Fault Generated Pseudotachylyte

8/9/2019 Fault Generated Pseudotachylyte

1/75

Fault generated pseudotachylytePseudotachylyte is a dark, aphanitic frictional melt rock that was formed during brittle faultingand preserves evidence of having undergone a melt phase (eg. Sibson, 1977 ; Magloughlin &

Spray, 1992 ; Sibson & Toy, 2006 ; Di Toro, 200 9). The occurrence of pseudotachylyte in faultszones is interpreted as fossil earthquakes ( Sibson, 1975 , 1989 ) and is largely found in faultzones that have been exhumed from


2/75

Figure 1. Illustration of fault vein and injection vein relationships for fault generated pseudotachylyte (after Sibson & Toy, 2006 ). a) pseudotachylyte fault vein offsetting foliation in banded gneisses, with associated injection veins; b) pseudotachylyte injection vein lying parallelto the foliation in banded gneisses with associated injection veins. NOTE: the injection veins, bydefinition, are entirely dilational features and will not show evidence of offset.

For a fault rock to be termed a pseudotachylyte it must show evidence of having undergone amelt phase, though this has been debated (eg. Philpotts, 1964 ; Wenk, 1978 ; Lin, 2008 ). Wenk(1978 ) first proposed that the main mechanism of formation pseudotachylyte formation wasultra- comminution , though this has been outweighed by a wealth of evidence for a melt phase. Ithas been shown that cataclasis does precede melting (eg. Spray, 1995 ) although evidence for amelt phase must be shown to be a true pseudotachylyte. Such evidence for a melt phase can beshown by any combination of the following features; rarely preserved glass; chill margins; flowtextures; vesicles and amygdules, variations of microlite and spherulite textures; melting and/orgrowth effects on clasts within the groundmass; and the dominance of angular quartz clasts overrounded and embayed feldspar clasts (Maddock, 1983 ; Maddock et al., 1987 ; Magloughlin &Spray, 1992 ; Sibson & Toy, 2006 ).

The relation of pseudotachylyte to other fault related rocks can be found in the classificationscheme of Woodcock & Mort (2008 ) which refers to pseudotachylyte as a cohesive, glassy ordevitrified non-foliated fault rock (Figure 2).
https://structuredatabase.files.wordpress.com/2014/08/pst-structure1.pnghttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23phttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23phttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23whttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23whttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23whttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23whttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23whttps://structuredatabase.wordpress.com/glossary/%23comminutionhttps://structuredatabase.wordpress.com/glossary/%23comminutionhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23whttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23whttps://structuredatabase.files.wordpress.com/2014/08/pst-structure1.pnghttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23whttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/glossary/%23comminutionhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23whttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23whttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23phttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.files.wordpress.com/2014/08/pst-structure1.png


3/75


4/75

Figure 3. Conceptual model of a major fault zone modified from Sibson (1977 ) and Rutter et al.(2001 ). Highlighted pseudotachylyte demonstrates maximum depth at which it can occur, ie.

within the frictional-plastic transitional regime.

The occurrence of pseudotachylyte at much greater depths (>60 km) has also been observedforming in continent collision and subduction zones ( Anderson & Austrheim, 2006 , Austrheim &Boundy, 1994 )(Figure 4). At higher crustal levels pseudotachylytes are preserved as brittle shearfractures overprinting the existing rock fabric while at the base of the seismogenic zone (brittle-ductile transition) rupture can occur during the development of shear fabrics so that the

pseudotachylytes can be reworked into the mylonitic foliation ( White, 1996 ; Sibson, 1980 ). Linet al. (2005 ) have also noted that the presence of pseudotachylyte does not necessarily mean thatrupture originated at this depth in the crust but may, instead be from the downward propagationof ruptures which nucleated at shallower depths.
https://structuredatabase.files.wordpress.com/2014/08/g3827.pnghttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23rhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23rhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23rhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23ahttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23ahttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23ahttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23ahttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23ahttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23whttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23whttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.files.wordpress.com/2014/08/g3827.pnghttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23whttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23ahttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23ahttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23ahttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23rhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.files.wordpress.com/2014/08/g3827.png


5/75

Figure 4. Estimated depths of formation for fault generated pseudotachylytes from Sibson & Toy(2006 ). Note the change in depth scales between groupings. (OHT = Outer Hebrides Thrust).

