HR: 17:15h
AN: T54A-06    [Abstracts]
TI: Experimental Measurements of Permeability Evolution During Brittle Deformation of Crystalline Rocks and Implications for Fluid Flow in Fault Zones
AU: * Mitchell, T
EM: t.mitchell@liverpool.ac.uk
AF: University of Liverpool, Rock Deformation Lab, Department of Earth and Ocean Sciences, 4 Brownlow Street, Liverpool, L69 3GP, United Kingdom
AU: Faulkner, D
EM: faulkner@liverpool.ac.uk
AF: University of Liverpool, Rock Deformation Lab, Department of Earth and Ocean Sciences, 4 Brownlow Street, Liverpool, L69 3GP, United Kingdom
AB: Detailed experimental studies of the development of permeability of crustal rock during deformation are essential in helping to understand fault mechanics and constrain larger scale models that predict bulk fluid flow within the crust. The strength, permeability and pore fluid volume evolution of initially intact crystalline rock (Westerly granite and Cerro Cristales granodiorite) under increasing differential load leading to macroscopic failure has been measured in a triaxial deformation apparatus. Experiments were run under pore water pressures of 50 MPa and varying effective pressures from 10 to 50 MPa. Permeability is seen to increase by up to and over two orders of magnitude prior to macroscopic failure, from 3.5 x 10^-21 to 9 x 10^-19 m² with the greatest increase seen at lowest effective pressures. Post-failure permeability is shown to be over 3 orders of magnitude higher than initial intact permeabilities, as high as 4 x 10^-18 m², and approaches lower the limit of measurements of in situ bulk crustal permeabilities. Increasing amplitude cyclic loading tests show permeability-stress hysteresis, with high permeabilities maintained as differential stress is reduced. The largest permeability increases are seen between 90-99% of the failure stress. Under hydrothermal conditions without further loading, it is suggested that much of this permeability can be recovered, and pre-macroscopic failure fracture damage may heal relatively faster than post-failure macroscopic fractures. Pre-failure permeabilities are nearly seven to nine orders of magnitude lower than that predicted by some high pressure diffusive models suggesting that microfracture matrix flow cannot dominate, and agrees with inferences that bulk fluid flow and dilatancy must be dominated by larger scale structures, such as macrofractures. It is suggested that the permeability of a highly stressed fault tip process zone in low-permeability crystalline rocks could increase by more than 2 orders of magnitude, while stress drops related to fracture propagation close damage zone cracks, whereupon some permeability is maintained due to hysteresis from permanent microfracture damage.
DE: 5104 Fracture and flow
DE: 5114 Permeability and porosity
DE: 5139 Transport properties
DE: 8010 Fractures and faults
DE: 8118 Dynamics and mechanics of faulting (8004)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting