A conceptual model for the origin of fault damage zone structures in high-porosity sandstone
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.
Section snippets
Field setting
The Big Hole and Blueberry faults in the Chimney Rock fault array, central Utah, have exceptional along-strike exposures of damage zone structures (Fig. 1). Both faults are several kilometres in length with maximum throws of tens of metres. Cowie and Shipton (1998) presented high-resolution measurements of throw on the Blueberry fault and Shipton and Cowie (2001) presented throw profiles and maps of the damage zone from both the Big Hole and Blueberry faults. The faults cut the Jurassic Navajo
The slip-patch model of fault growth
Most seismically active faults rarely slip along their entire length; instead, portions of the fault surface rupture in one earthquake (e.g. Crone and Haller, 1991). It is therefore not appropriate for fault models to be constrained by assuming that failure always occurs over the entire fault surface (e.g. Pollard and Segall, 1987, Cowie and Scholz, 1992, Scholz et al., 1993, Bürgmann et al., 1994). Although earthquake ruptures can often terminate at segment boundaries (see Machette et al., 1991
A modified slip-patch model for the Big Hole and Blueberry faults
The structures in and around the Big Hole and Blueberry faults are consistent with the two main assumptions of the slip-patch model, i.e. that the fault slips repeatedly in small patches and that healing occurs, which allows stress to be supported on the fault surface. Microstructures in the fault zone illustrate that it has had a complex history of overprinting slip events. Very low porosity (a high degree of grain crushing) is seen in the cataclasite adjacent to the main fault slip-surface
Discussion
Our field observations, coupled with the slip-patch model of Cowie and Shipton (1998), suggest a new integrated model for displacement accumulation and formation of a damage zone along faults in high-porosity sandstones. We find that the evolution of the damage zone and the scaling of damage zone width with throw are strongly controlled by the deformation mechanisms operating at the grain-scale and thus the lithology and porosity of the host rock. Strain hardening is a vital component of the
Conclusions
We review detailed field observations, previously published by Shipton and Cowie (2001), of damage zone structures developed along kilometre-scale faults in the high-porosity Navajo Sandstone of central Utah. These data show a positive correlation between damage zone width and fault throw, for values of throw ranging from 0 to 30 m. This correlation, plus the orthorhombic geometry of structures in the damage zone, indicate that the damage zone has increased in width in response to displacement
Acknowledgements
This work benefited greatly from discussions with Bryne Ngwenya, Gerald Roberts and Jim Evans. Noelle Odling and Alistair Welbon provided reviews of the manuscript. Thanks to Jon Perry for help with Erdas Imagine. Jan Vermilye, John Mayers, Steve Schulz, Amy Hochberg, Kathryn Hardacre, Richard Jackson, Bertrand Maillot, Clare Bond, Kim Robeson, Jonathan Lim and Steve Thurber assisted with collecting the field data. Z. Shipton was supported by NERC studentship grant GT4/95/91/E. P. Cowie is
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