During Leg 190, accreted sediments at an evolved portion of the Nankai accretionary prism were penetrated. Deformation structures and anisotropy of magnetic susceptibility (AMS) data from Site 1178 indicate that structural and magnetic fabrics formed at the frontal part of the prism have been preserved, even ~65 km landward of the deformation front. As the sediments approached the deformation front, the bedding/fissility-parallel magnetic fabric took on a northeast-southwest-trending subhorizontal maximum AMS axis because of northwest-directed bedding-parallel compression. Subsequent faulting, represented by brecciated zones, took place at the frontal part of the accretionary prism, which is presumably equivalent to the imbricate thrusts in the prism. During this process, a steeply dipping bedding-oblique foliation developed in a fault zone, >100 m in thickness. The foliation was formed as a flattening plane associated with shearing deformation and alternates with brecciated zones, giving a geometric relationship similar to S-C or P-Y fabric. The magnetic fabric oriented parallel to the bedding-oblique foliation records pervasive strain related to faulting. Bedding and bedding-parallel magnetic fabric together with fissility were rotated about a horizontal axis during frontal accretion, resulting in a seaward-verging magnetic fabric with northeast-southwest-trending subhorizontal maximum AMS axes. Overprinting of the deformation and/or modification of preexisting fabrics during subsequent rapid seaward growth of the Nankai accretionary prism has not been recorded in the accreted sediments at Site 1178, suggesting that later penetrative deformation was not significant at shallow burial depths in the prism.
1Ujiie, K., Hisamitsu, T., Maltman, A.J., Morgan, J.K., Sánchez-Gómez, M., and Tobin, H.J., 2003. Deformation structures and magnetic fabrics at Site 1178: implication for deformation history recorded in accreted sediments at an evolved portion of the Nankai accretionary prism. In Mikada, H., Moore, G.F., Taira, A., Becker, K., Moore, J.C., and Klaus, A. (Eds.), Proc. ODP, Sci. Results, 190/196 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/190196SR/202/202.htm>. [Cited YYYY-MM-DD]
2Institute for Frontier Research on Earth Evolution, Japan Marine Science and Technology Center, 3173-25 Showa-machi, Kanazawa-ku, Yokohama 236-0001, Japan. ujiiek@jamstec.go.jp
3Institute for Frontier Research on Earth Evolution, Japan Marine Science and Technology Center, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan.
4Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, Aberystwyth SY23 3DB, United Kingdom.
5Department of Geology and Geophysics, Rice University, MS-126, 6100 South Main Street, Houston TX 77005-1892, USA.
6Departamento de Geología, Universidad de Jaén, Virgen de la Cabeza 2, 23071 Jaén, Spain.
7Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro NM 87801, USA.
Initial receipt: 13 August 2002
Acceptance: 23 April 2003
Web publication:
26 August 2003
Ms 190SR-202