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Borehole image analysis of the Nankai accretionary wedge, ODP Leg 196: structural and stress studies

Record type: Article

Electrical images recorded with Resistivity-At-Bit (RAB) from two sites drilled during Ocean Drilling Program (ODP) Leg 196 were analyzed to study the effects of subduction at the Nankai margin. For the first time in the history of scientific deep-sea drilling in ODP, in situ complete borehole images of the décollement zone were obtained. Analyses of all drilling-induced fracture data indicated that the maximum horizontal compressive stress (SHmax) axes have an azimuth of 303°, and analyses of breakout data from RAB images indicated an azimuth of 310°. These azimuths approximate the convergence direction of the Philippine Sea plate towards the Eurasian plate. The frontal thrust at Site 808 was encountered at about 389 mbsf. Density, porosity, resistivity, and gamma ray data change across the frontal thrust. The décollement zone at the deformation front was identified between 937 and 965 mbsf. The base of the décollement is sharply defined as the maximum extent of conductive fracturing and is marked by abrupt changes in physical properties [Mikada, H., Becker, K., Moore, J.C., Klaus, A., Austin, G.L., Bangs, N.L., Bourlange, S., Broilliard, J., Brückmann, W., Corn, E.R., Davis, E.E., Flemings, P.B., Goldberg, D.B., Gulick, S.S., Hansen, M.B., Hayward, N., Hills, D.J., Hunze, S., Ienaga, M., Ishiguro, H., Kinoshita, M., Macdonald, R.D., McNeill, L., Obana, S., Hong, O.S., Peacock, S., Pettigrew, T.L., Saito, S., Sawa, T., Thaiprasert, N., Tobin, H.J., Tsurumi, H., 2002. Proc. ODP, Initial Rep., 196, College Station, TX, (Ocean Drilling Program)]. The upper boundary of the décollement is marked by several sets of conductive fractures and by high variability in physical properties. The décollement zone is characterized by intense brittle fracturing. These fractures are considered to be the consequence of cyclic stresses and high fluid pressures in this zone. We analyzed fracture dips and their orientations at both sites and found that they are all consistent with a unique stress field model surrounding the two sites.

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Citation

Ienaga, M., McNeill, L.C., Mikada, H., Saito, S., Goldberg, D. and Moore, J.C. (2006) Borehole image analysis of the Nankai accretionary wedge, ODP Leg 196: structural and stress studies Tectonophysics, 426, (1-2), pp. 207-220. (doi:10.1016/j.tecto.2006.02.018).

More information

Published date: 2006
Keywords: décollement, borehole breakout, Nankai Trough, LWD, RAB, ODP

Identifiers

Local EPrints ID: 55792
URI: http://eprints.soton.ac.uk/id/eprint/55792
ISSN: 0040-1951
PURE UUID: 2f011b9a-1628-4c07-bc76-f4260e2d7459

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Date deposited: 06 Aug 2008
Last modified: 17 Jul 2017 14:32

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Contributors

Author: M. Ienaga
Author: L.C. McNeill
Author: H. Mikada
Author: S. Saito
Author: D. Goldberg
Author: J.C. Moore

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