Host rock type and fabric

Pseudotachylyte have been described in a variety of rock lithologies although the majority(>95%) of host rocks are crystalline rocks of metamorphic or plutonic assemblages (Sibson &

Toy, 2006 ). The most common assemblages are metamorphic assemlages from the amphibolitefacies, greenschist and granulite facies followed by plutonic igneous rocks with very fewoccurrences in sedimentary rocks. Crystalline host rocks generally have a low porosity and highcompetence which allows for a high tolerance of elastic strain energy to be stored before brittlefailure occurs. Many host rocks contain some form of planar heterogeneities such as a myloniticfoliation, gneissic banding or schistosity which may form anisotropic shear strengths (Sibson &Toy, 2006 ). Paterson & Wong (2005 ) have shown that the brittle shear strength of rockscontaining a strong planar anisotropy is about half of cross-foliation strength. They alsodemonstrate that preferential failure along a foliation can occur over a wide range of foliationorientations with respect to the stress field (20 0 from the optimally orientated plane of shearfailure).

Production of pseudotachylyte

Melt through friction

During coseismic slip the elastic strain energy is released from the wall rocks and is released asfrictional work. Beke (1964 ) demonstrated that 97% of energy consumed during grinding wasconverted to heat. It is assumed that a similar situation holds for coseismic slip where
https://structuredatabase.files.wordpress.com/2014/08/capture2.pnghttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23phttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23phttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23bhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23bhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23bhttps://structuredatabase.files.wordpress.com/2014/08/capture2.pnghttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23bhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23phttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.files.wordpress.com/2014/08/capture2.png


6/75


7/75

Figure 5. Time/temperature curve for a for an idealised coseismic thermal spike associated withfrictional melting and pseudotachylyte production (after Swanson, 1992 ). Various featuresassociated with different stages of development are discussed in text.

Rupture propagation

A frictional melt rupture propagation model was developed by Swanson ( 1992 ) who described itin terms of a slip window and friction-melt window (Figure 6). The leading edge of the slipwindow represents the rupture front during propagation with cessation of rupture occurring at thetrailing edge. The friction melt window is defined by leading edge of melting and a trailing edgeof solidification. The melting front must follow some distance behind the rupture front so toallow the right level of surface refinement, slip velocity, slip duration and generation of frictionalheat (Swanson, 1992 ). Melt survives after the cessation of slip as the initial rupture periodcreated a great enough temperature rise for melt to continue to occur, though in a cooling phase.The cooling times of this friction-melt window is dependant on depth. At shallow depths thefriction melt window is not present and rupture occurs without presence of a melt phase as aresult of lower confining pressures.
https://structuredatabase.files.wordpress.com/2014/08/capture6.pnghttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.files.wordpress.com/2014/08/capture6.pnghttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.files.wordpress.com/2014/08/capture6.png


8/75

Figure 6. Rupture model of pseudotachylyte production and development (after Swanson, 1992 ).See text for details.

The mechanisms of rupture propagation vary between pseudotachylytes forming solely in the brittle regime and those that form where some plastic deformation occurs, in the brittle-ductiletransition regime. This has lead to the development of an abrasive wear-dominant model andadhesive wear-dominant model for higher crustal levels and lower crustal levels respectively (seeSwanson, 1992 ).

