Fault breccias, or tectonic breccias, are a type of cataclastic rock formed by mechanical deformation during crustal movements (Glossary of Geology, 2005). They are further classified as chaotic, mosaic, or crackle breccias based on percentage of large (>2mm) clasts (Woodcock and Mort, 2008).
Spry (1969) and Higgins (1971)
As discussed in Fault Gouge, cataclastic fault rocks are a spectrum of rock types defined by texture including fault breccia, fault gouge, pseudotachylyte, and mylonites. Early work on cataclastic rocks used a variety of inconsistent terms to define fault rocks, Spry (1969) review of metamorphic deformation mechanisms and textures and Higgins (1971) review of existing terminology provided the basic fault rock nomenclature used today. Higgins defined the terms “fault breccia” and “fault gouge” as separate from the mylonite sequence. He classified cataclastic rocks by the presence or absence of primary coherence (Figure 1) and used the umbrella term “cataclastic rocks” for all rocks formed by cataclasis.
Sibson (1977) to 2008
The seminal Sibson (1977) classification scheme (Figure 2) was strongly based on Spry (1969) and Higgins (1971). Sibson recognised that cataclasis was not the dominant process in the formation of Higgen’s cataclastic rocks and instead uses the collective term fault rocks for rocks found in zones of shear dislocation. Sibson’s classification scheme is divided by fabric and primary cohesion; here fault breccia is defined as incohesive with a random fabric and visible rock fragments >30% of the rock mass. Sibson also recognised that there are cohesive breccias and included a series of crush breccias defined as a range of fragment size with 0 – 10% matrix.
Fault breccias underwent a series of further classification since Sibson (1977). Some key schemes include Sibson (1986) which subdivides fault breccia into attrition, distributed crush, and implosion breccias based on mechanical processes during brecciation. Using hydrothermal breccias Jebrak (1997) classified breccias by brecciation mechanisms based on particle size distribution. Killick (2003) modified the Sibson (1977) classification scheme to remove crush breccias, instead including them as protocataclasites.
Woodcock and Mort (2008)
Fault rock classification with a focus on fault breccias was most recently revised by Woodcock and Mort (2008). Here fault breccia definition approximates usage for sedimentary rocks, relying on clast size as the primary criterion and clast proportion as a secondary criterion rather than differentiating by cohesive nature and matrix percentage (Figure 3). Classification by clast size is preferable due to the difficulty in identifying primary vs secondary (post-faulting cementation) cohesion and the fact that cohesive fault breccias have been widely recognised (Brodie, Fettes & Harte, 2007; Higgins, 1971; Killick, 2003; Sibson, 1977) but do not comfortably fit in earlier classification schemes. Proportion of clast or matrix is relegated to a secondary criterion as it is often difficult to determine proportions in the field. Clasts are defined as > 2mm in diameter. The Woodcock and Mort (2008) classification can therefore be used for fault breccias which have any combination of cohesion, cementation, foliation, and/or variable clast size.
Like Killick (2003), the Woodcock and Mort (2008) classification opts to group Sibson (1977) crush breccias in with protocataclasites. Using nomenclature from cave-collapse literature (e.g. Loucks, 1999) fault breccias are then subdivided by how well clasts fit together into crackle, mosaic, and chaotic breccias (Figure 4a). Crackle breccias have clasts with little to no rotation relative to each other and thin cement or matrix seams between clasts. Mosaic breccias show increased separation and rotation of clasts. Chaotic breccia clasts have been strongly rotated and show little to no geometric fit with adjacent clast. Mort and Woodcock (2008) show that clast area as a percentage of total area can be used as a proxy for clast tesselation (Figure 4b) although it is important to remember that this textural spectrum is variable. Therefore, using Woodcock and Mort (2008) classification scheme for fault rocks, fault breccia is defined as having at least 30% of its volume being clasts larger than 2 mm in diameter and can be further subdivided into the crackle-mosaic-chaotic breccia spectrum approximately corresponding to large (>2 mm) clast percentages of 75% and 60%.
Sibson (1977) describes a bimodal deformation model for fault zones (Figure 5) where deformation can be friction dominated, elastico-frictional, or quasi-plastic where rocks can readily deform by crystal plasticity. Fault breccias exist purely in the elastico-frictional domain with incohesive breccias restricted to the near-surface, between 1-4km depth, while cohesive breccias are found between 10-15km depth (Sibson, 1977). Using patterns of shallow earthquake surface-rupture and aftershock distributions Sibson (1986) shows that location within the fault zone is also affected by the infrastructure of the principle slip zone of a fault (Figure 6) with brecciated zones at dilational (implosion breccias), neutral (attrition breccias), and antidilational (distributed crush breccia) sites.
