Rock Mechanics

Q-P diagram for Wingate sandstone defined from results of triaxial compression testing.

    In porous rock, such as sandstone, limestone, or tuff, the onset of irrecoverable strain in an overall compressive stress state is manifest through a set of distinct of geologic structures that are interrelated through a common strength envelope for plastic yield. Thus dilation bands [Issen and Rudnicki, 2000; Du Bernard et al., 2002], deformation bands (dbs) [Aydin, 1978; Aydin and Johnson, 1978; Borja and Aydin, 2004], cataclastic flow [Wong et al., 1997; Wu et al., 2000] and compaction bands [Mollema and Antonellini, 1996; Issen and Rudnicki, 2000] are understood as plastic yield structures that develop within a specific range of effective mean, P, and differential, Q, stresses at yield [Issen and Rudnicki, 2000; Wong et al., 2004]. The causative mode of plastic yielding for each geologic structure entails mechanical alteration of the porosity and permeability of the host rock [Wong et al., 1992; Zhu and Wong, 1997; Vajdova et al., 2004] and can therefore significantly influence patterns of fluid flow [Antonellini and Aydin, 1994; Sigda and Wilson, 2003] and brittle faulting [Aydin and Johnson 1978; Shipton and Cowie 2001].

Normal fault-related deformation band damage zone at Goblin Valley, Utah. Deformation bands are the linear positive relief discontinuities within the upper-most, tan-colored Entrada sandstone.

    Fault–related deformation bands nucleate and propagate in well–defined damage zones surrounding the causative fault [Jamison and Stearns, 1982; Shipton and Cowie, 2001, 2003], a process analogous to the development of secondary fault–related fractures in crystalline rock. The yield strength of a deformation band however, is lower in magnitude than the peak strength of its host rock [Wong et al., 1997; Borja and Aydin, 2004]. Therefore deformation bands lack the fully–yielded fracture surfaces typical of joints and faults in crystalline rock. Instead, deformation bands are characterized by changes in porosity and permeability due pore volume change and shear [e.g. Antonellini and Aydin, 1994; Wong et al., 1997; Sigda and Wilson, 2003]. Thus in addition to controlling fault geometry, deformation bands and the damage zones they can form important reservoir–scale barriers or conduits to matrix dominated fluid flow depending on the causative mode of plastic yielding. From a fluid–flow perspective, understanding the growth of deformation band damage zones is especially pertinent to the petroleum industry since important reserves commonly occur within faulted porous granular rocks [e.g. Antonellini and Aydin, 1994; Fossen and Hesthammer, 1998].

Photomicrograph of a dilational deformation band created through triaxial compression testing of Wingate sandstone. Blue resin fills the pore spaces between lightly-cemented quartz and feldspar grains, as well as within the band itself.

    I investigate the geometries, intensities and propagation tendencies of fault–related deformation band damage zones using numerical model predictions of both distortional and volumetric strain energy density. These criteria are shown to successfully predict the growth of a classic outcrop of fault–related deformation band damage zones within the in the Laramide–aged Uncompahgre fold, in western Colorado [Stearns and Jamison, 1977; Jamison, 1979]. In these numerical models, the geometry of the causative thrust faults is based on field exposures and previous observations [e.g. Jamison and Stearns, 1982]. The strength and deformability of the host rock (Jurassic Wingate sandstone) is determined through a series of triaxial and uniaxial tests conducted here at UNR. These tests are also used to determine the critical values of volumetric and distortional strain energy density at which deformation band growth is predicted. Volumetric strain energy density is used to predict the tendency for deformation band nucleation, the growth stage at which the deformation bands are defined by pore space collapse. Deformation band propagation, where shear occurs along the band, is predicted by distortional strain energy density.

Limb of the Uncompahgre fold where exposures of deformation band damage zones are mapped. The Wingate sandstone forms the uppermost tan-colored unit, which is underlain by the finer-bedded and red-colored mudstone of the Chinle formation. An out crop of the gray-colored metamorphic basement is visible in the lower right corner.

    Comparison of these model results with field observations at the Uncompahgre fold show that enhanced deformation band nucleation tendencies are predicted and observed to occur within the upper hanging wall and ahead of the causative thrust fault, as well as along the frictionally–slipping base of the Wingate. Additionally, enhanced deformation band propagation tendencies are predicted ahead of and slightly within the footwall of the thrust. The predicted tendencies for deformation band propagation are consistent with the observed distributions of compressive mode II deformation band stepover structures, which occur solely between propagating deformation bands. This work demonstrates how the geometries, intensities, and propagation tendencies of deformation band damage zones within km–scale fault–related folds is systematic and predictable based on field observations and laboratory testing. Since these deformation band damage zones control the subsequent geometries of through–going faults, this work establishes a conceptual model for investigating the growth of fault–related folds in porous granular rocks.

Numerical model simulations of deformation band damage zones at the Uncompahgre fold. The tendency for deformation band nucleation is shown on the left, and the tendency for deformation band nucleation is shown on the right. Predicted tendencies increase toward red.

    Publications of my rock mechanics research are currently under review (see the list on my Publications page).