HR: 0800h
AN: T41B-0582    [Abstracts]
TI: Localized Versus Distributed Deformation as a Control on the Evolution of Permeability in Anhydrite Rocks
AU: Collettini, C
EM: colle@unipg.it
AF: GSG, Dipartimento di Scienze della Terra, Universita' di Perugia, P.zza Universita 1, Perugia, 06100, Italy
AU: * De Paola, N
EM: nicola.de-paola@durham.ac.uk
AF: RRG, Earth Sciences Department, University of Durham, South Road, Durham, DH1 3LE, United Kingdom
AU: Faulkner, D
EM: faulkner@liv.ac.uk
AF: Rock Deformation Lab, Earth and Ocean Sciences Department, University of Liverpool, 4 Brownlow Street, Liverpool, L69 3GP, United Kingdom
AB: We have taken an experimental approach to understand and quantify the deformation processes and fluid flow within anhydrite-bearing fault damage zones during the seismic cycle. Triaxial loading tests have been performed on borehole samples of anhydrites at room temperature, 100 MPa confining pressure (Pc), and range of pore fluid pressures (Pf). Permeability and porosity development was continuously measured throughout the deformation experiments. The tests were conducted on samples with different grain sizes (10 microns to 1 mm) that were cored in different directions relative to the macroscopic foliation. Static permeability measurements have been carried out to determine the permeability anisotropy and sensitivity of the permeability on the effective pressure (Pc – Pf). Our results show that the brittle-ductile transition occurs for effective pressures (Pe) between 20 to 40 MPa and is almost independent of fabric orientation and grain size. Brittle failure is localized along discrete fractures and is always associated with a sudden stress drop. Conversely, ductile failure occurs by distributed deformation along cataclastic bands. In this case no stress drop is observed. Static permeability measurements show increasing values of permeability for decreasing values of Pe, (k = 10E-20 – 10E-22 m2). During single cycle loading tests, the evolution of the permeability is controlled by the failure mode: permeability begins to increase significantly at 40% and 80% of the max load for samples displaying brittle and ductile behaviour, respectively. The permeability values, immediately prior to failure, are about three orders of magnitude higher than the initial values. Multiple cycling tests, within the ductile field, show that permeability starts increasing at only 40% and 30% of the max load during the second and third loading cycle, respectively. Our results show that the history of deformation and the mode of deformation can control the evolution of the permeability, and that they are more significant than other factors such as fabric and grain size. In natural environments, fluid pressure fluctuations, such as might be experienced during the seismic cycle, can promote a switch from localized (brittle behaviour) to more distributed (ductile behaviour) deformation, leading to complex permeability patterns.
DE: 8034 Rheology and friction of fault zones (8163)
DE: 8163 Rheology and friction of fault zones (8034)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting