HR: 0800h
AN: T21B-0464    [Abstracts]
TI: A change in slip mechanism and subsequent weakening along a fault zone in in-situ magmatic oceanic crust, 30° N Mid-Atlantic Ridge
AU: * Michibayashi, K
EM: sekmich@ipc.shizuoka.ac.jp
AF: Institute of Geosciences, Shizuoka University, Ohya 836, Shizuoka, 422-8529 Japan
AU: Escartin, J
EM: escartin@ipgp.jussieu.fr
AF: Marine Geosciences, CNRS/IPGP, 4 Place Jussieu, Paris, 75252 France
AU: Delius, H
EM: hd21@leicester.ac.uk
AF: Department of Geology, University of Leicester, University Road, Leicester, LE1 7RH United Kingdom
AU: Linek, M
EM: m.linek@geophysik.rwth-aachen.de
AF: Applied Geophysics, RWTH, Aachen University, Lochnerstrase 4-20, Aachen, 52056 Germany
AU: Nozaka, T
EM: nozaka@cc.okayama-u.ac.jp
AF: Department of Earth Sciences, Okayama University, Tsushima, Okayama, 700-8530 Japan
AU: Hirose, T
EM: hirose@erdw.ethz.ch
AF: Department of Geology and Mineralogy, Kyoto University, Sakyo-ku, Kyoto, 606-8501 Japan
AU: Ohara, Y
EM: ohara@jodc.go.jp
AF: Hydrographic and Oceanographic Department of Japan, Chuo-ku, Tokyo, 104-0045 Japan
AU: Scientific Parties, I
EM: miller@iodp.tamu.edu
AF: Integrated Ocean Drilling Project, College Station, College Station, TX 77845-9547 United States
AB: The study of an in situ fault zone within Atlantis Massif oceanic core complex (Mid-Atlantic Ridge) provides clues into deformation mechanisms and their evolution in the oceanic crust. IODP EXP304/305 drilled a succession of gabbroic lithologies down to a final depth of 1415 meters below sea floor (mbsf), with very high recovery rates (~80%), up to 100%. We have identified an intra-crustal fault zone between 720 and 780 mbsf, in a section consisting of massive gabbro, olivine gabbro and oxide gabbro units, with minor diabase intrusions. Of particular interest is the section between 744 and 750 mbsf, which is portrayed by low recovery (17%). Electrical borehole wall images show a 1-m thick zone of west-dipping fractures within this interval that is otherwise dominated by N-S structures. Despite a high fracture density the section shows smooth walls with rare breakouts thus suggesting that the low recovery is due to a lithology change and not to well conditions. Logging data suggest that the gabbroic rocks in this interval are rich in hydrous phases, corroborating increased amounts of chlorite found in the core. The neutron porosity shows high values ranging between 10% to 15%, accompanied by a density reduction from 2.9 to 2.66 g/cm³, and a resistivity change from 144 to 28 Ohm m; all these marked changes compared to the rocks above and below this interval are consistent with the fault zone associated with hydrous minerals. Onboard structural analyses of the cored fault zone rock reveal a change in slip mechanism resulting in weakening along this intracrustal shear zone. Syntectonic fluid-assisted overgrowth of chlorite-rich films have replaced earlier cohesive 'ultracataclasite' and possibly incohesive 'fault gouge' formed in the greenschist facies regime. In contrast to the gabbro protolith, which was deformed in the brittle regime, the replacing films with (001) cleavages subparallel to the fault plane resulted in plastic strain localization along slip zones within the fault zone. This is thus a frictional to plastic transition, so that the later plastic flow is likely to enhance slip-weakening within the fault zone. As a consequence, seismic frictional slip could be succeeded by aseismic slow slip with a wide distribution of hydrous phases such as chlorite as revealed by the logging results. Moreover, since the friction coefficient of chlorite is lower than that predicted by Byerlee_fs law, and the presence of fluid can reduce it further, the established thin chlorite-rich 'mylonite' likely induces long-term weakening. This friction-to-plastic deformation evolution, similar to that reported for mantle rocks in the presence of talc or serpentinite, could control fault behavior within the magmatic oceanic crust at shallow levels, and operate in a wide range of tectonic environments from the ridge axis to the subduction zone.
DE: 7220 Oceanic crust
DE: 7245 Mid-ocean ridges
SC: Tectonophysics [T]
MN: Fall Meeting 2005