HR: 1330h
AN: T52C-0299    [PDF]
TI: A Normal-faulting Paleostress in the Vicinity of Up-dip Limit of Seismogenic Zone Detected by Meso-scale Fault Analysis in a Tectonic M\'{e}lange
AU: * Sato, K
EM: sato@solid.eps.s.u-tokyo.ac.jp
AF: Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033 Japan
AU: Ikesawa, E
EM: a-say@solid.eps.s.u-tokyo.ac.jp
AF: Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033 Japan
AU: Kimura, G
EM: gaku@eps.s.u-tokyo.ac.jp
AF: Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033 Japan
AU: Kimura, G
EM: gaku@eps.s.u-tokyo.ac.jp
AF: Institute for Frontier Research on Earth Evolution Japan Marine Science and Technology Center, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001 Japan
AB: The Mugi m\'{e}lange in the Shimanto Belt, SW Japan, is a mixture of terrigenous and oceanic materials of late Cretaceous to Paleocene. Intermittent bedding planes trend ENE-WSW to E-W (subparallel to the Nankai trough axis) and dip steeply northward. The Mugi m\'{e}lange consists of several duplex units accompanied by shear zones of basalt layers at their boundaries. Systematic shear fabrics and P-T conditions estimated from analyses of vitrinite reflectance and fluid inclusions indicate that the Mugi m\'{e}lange had once been subducted to a significant depth (6-7 km below sea floor, which appears to coincide with the up-dip limit of the seismogenic zone), then underplated to the Shimanto accretionary prism, and is now exhumed on ground surface. In this study, for the purpose of determining paleostress fields related to the processes in which subducted materials were deformed, underplated and uplifted to surface, orientations of meso-scale faults and striations were analyzed. Stress inversion techniques including Angelier's Inversion, Multiple Inversion and Ginkgo Method were applied to fault-slip data obtained in each duplex unit of the Mugi m\'{e}lange, and the results were almost consistent with each other. Most of the resultant $\sigma$1 axes trend N-S horizontally, and are parallel to poles of shale cleavages, which are roughly parallel to bedding planes. Although the cleavages slightly vary their orientations according to later rotation, $\sigma$1 axis changes together with them. This cleavage-controlled paleostress has a low Bishop's stress ratio (i.e. low magnitude of $\sigma$2), therefore is an axial compressional stress normal to cleavages. The restored paleostress was probably exerted just before or at the same time of the formation of duplex structure and the rotation of bedding planes. The meso-scale faults appear to have been formed as normal ones due to overburden. P-T conditions estimated by analysis of fluid inclusions, which occur in the mineral veins sealing measured faults, and cross-cutting relationships between the faults and unit boundary shear zones indicate the simultaneity of these faulting and duplexing. The duplex structure is thought to be formed at the moment of underplating and be caused by stepdown of the d\'{e}collement. A great variety of drastic changes in properties of material and circumstance such as stress field may occur at the very point of the stepdown, underplating of subducted material, and the up-dip limit of the seismogenic zone.
DE: 8000 STRUCTURAL GEOLOGY (New field, replaces single entry 8165)
DE: 8010 Fractures and faults
DE: 8025 Mesoscopic fabrics
DE: 8150 Plate boundary--general (3040)
DE: 8164 Stresses--crust and lithosphere
SC: Tectonophysics [T]
MN: 2003 Fall Meeting