紀伊半島中央部に分布するジュラ紀秩父付加体と白亜紀四万十付加体の境界周辺の地質構造と砕屑性ジルコンU-Pb年代

摘要

We investigated the geological structure around the boundary between the Jurassic Chichibu accretionary complex (AC) and the Cretaceous Shimanto AC in the Yoshino area (central Kii Peninsula, SW Japan). The Chichibu AC in the study area is divided into the Daifugendake and Wasabidani complexes. Bedding plane in the Chichibu AC strikes E-W and dips gently to the north or south. The Shimanto AC, which consists of the Mugitani Complex, is characterized by the WNW-ESE striking and moderately northward dipping foliation. The boundary between the Chichibu AC and the Shimanto AC is a low-angle fault (Omine-Odai Thrust), which strikes WNW-ESE and dips gently to the south. To constrain the depositional ages of the Chichibu AC, we obtained detrital zircon U-Pb ages from four sandstone samples in the Daifugendake Complex (two samples) and the Wasabidani Complex (two samples). The youngest single grain and youngest cluster ages in the Daifugendake Complex are ca. 175-164 Ma,and those in the Wasabidani Complex are ca. 168-153 Ma. The ages are significantly older than previously reported youngest U-Pb ages from the Mugitani Complex (ca. 110-100 Ma), indicating that an age gap of ca. 55 Myr exists across the Omine-Odai Thrust. This age gap is considerably wider than the previously suggested age gap of ca. 20-30 Myr across the boundary between the Chichibu AC and the Shimanto AC in other areas (e.g., Shikoku and Kanto Mountains).