Earth Planets Space, Vol. 56 (No. 12), pp. 1331-1338, 2004
Driss Elouai1, Hiroshi Sato2, Naoshi Hirata2, Shinji Kawasaki3, Toru Takeshita1, Naoko Kato2, and Tetsuya Takeda2
1Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan
2Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo, Tokyo 113-0032, Japan
3JGI Inc., 1-5-21 Otsuka, Bunkyo, Tokyo 112-0012, Japan
(Received June 7, 2004; Revised November 25, 2004; Accepted December 3, 2004)
The Northern Fossa Magna is a Miocene failed rift and due to subsequent shortening, its basin-fill forms a fold-belt associated with active faults. Seismic reflection data across the middle part of the northern Fossa Magna acquired in late 1990s were reprocessed to reveal the deep geometry of active faults. The reprocessed seismic sections portray the folded and faulted structure of the Neogene basin-fill. The deeper extension of the Western Nagano Basin active Fault (WNBF), which has been revealed for the first time, can be traced down to 4 km, as a reverse fault dipping 40° westward. In the western part, the Itoigawa-Shizuoka Tectonic Line (ISTL) active fault is presented as an emergent thrust dipping 30-35° eastward. Based on the seismic profiles, surface geology and well data, the balanced geologic cross section was constructed. Using simple-shear model of the basin formation, the total amount of Miocene extension is calculated to be ca. 27 km and the total amount of late Neogene to Quaternary shortening is ca. 11 km. The basin formation and shortening deformation are well explained by the tectonic inversion model and fault reactivation.
Key words: Seismic reflection, West Nagano Basin Fault, Itoigawa-Shizuoka Tectonic Line, Northern Fossa Magna, Active faults, Japan.