HR: 10:55h
AN: T42B-03 INVITED     [Abstracts]
TI: Fluid flow properties of fault rocks during deformation: dynamic fault zone permeability structure
AU: * Wibberley, C A
EM: wibbs@geoazur.unice.fr
AF: Université de Nice - Sophia Antipolis, Géosciences Azur CNRS, 250 rue A. Einstein, Valbonne, 06560 France
AB: Fluids and the migration of fluids fundamentally affect the mechanical and seismological properties of the crust. Fault zones are both the sites of seismogenic activity and the focus of fluid flow, making the study of fluid flow properties of crustal fault zones highly important in understanding the link between fluid behaviour and seismogenesis. Recent studies of fault zone structure and permeability have shown the presence in many cases of large faults, often with zones of fractured rock around the core zone and/or contained as lenses within it. Hence the permeability structure depends on the distribution of these zones, dependant on the lithologies involved and the deformation history to which they are subjected. Geological evidence shows that the fractured zones behave as transient fluid conductors during earthquake slip, whereas the low permeability fault gouges are considered to be typically across-fault barriers to fluid flow. Most laboratory measurements of the fluid flow properties of these fault rocks are undertaken under isotropic states of stress, i.e. without subjecting the sample to deformation in the laboratory other than exerting a confining pressure. This contribution presents new data on the evolution of permeability and porosity of different fault rocks during deformation experiments. The data show that the fault gouges suffer much more compaction (porosity and permeability decrease) if effective mean stress is increased by increasing stress in one direction only (i.e. by increasing differential stress) than if it is increased by an increase in confining pressure. This is typical behaviour of porous granular materials, allowing us to use critical state soil mechanics to predict the relationship between differential stress, effective mean stress and the evolving porosity of the gouge during deformation. Laboratory work on fractured crystalline rocks on the other hand shows the importance of elastic crack closure followed by fracture dilatancy. Thus the distribution of fault rocks in a complex fault zone gives rise not only to a given permeability structure in the static case but also the need for different approaches to modeling fluid flow in different parts of the fault zone during continued deformation, i.e. a dynamic permeability structure.
DE: 1822 Geomechanics
DE: 5104 Fracture and flow
DE: 5114 Permeability and porosity
DE: 8010 Fractures and faults
DE: 8045 Role of fluids
SC: Tectonophysics [T]
MN: Fall Meeting 2005