POST-LATE MIOCENE CURVATURE OF ACCRETIONARY PRISM IN THE SOUTHERN PART OF THE BOSO PENINSULA IN THE CONTEXT OF IZU FOREARC COLLISION ZONE

Non-metamorphosed and highly porous acretionary prism sediments are well exposed in the southern part of the Miura and Boso Peninsulas, central Japan. Neogene deep marine volcaniclastic sedimentary sequences, the Misaki and Nishizaki Formations, are composed of forearc sediments accreted to another forearc during the Izu arc collision against the Honshu arc. Geologic structures in the southern part of the Boso Peninsula are characterized by an east-west trending, and south verging fold and thrust belt. Exceptionally, in the western part of the Nishizaki Formation, the fold belt trends northwest to southeast. Detailed structural, paleomagnetic, and biostratigraphic analyses were applied to elucidate the northward convex curvature of the fold and thrust belt. Paleomagnetic study of the Nishizaki Formation shows the western part where general east-west trending of fold axes swing to northwest-southeast, rotates clockwisely by approximately 70 degrees, whereas the rotations were estimated at 20 to 30 degrees at the middle and eastern parts where the fold axes trend closer to the general east-west direction. In the Kagamigaura Formation which unconformably overlies the Nisizaki Formation, approximately 25 degrees of clockwise rotation occurs in the western part, 21 degrees and 12 degrees in the the middle and eastern parts, respectively. Paleomagnetic information indicates that the rotation of the wetern part of the Nishizaki Formation started before the sedimentation of the Kagamigaura Formation. Radiolarian biostratigraphic study shows the depositional period of the Nishizaki Formation corresponds to the interval from the Cannartus petterssoni to the Stichocorys peregrina Zone, estimated as 10-6.8 Ma, whereas Sphaeropyle langii Zone, estimated as sometime during 4.8-4.2 Ma of the Kagamigaura Formation. As the results of these studies, the age of the rotation was limited around 6-5 Ma. The curvature of the Nishizaki Formation was formed by Izu arc collision and possible associated with drag along the right lateral faults.