1) Attritional wear. Attritional wear is a combination of frictional wear and grain cataclasis associated with major shear separation and can occur during both seismic and aseismic events. Frictional wear is described by Sibson (1977; 1986) as including brittle shearing of asperities, asperity indentation, and sidewall plucking. Grain cataclasis is the “comminution [of grains] by inter- and intra-granular cracking” (Sibson, 1986) and grain rotation (Sibson, 1977; 1986) around the propagating tip of a fracture (Knipe, 1989; Stewart and Hancock, 1990). The primary controls of cataclastic deformation is frictional grain boundary sliding and fracturing (Knipe, 1989). Attrition leads to the development of sidewall breccias and fault gouge (Higgins, 1971; Scholz, 1987; Sibson, 1977; 1986; Stewart and Hancock, 1990).
For a more in depth discussion of attritional wear see Fault Gouge.
2) Distributed cataclastic crushing. Cataclastic crush brecciation involves microcracking and microfaulting over a broad area in the vicinity of antidilational jogs to produce a structureless microbreccia. Crush brecciation is associated with minor shear separation and are more pervasive as effective overburden pressure increases. [Sibson (1986)]
3) Implosion brecciation. Implosion brecciation occurs when cavities open during rapid slip, often at dilational jogs, allowing wall rock to infill the cavity. Interestingly, the presence of wall rock breccia in psuedotachylyte matrix provides evidence that cavity opening can be co-seismic and can occur at several kilometers depth. Texturally breccias formed by implosion are the most recognisable showing good fit with adjacent clasts and little sign of frictional attrition, what would be considered crackle to mosaic breccias by Woodcock and Mort (2008). Texture is particularly important in implosion breccias as it acts as a record of incremental or multi-episode formation. This, combined with the common presence of hydrothermal minerals in veins or matrix, indicates that the common mechanism is hydraulic implosion due to rapid generation of fluid pressure differentials during rupture arrest and enhanced fracture permeability. Therefore, unlike attrition and crush brecciation, implosion brecciation is directly associated with local extensional environments and often hosts extensive vein networks. [Sibson (1986)]
Other mechanisms for generation of a fault breccia include;
Fault void collapse. Fault void collapse is gravity driven collapse of hanging wall material into persistent voids as opposed to implosion into transient voids (Woodcock et al., 2006) as shown in Figure 8. Persistent fault voids are generated by geometric mismatch of opposing fault walls at shallow depths with low confining pressure (Loucks, 1999). Fault void collapse creates volumes of chaotic, hanging wall clast supported, breccias larger than void volume (Loucks, 1999) and may display bedding defined by clast shape and size at the angle of repose for scree (Woodcock et al., 2006).
Fluidization. Described in Smith et al., (2008) fluidization is related to grain cataclasis in attritional wear processes (Knipe, 1989) and defined as “the state in which grains fly around with a mean free path like gaseous molecules” (Monzawa and Otsuki, 2003). Fluid overpressure builds until it reaches a critical state triggering embrittlement of the fault core and fracturing in the surrounding rock as recognised in the Zuccale Fault, Elba, Italy by Smith et al., (2008). Characteristics of the fluidized breccia noted in the Zuccale Fault include 1) irregular interface between breccia and fault core, 2) lack of evidence for the traditional frictional deformation mechanisms mentioned above, 3) indication of dissolution from fluid migration, and 4) preferred orientation of clasts due to fluid migration. Fluidization was able to occur in Zuccale Fault because of the presence of CO2 bearing fluids in pre-existing footwall breccias (Figure 9a). These pre-existing breccias experienced dissolution leading to loss of cohesion allowing clasts to be fluidized during slip events on local footwall faults. The impermeable nature of the main fault core means fluid was trapped and forced to spread laterally causing deformation of the breccia-fault core interface (Figure 9b). Rising fluid overpressure acts to embrittle the fault core leading to hydrofracturing and draining of fluid into the hanging wall (Figure 9c). The system can then heal during the inter-seismic phase allowing the system to repeat. [Smith et al., (2008)]
Sibson (1987) noted the importance of fault breccias as hosts of secondary, often hydrothermal, mineralisation in fault zones. Fluid migration through fault breccias often induces cementation, defined as “crystalline material grown in place, either as infill of void space or as a replacement of clasts or matrix” (Woodcock and Mort, 2008), reducing permeability or dissolution and increased permeability (Jebrak, 1997). Episodic fluid migration and associated mineralisation in fault zones is intrinsically linked to rapid changes in fluid pressure at dilational jogs where fluid influx is assisted by fluid pressure drop as fractures open (Micklethwaite and Cox, 2004; Sibson, 1987; Tarasewicz et al., 2005; Woodcock and Mort, 2008), see below for more details.
