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Abstract

Deformation bands form in porous, clay-poor, sandstones in the top few kilometres of the Earth’s surface, involving the sequential growth of a set of discrete fault strands with minimal individual offset, ultimately culminating in the development of a slip surface with a large offset. We review some of our recent experimental results designed to reproduce the early stages of this sequence, obtained at room temperature and low confining pressure (P < 70MPa) in a large-capacity (10 cm core diameter) deformation rig. We examine the physical weakening and strengthening mechanisms at work in the experiments, and discuss the implications for fault sealing. We describe laboratory experiments where deformation occurs by the progressive formation of new bands with a finite small offset and a relatively constant fault gouge grain-size distribution, at a relatively constant stress measured at the sample boundaries. The friction coefficient is 0.6, i.e. within the standard range. No large-offset slip surfaces were observed. Cross-fault permeability is transiently increased during dynamic stress drop, associated with the ‘suction pump’ provided by rapid near-fault dilatancy under conditions of constant flow rate. As the deformation band develops quasi-statically, permeability is then reduced further by up to two orders of magnitude as a result of shear-enhanced compaction and porosity loss of the poorly sorted gouge fragments. A simple microstructural model successfully predicts the physical sealing rates in the post-failure stage. Finally, we estimate the chemical sealing rates from mass balance calculations based on direct measurement of the pore fluid chemistry during constant flow experiments at temperatures up to 120° C. When extrapolated to longer timescales, these account quantitatively for the differences between permeability reductions measured in the laboratory and in the field.

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