HR: 17:30h
AN: V14A-07    [Abstracts]
TI: The Geology of Weak Fault Zones: Long Term and Short Term Processes
AU: Holdsworth, R E
EM: R.E.Holdsworth@durham.ac.uk
AF: Department of Earth Sciences, University of Durham, Durham, DH1 3LE United Kingdom
AU: Collettinni, C
EM: colle@unipg.it
AF: Dipartimento di Scienze della Terra, Universit… di Perugia, Piazza dell'Universit… 1, Perugia, 06100 Italy
AU: * Imber, J
EM: jonathan.imber@durham.ac.uk
AF: Department of Earth Sciences, University of Durham, Durham, DH1 3LE United Kingdom
AU: Jefferies, S P
EM: s.p.jefferies@durham.ac.uk
AF: Department of Earth Sciences, University of Durham, Durham, DH1 3LE United Kingdom
AB: Crustal- or larger-scale fault zones form important structural anisotropies that cut across the main load-bearing mechanical layers in the lithosphere. Geological and geophysical studies consistently demonstrate that such fault zones localise displacements during lithosphere deformation at all scales, often repeatedly and over very long time periods, particularly in the continents. These observation suggest that such faults are weak relative to the surrounding regions of intact rock and, in many cases may be weak in an absolute sense, i.e. slip can occur even when sigma 1 is oriented at high angles (up to approximately 90 degrees) to the fault zone. In this presentation, we will examine deformation-related weakening processes caused by grain-scale fluid-rock interactions along examples of two classes of fault: 1) a reactivated plate boundary fault - the Median Tectonic Line, Japan; and 2) a low angle normal fault - the Zuccale detachment, Isle of Elba, Italy. In both fault types, early cataclasis facilitates fluid influx into the active fault zone, leading to widespread alteration of the crushed material (reaction softening) and simultaneously triggering the onset of stress-induced solution-precipitation deformation mechanisms such as pressure solution. These processes lead to the development of a core region of foliated fault rocks (`phyllonites') that localise most of the later displacements. The textures in the phyllonites are similar to those observed in rock analogue materials deformed in laboratory experiments designed to examine the role of phyllosilicate foliation development and pressure solution creep in fault zones. A microphysical model based on the results of the deformation experiments can be used to generate synthetic strength profiles that predict profound long-term weakening in the fault core region of the two natural faults studied (assuming steady-state slip). The large size and high interconnectivity of these fault zones means that they quickly form highly effective interconnected weak layers, thereby facilitating rapid transfer of grain-scale weakening effects up to lithosphere scales. On shorter timescales, the impermeable character of the foliated fault cores also facilitates more transient weakening effects due to the local short lived attainment of fluid overpressures, leading to widespread hydrofracturing and mineralisation.
DE: 8000 STRUCTURAL GEOLOGY
DE: 8010 Fractures and faults
DE: 8030 Microstructures
DE: 8035 Pluton emplacement
DE: 8045 Role of fluids
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005