Role of Fault Breccias in Fluid Migration
Fault breccias play an integral role in fluid migration within fault zones. Sibson (1990) describes the fault-valve model (Figure 10) as impermeable faults which act as fluid barriers except for a short period post-failure when the faults become fluid conduits and, on intersection with high fluid pressure differentials, become “fluid-pressure-activated valves.” Changes in fluid pressure are therefore intimately linked to the earthquake cycle as fluid pressure cycling affects fault behaviour and strength (Sibson, 1990; 1992; Smith et al., 2008).
Caine et al. (1996) showed that the percentage of fault rocks, fault breccias included, in total fault width relative to the percentage of damage structures (e.g. faults, fractures, veins) can be used to qualitatively model fluid flow in and around fault zones. Caine’s scheme indicates that well developed fault rocks without well-developed damage structures act as a barrier to fluid flow but as a conduit-barrier in the presence of extensive damage structures (Figure 11). Gudmundsson (2001) reaffirms the idea that faults can act as barriers or conduit-barriers in small faults with breccia cores and demonstrates that the fault breccias act as barriers to vertical groundwater flow. Furthermore, as breccia thickness varies along fault it acts to channel fluid along the fault and into the footwall at thin breccia zones allowing deeper crustal flow (Figure 12) or it can locally trap upward migrating fluids increasing fluid pressure gradients across the fault. Breccia thickness, and any fluctuations in breccia thickness, will therefore significantly impact the groundwater system around the fault.
Transient Permeability in Fault Breccias
Several people including Sibson (1996), Micklethwaite and Cox (2004), and Woodcock et al., (2007) expand on the transient nature of increased permeability from dilation brecciation. Micklethwaite and Cox (2004) hypothesize that permeability is enhanced immediately after fault rupture followed by a decrease in porosity on the main rupture surface as fluid pathways are sealed and protracted permeability increase in aftershock zones . Woodcock et al. (2007) present a case study from Dent Fault zone in NW England demonstrating this transient permeability.
Dent Fault zone includes 10-100m, mostly single phase breccias in a near-random carbonate cement sealed fracture mesh (Woodcock et al., 2007). The sealing cement is granular rather than fibrous indicating that fractures opened faster than the rate of carbonate cement growth so breccias are likely to have developed during seismic phases and resealed during the inter-seismic phase (Tarasewicz et al. , 2005). The large extent and single phase nature of the Dent Fault breccias point to inter-seismic resealing hardening the breccia causing later brecciation to occur in nearby weak intact rock, permeability would therefore be greatest at the end of the post-seismic phase and progressively decrease from there as shown in Figure 13 (Woodcock et al., 2007).
resultant permeability to the phases of the earthquake cycle from Figure 13. The relationship of brecciation, reseal andWoodcock et al. (2007)[/caption]
Another case study on the Talhof fault (Figure 14), a 15 km long segment of the Salzach-Ennstal-Mariazell-Puchberg (SEMP) fault system in the Eastern Alps, is presented by Hausegger et al. (2010). The Talhof fault is in layered, anisotropic carbonates which developed by layer parallel shear along the shear zone, forming a penetrative fracture cleavage. Fractures in zones parallel to the shear zone boundary formed to accommodate strain and later hosted cataclastic flow as shear became localised and the evolution of a fault core. Infiltration of fluid into the fault zone lead to cementation of breccias with reduced porosity and permeability (strain hardening) forcing shear localisation to adjacent zones (strain weakening). As brittle deformation continues the fault breccias undergo cyclic healing (cementation) and fracturing (brittle failure) which can be equated to a cycle of closed and open fluid systems respectively. [Hausegger et al., (2010)]
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