Analysis of the growth of strike-slip faults using effective medium theory

Abstract

Increases in the dimensions of strike-slip faults including fault length and thickness of fault rock and the surrounding damage zone collectively provide quantitative definition of fault growth and are commonly measured in terms of slip. A vast amount of field data shows that fault dimensions increase in some fashion as the slip across faults increases though these relationships may not be simple. The field observations also indicate that a common mechanism for fault growth in the brittle upper crust is fault lengthening by linkage and coalescence of neighboring fault segments or strands. In addition, the mechanism of fault zone widening is spreading of fault rock via cataclastic deformation into highly fractured inner damage zone. The most important underlying mechanical reason in both cases is prior weakening of the rocks surrounding a fault's core and between neighboring fault segments by fault-related fractures. Using field observations together with effective medium models, this paper attempts to constrain the reduction in the effective elastic properties of rock in terms of the orientation and density of the fault-related brittle fractures. Calculated and extrapolated values of the Young's and shear moduli for idealized fractured rock masses at fault steps and inner damage zones provide insight into the degree of strength degradation as a function of the angle between fracture sets and fracture density and how increasing fracture density may eventually facilitate fault growth via cataclastic deformation of fractured rock masses.

Permeability of a fault zone crosscutting a sequence of sandstones and shales and its influence on hydraulic head distribution in the Chatsworth Formation, California, USA

“…The widths of the stepovers, which are expressed by a negative relief, define approximately the fault zone width varying from 30 to 150 m at the surface. These observations are consistent with the inferred dominant left-lateral slip component across the fault zone and its growth by the linkage of neighboring segments similar to those described by other authors at various locations (Cartwright et al 1995;de Joussineau and Aydin 2007;Aydin and Berryman 2010).…”

Propagation energies inferred from deformation bands in sandstone

“…The scaling relations may be interpreted as involving a process of band thickening during band growth at a constant value of plastic normal strain. This interpretation is consistent with the findings of Chemenda (2011) in his analyses of band localization and growth as a consequence of discontinuous bifurcation of material properties within a deforming host rock and qualitatively with the widening of shear zones in porous rocks as discussed by Aydin and Berryman (2010). The scaling of compactional shear bands is given simply by D s = AL 1/2 (Schultz et al 2008) where A may be related to host-rock stiffness and grain toughness.…”

Micro-texture and petrophysical properties of dilation and compaction shear bands in sand

“…The deformation mechanisms involved in the bands produce different micro-texture which may variably affect fluid flow potential of the reservoir. 12,37,38 For example dilation shear bands may cause permeability enhancement. 17 However, the permeability difference between dilation shear band and host rock is relatively low, and these bands do not have an important influence on permeability of reservoir.…”