A conceptual model for the origin of fault damage zone structures in high-porosity sandstone

Abstract

We present a conceptual model to explain the development of damage zones around faults in high-porosity sandstones. Damage zone deformation has been particularly well constrained for two 4-km-long normal faults formed in the Navajo Sandstone of central Utah, USA. For these faults the width of the damage zone increases with fault throw (for throws ranging from 0 to 30 m) but the maximum deformation density within the damage zone is independent of throw. To explain these data we modify a previously published theoretical model for fault growth in which displacement accumulates by repeated slip events on patches of the fault plane. The modifications are based on field observations of deformation mechanisms within the Navajo Sandstone, the throw profiles of the faults, and inferences concerning likely slip-patch dimensions. Zones of enhanced stress are generated around the tips of each slipping patch, raising the shear stress on adjacent portions of the fault as well as potentially causing off-fault damage. A key ingredient in our model for off-fault damage accumulation is the transition from strain hardening associated with deformation band development, to localised strain softening as a slip-surface develops. This transition occurs at a critical value of deformation density. Once a new slip-surface develops at some distance from the main fault plane and it starts to accumulate throw it can, in turn, generate its own damage zone, thus increasing the overall damage zone width. Our approach can be applied to interpret damage zone development around any fault as long as the host-rock lithology, porosity and deformation mechanisms are taken into consideration.

Introduction

Faults are often surrounded by a zone of subsidiary structures referred to as the damage zone (Chester and Logan, 1986). Possible origins for damage zone structures include flexure of beds around faults (Jamison and Stearns, 1982, Antonellini and Aydin, 1994), repeated slip on a fault surface (Vermilye and Scholz, 1998), enhanced stress at fault tips (Cox and Scholz, 1988, McGrath and Davison, 1995, Martel and Boger, 1998, Vermilye and Scholz, 1998) and strain at zones where adjacent fault segments link (Peacock and Sanderson, 1991, Childs et al., 1995). Recognition of systematics in the geometry of damage zone structures would aid in the prediction of sub-seismic fault distribution (e.g. Knipe et al., 1998) and the characterisation of fluid flow within and around fault zones (e.g. Caine et al., 1996, Shipton et al., 2002).

A detailed investigation of damage zone structure around kilometre-scale faults in the high-porosity (20%) Navajo Sandstone found that damage zone width was proportional to the total fault throw (Shipton and Cowie, 2001). Similar positive correlations have also been found in other high-porosity sandstones (Knott et al., 1996, Beach et al., 1997, Beach et al., 1999, Myers and Aydin, 1998, Fossen and Hesthammer, 2000) and in mixed sedimentary sequences (Wallace and Morris, 1986). Knott et al. (1996) discussed the effect of extensional and compactional quadrants around growing faults on the width of the resulting damage zone. However, the role of deformation mechanisms in controlling the scaling of damage zone width and displacement has not previously been discussed.

A model proposed by Cowie and Shipton (1998) conceives of fault growth as occurring by repeated slip on many small patches of the fault surface. This model successfully demonstrated that observed fault displacement profiles (e.g. Dawers et al., 1993, Cartwright and Mansfield, 1998>, Muraoka and Komata, 1983) can be modelled by the summation of many small slip events without creating unrealistic stress concentrations at the fault tip (Cowie and Scholz, 1992). As we show here, this model also has important implications for the development of damage zone structures. When a slip-patch ruptures, stress is enhanced in a volume around the slip-patch tip. In addition to re-loading the fault plane along strike, this could cause failure of the rock out of the plane of the fault. In this paper we investigate the implications of this slip-patch model for the distribution, geometry and evolution of damage zone structures.

We start by presenting field data collected from two 4-km-long faults that provide a particularly well-constrained example of fault damage zone structures in high-porosity sandstone (Shipton and Cowie, 2001). We then summarise the slip-patch model presented in Cowie and Shipton (1998) and discuss the implications of the modified slip-patch fault growth model that includes strain hardening to explain the generation of the damage zone in our study area. Finally we discuss the potential for applying this model to other faults.