Fault zone architecture and its scaling laws: where does the damage zone start and stop?

Abstract

Damage zones of different fault types are investigated in siliciclastics (Utah, USA), carbonates (Majella Mountain, Italy) and metamorphic rocks (western Norway). The study was conducted taking measurements of deformation features such as fractures and deformation bands on multiple 1D scanlines along fault walls. The resulting datasets are used to plot the frequency distribution of deformation features and to constrain the geometrical width of the damage zone for the studied faults. The damage-zone width of a single fault is constrained by identifying the changes in the slope of cumulative plots made on the frequency data. The cumulative plot further shows high deformation frequency by a steep slope (inner damage zone) and less deformation as a gentle slope (outer damage zone). Statistical distributions of displacement and damage-zone width and their relationship are improved, and show two-slope power-law distributions with a break point at c. 100 m displacement. Bleached sandstones in the studied siliciclastic rocks of Utah are associated with a higher frequency of deformation bands and a wider damage zone compared to the unbleached zone of similar lithology. Fault damage zones in the carbonate rocks of Majella are often host to open fractures (karst), demonstrating that they can also be conductive to fluid flow.

Review of Modeling Approaches to Groundwater Flow in Deformed Carbonate Aquifers

“…Normal and strike slip faults that develop in lowporosity carbonates are characterized by fault cores of cataclasites and clay smears and areas of intense fracturing defined as damage zones (see conceptual model Figure 5a). Low-displacement (throw < ∼100 m) faults tend to primarily develop damage zones that are dominated by highly conductive open fractures (Figure 5; Caine et al 1996;Bauer et al 2016;Torabi et al 2019b). Indeed, the fault throw versus damage zone width curve flattens for throw values greater than approximately 100 m in carbonate rocks.…”

“…Indeed, the fault throw versus damage zone width curve flattens for throw values greater than approximately 100 m in carbonate rocks. Torabi et al 2019aTorabi et al , 2019b. Cataclastic fault cores are characterized by low-porosity fine-grained matrix generated by processes such as grain fracturing, chipping, and further abrasion, which reduce the grain size of the host rock.…”

“…Damage zone of faults are characterized by fractures, veins, and deformation bands depending on the initial porosity of the carbonate rock under deformation. Porous carbonate rocks are less prone to fracturing and tend to primary develop low-permeability cores with increasing fault throw, while the damage zone is less fractured than that of faults developed within low-porosity carbonates (Antonellini et al 2014;Torabi et al 2019aTorabi et al , 2019bTorabi et al , 2020.…”

“…Such tectonic structures are characterized by permeabilities up to two order of magnitude higher than the host rock in fractured carbonates (Aldrick 1978;Allen et al 1997;Medici et al 2019b). However, evidence of hydraulic barrier behavior across normal faults is more common in relatively high-displacement (throw > ∼100 m) faults due to presence of continuous, relatively thick, and low permeable cataclastic and clay-rich fault cores (Figures 5a and 6b; Caine et al 1996;Billi et al 2003;Micarelli et al 2006;Antonellini et al 2008;Faulkner et al 2010;Smeraglia et al 2016aSmeraglia et al , 2016bOrtiz et al 2018;Torabi et al 2019aTorabi et al , 2019b. Largedisplacement normal faults in southern France and central Italy displacing Meso-Cenozoic limestones compartmentalize aquifers (Celico et al 2006;Bucci et al 2014).…”

“…Synclines are also exposed to intense karstification, that is, formation of steep valleys that drive rapid groundwater flow in the subsurface (Goldscheider 2005;Göppert et al 2011;Chen and Goldscheider 2014). As a consequence of this, caves and cavities tend to develop parallel to the strike of normal and transcurrent faults and syncline axis (Figure 7c-e; Torabi et al 2019b). The coupling of conduits and high hydraulic gradients in these three tectonic structures causes local occurrence of turbulent flow.…”