HR: 15:05h
AN: U12A-05    [PDF]
TI: Fluid pressure transients and mechanical coupling in decollement zones
AU: Henry, P
EM: henry@geologie.ens.fr
AF: CNRS-College de France, Batiment Laennec Europole de l'Arbois, Aix-en-Provence, 13545 France
AU: * Bourlange, S
EM: henry@geologie.ens.fr
AF: Laboratoire de Geologie de l'ENS, 24 rue Lhomond, Paris, 75005 France
AB: Accretionary wedges syn-sedimentary decollements are low friction fault zones and generally lie in the aseismic portion of the subduction plane updip of the seismogenic zone. Understanding the role of fluid migration and fluid pressure has been an important target of ODP, with legs 110 and 156 on Barbados accretionary wedge and legs 131, 190, 196 on Nankai accretionary wedge. In both settings, arguments based in great part on microstructural observations suggested pore pressure cycling. Logging while drilling data also indicate that decollements can keep a high water content in spite of the generally compactive nature of the deformation observed at the micro-scale. Theory and modeling show that high pore pressure fronts may propagate spontaneously along a decollement under certain conditions, which include a strong dependency of permeability on pore pressure. However, there are important differences between the Barbados case and the Nankai case. In Barbados, pore fluid chemistry indicates transient fluid migration along the decollement from a smectite diagenesis fluid source at depths. On the Nankai Muroto transect, smectite diagenesis occurs in the trench and there is no clear fluid migration signal at decollement level. This suggests that pressure cycling may occur without significant horizontal flow along the decollement. A conceptual model is proposed in which transient fluid pressure transfer from the underthrust sequence to the decollement and pore pressure in the decollement are controlled by the stress state in the underthrust sequence. Studies of anisotropic physical properties indicate strong decoupling at the decollement level and suggest an extensional state of stress is maintained in the underthrust sequence. Slip along the decollement increases the extensional stress in the underthrust sequence at the tip of the slipping zone and promotes fluid flow into the slipping zone.
DE: 8010 Fractures and faults
DE: 8045 Role of fluids
DE: 8100 TECTONOPHYSICS
SC: U
MN: 2003 Fall Meeting