite the Fudodani Granite that intrudes the granite porphyry. Among them, the Kumano Granite Porphyry, comprising most of the KARs, occurs as a ring dike and a subhorizontal sheet intrusion. The ring dike, some km or less wide and approximately km in size, is found at the northern end of the mapped area. The subhorizontal sheet intrusion approximately km or more in the maximum thickness, the eastern half of which is hidden in the sea. The roof of the sheet intrusion is observed in the northwestern part of the mapped area. Detailed mapping reveals a cauldron structure and indicates that a part of the sheet intrusion branches off from the ring dike at depth. Our observations argue against the previous interpretation that the Kumano Granite Porphyry formed as a gigantic lava lake body, instead we suggest a new view in which voluminous magma intrusions in the form of a ring dike-subhorizontal sheet interconnection are likely to be an intriguing feature of the volcano-plutonic complex at depth.
Abstract
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