Caine et. al. (1996) is a cornerstone paper which describes, compares and quantifies brittle fault zones and their permeability (fluid flow) properties from observations. Doing so, the paper initiates the accumulation of information on the permeability of brittle fault zones described previously (Randolph and Johnson 1989; Scholz 2019; Byerlee 1993).
The study begins by defining fault zones as composed of two units with distinct structural and hydrogeologic properties, which result from the material properties and deformation conditions during faulting: the fault core and the damage zone. The fault core is the region of the fault zone that accommodates most of the displacement, and the damage zone is the region mechanically related to that displacement, expressed by the secondary structures that lead to heterogeneity and anisotropy in the fluid flow and elastic characteristics around the fault zone. The contrasting distribution of structure and grain size in these two zones lead to their different permeability capabilities in each zone and in the fault zone itself, which can act as a conduit, barrier, or combined conduit-barrier system to fluid flow.
Caine et al., (1996) uses fault zone architecture and permeability plots for a variety of field data to show that fluid flow in fault zones is determined by the interior structure and composition of the fault zone core. On the basis of this, the study proposes two “schemes” to understand permeability in fault zones. The qualitative scheme compares the percentage of fault core materials (e.g., anastomosing slip surfaces, clay-rich gouge, cataclasite, and fault breccias) to the percentage of subsidiary damage zone structures (e.g., kinematically related fracture sets, small faults, and veins), within the total fault zone width. The quantitative scheme defines indices to characterize the fault zone architecture (Fa) and its spatial distribution across the fault zone (Fs), so that the permeability along the fault and damage zone can be quantified.
The paper further proposes that fault zone permeability can be understood as a conduit-barrier system dependent on the fault zone architecture. Furthermore, the research concludes with the idea that there is a relationship between lithology and fault core material, both of which have a direct influence on fluid flow in these zones.
The main take-home message of Caine et al. (1996) is that the architecture of upper-crustal, brittle fault zones controls their permeability, as dictated by the relative percentage of fault core and damage zone structures and the inherent variability in grain scale and fracture permeability.
The research was one of the first to highlight the relevance of fault zone architecture in fluid circulation. This is key to various technologies such as geothermal energy, oil production, groundwater exploitation, and induced-seismicity risk assessment. Since this must read paper, several authors have examined the hydraulic behaviours of complex fault networks in order to produce exceedingly anisotropic flow characteristics (e.g. Lunn et al. 2008; Evans and Chester 1995). Furthermore, recent research has called into question the fundamental structure of a single fault core and the damage zone by observing multiple fault zones, and thus multicore high-strain materials around the damage zone (e.g. Faulkner 2004). As a result, the standard quantitative representation of the fault zone now also takes into account 3D fracture density variation instead of the one proposed by Caine et. al. (1996).
Written by Utsav Mannu, David Fernández-Blanco, and the TS Must Read team
Caine, J.S., Evans, J. P., and Forster, C.B. 1996. Fault zone architecture and permeability structure. Geology, 24(11): 1025-1028 doi: http://dx.doi.org/10.1130/0091-7613(1996)024%3C1025:FZAAPS%3E2.3.CO;2
Evans, James P., and Frederick M. Chester. 1995. “Fluid-Rock Interaction in Faults of the San Andreas System: Inferences from San Gabriel Fault Rock Geochemistry and Microstructures.” Journal of Geophysical Research: Solid Earth. https://doi.org/10.1029/94jb02625.
Faulkner, D. R. 2004. “A Model for the Variation in Permeability of Clay-Bearing Fault Gouge with Depth in the Brittle Crust.” Geophysical Research Letters 31 (19). https://doi.org/10.1029/2004gl020736.
Faulkner, D. R., C. A. L. Jackson, R. J. Lunn, R. W. Schlische, Z. K. Shipton, C. A. J. Wibberley, and M. O. Withjack. 2010. “A Review of Recent Developments Concerning the Structure, Mechanics and Fluid Flow Properties of Fault Zones.” Journal of Structural Geology 32 (11): 1557–75.
Lunn, Rebecca J., Jonathan P. Willson, Zoe K. Shipton, and Heather Moir. 2008. “Simulating Brittle Fault Growth from Linkage of Preexisting Structures.” Journal of Geophysical Research 113 (B7). https://doi.org/10.1029/2007jb005388.
Randolph, L., and B. Johnson. 1989. “Influence of Faults of Moderate Displacement on Groundwater Flow in the Hickory Sandstone Aquifer in Central Texas.” In Geological Society of America Abstracts with Programs, 21:242.