HR: 09:15h
AN: S31D-06    [Abstracts]
TI: Transition from cataclastic flow to aseismic brittle failure in Carrarra marble
AU: Walker, E
EM: walker.e@noos.fr
AF: EOST, Ecole et Observatoire des Sciences de la Terre de Strasbourg, Strasbourg, 67000 France
AU: * Schubnel, A
EM: alexandre.schubnel@utoronto.ca
AF: Lassonde Institute, University of Toronto 170 College street, Toronto, On M5S 3E3 Canada
AU: Thompson, B
EM: B.Thompson@liverpool.ac.uk
AF: Lassonde Institute, University of Toronto 170 College street, Toronto, On M5S 3E3 Canada
AU: Fortin, J
EM: fortin@geologie.ens.fr
AF: Laboratoire de Geologie - ENS, Ecole Normale Superieure 24 rue Lhomond, Paris, 75005 France
AU: Nasseri, M
EM: nasseri@ecf.utoronto.ca
AF: Lassonde Institute, University of Toronto 170 College street, Toronto, On M5S 3E3 Canada
AU: Young, R
EM: paul.young@utoronto.ca
AF: Lassonde Institute, University of Toronto 170 College street, Toronto, On M5S 3E3 Canada
AB: Interest in the brittle-ductile transition has increased considerably in recent years, in large part due to the fact that the maximum depth of seismicity corresponds to a transition in the crust and in the upper mantle from seismogenic brittle failure to aseismic cataclastic flow, i.e. from localized to homogeneous deformation. The mechanics of the transition depends both on some extrinsic variable (state of solid stress, pore pressure, temperature, fluid chemistry and strain rate) and intrinsic parameters (crack and dislocation density, modal composition of the rock or porosity for example). In the present study, two triaxial experiments were performed on Carrara marble at room temperature. The rock samples were first deformed in the cataclastic domain (up to $\sim5%$ axial strain) until they exhibited severe damage accumulation, i.e. wavespeed attenuation. Damaged samples were then brought back {\bf at constant differential stress} into the the brittle field by solely reducing the effective mean stress. Throughout both experiments, compressional wave velocities were measured along several raypaths. Acoustic Emissions, when any, were monitored and localized after testing. A complete 2 minutes failure recordings of failure (12 channels at 10MHz sampling frequency) was also obtained using ESG's Hyperion gigarecorder during one of the experiment. Our new set of data shows that during cataclastic deformation, elastic wave velocities show large variations, but only a small degree of elastic anisotropy when compared to what is generally observed in typical brittle materials such as granite or sandstones. After sufficient damage accumulation and when reducing the mean stress, both samples exhibited a fast acceleration in axial strain. Tertiary creep was followed by the nucleation of a brittle failure. Observed differential stress drops during rupture propagation were of the order of $150\,MPa$. Although failure occurred with large slip and stress drop, only very few AEs could be detected. The complete recording shows that rupture nucleated, initiated and propagated almost aseismically. Elastic properties, macroscopic strain, AE recordings and post-deformation microstructural analysis are here put together and show what we believe are the first experimental evidence of aseismic failure in the laboratory. Those results may have direct implications for the understanding of silent earthquakes and aseismic slips in the field. Limestones, because they can deform plastically at room temperature, may be good deep fault gouge analogs and thus their behavior could have direct consequences on the understanding of fault zones and the earthquake cycle.
UR: http://www.lassondeinstitute.utoronto.ca/young/people/alex2.htm
DE: 8159 Rheology--crust and lithosphere
DE: 5102 Acoustic properties
DE: 7209 Earthquake dynamics and mechanics
DE: 3902 Creep and deformation
DE: 3909 Elasticity and anelasticity
SC: Seismology [S]
MN: 2004 AGU Fall Meeting