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Geological Society, London, Special Publications; 2007; v. 270; p. 43-53;
DOI: 10.1144/GSL.SP.2007.270.01.03
© 2007 Geological Society of London

Outcrop studies

Transient permeability and reseal hardening in fault zones: evidence from dilation breccia textures

N. H. Woodcock, J. A. D. Dickson & J. P. T. Tarasewicz

Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK(email: nhwl{at}esc.cam.ac.uk)

It is well established that earthquake faulting can create permeability along a fault zone in high competence rocks - by mismatch of the fault walls and by secondary fracture in a surrounding damage zone - and that this permeability is created repeatedly during successive earthquake cycles. Less well proven is that such permeability is transient, being episodically reduced by precipitation of cements in the fracture porosity. The textures of carbonate dilation breccias, formed at around 1.7 km depth on the Dent Fault zone (NW England), lend support to this economically important concept of transient fracture permeability. The key observation is that many breccias reflect only a single episode of brecciation and reseal. A generally applicable explanation of such single-phase breccias is that they were resealed in the interval between major earthquakes, that this reseal made the breccia stronger that the intact rock, and that subsequent brecciation in the same rock volume was inhibited. This reseal-hardening model implies that transient permeability in fault zones may last no longer than the recurrence times of large earthquakes, and that the permeability conduits will change position in the damage zone with time, unless focused at a major fault jog or termination.





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N.H. WOODCOCK, N.J. SAYERS, and J.A.D. DICKSON
Fluid flow history from damage zone cements near the Dent and Rawthey faults, NW England
Journal of the Geological Society, 2008; 165: 829 - 837.
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