Slip/strain rates

It is widely accepted that fault-generated pseudotachylyte is an indicator of high-velocity slipduring earthquake rupture, ie. at slip rates of 1 10 m/s (McKenzie & Brune, 1972 ; Sibson,1975 ; Passchier, 1982 ; Magloughlin, 1992 ). Once the elastic strain energy stored in thewall rocks that drives the propagation of the rupture is released, there is a decrease in frictionalresistance (decreasing slip rate) with progressive displacement as the melt layer developssmothering low amplitude asperities to decrease the contact area between opposing walls(Sibson, 1975 ). It is shown in Figure 7 that with an pseudotachylyte formation is strain ratedependant and with an increase in strain rate more melt will be produced (Spray, 1995 ).
https://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23phttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.files.wordpress.com/2014/08/g3870.pnghttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23phttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.files.wordpress.com/2014/08/g3870.png


9/75

Figure 7. Relation amongst comminution, melting and strain rate. Strain rate = v/ (per s), wherev = velocity and = slip-zone thickness.(a fter Spray, 1995 )

Temperature estimates

The temperature of pseudotachylyte formation is generally estimated from mineralgeothermometers applied to microlites or chemical composition of the matrix (cf. Lin, 2008 ).Estimated temperatures of frictional melt range from 650 1730 oC with an average of 1200 oC(Sibson & Toy, 2006 ). The temperature rise associated with melt production increases withincreasing slip velocity (Sibson, 1975 ).

Fault slip and vein thickness relationship

With an offset marker estimates of apparent or true lateral displacement along a pseudotachylyte-generating fault can be made (Sibson, 1975 ; Di Toro & Pennanchioni, 2005 ). Sibson (1975 ) quantified a relationship between the displacement (d) and the thickness of the melt layer (a) inthe Outer Hebrides Thrust, Scotland along single jerk displacments. From this he was ablesolve an empirical relationship where,

d = 436a 2
https://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23S


10/75


11/75

It is still debated whether frictional melting occurs under hydrous or anhydrousconditions ( Sibson, 1973 , 1975 ; Magloughlin & Spray, 1992 ; OHara and Sharp, 2001 ). The

presence of fluid inclusions, vesicles and amydules (eg. Maddock et al., 1987 ) suggest that waterfluids must, in some cases, be present at the time of pseudotachylyte generation. However it isnot clear whether water is present as pore fluid or was bound up in hydrous minerals and

released upon their breakdown during frictional heating. The presence of intergranular porefluids during slip should impede temperature rise needed to produce a frictional melt as fluidscan undergo thermal pressurization inducing a decrease in the effective normal stress anddynamic friction upon the fault plane thus restricting a temperature increase (Sibson, 1975 ; Sibson & Toy, 2006 ; Lin, 2008 ; Di Toro, 2009 ). However, it has also been argued that hydrousconditions may favour melting by lowering the melting point of the host rock and associatedminerals forming a low-viscosity melt ( Allen, 1979 ; Maddock, 1992 ). Okamoto et al. (2006 ) found that pseudotachylyte had formed in a water-rich environment where thermal pressurisationof the fluid induced fracturing of the wall rocks, allowing for the expulsion of fluids. This

process increased the effective stress of the fault plane which lead to frictional melting, so,melting occurred after the expulsion of fluid in anhydrous conditions.

Characteristics of fault generated pseudotachylyte

Mesoscale features

Pseudotachylytes commonly occur as sharply bounded fault veins that often pinch and swell asa result of surface irregularities in fault planes (Sibson, 1975 ) although regularly range from afew millimetres to several centimetres in width, though much larger veins have been reported (DiToro, 2009 ; Camacho et al., 1995 ). Fault veins often pinch and swell Injection veins areinterpreted as dilational fractures induced by fault pressurisation due to thermal expansion duringfrictional melt generation and were subsequently infilled with melt (Sibson,

1975 ). Pseudotachylyte may occur in a variety of complex geometric arrangements which appearto develop as a function of the degree of host rock anisotropy (Swanson, 2006 )(Figure 9). Asseismic slip commonly occurs along pre-existing planes of weakness, the geometry of thesestructures plays a large role in determining the form of pseudotachylyte networks.
https://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23ohttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23ohttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23ahttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23ahttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23ohttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23ohttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23ohttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23chttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23chttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23ohttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23ahttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23ohttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23S


12/75

Figure 9. Varying forms of pseudotachylyte structure. a) single generation zone with fault veinoccurring along generation fault plane, injection veins intrude wall rock away from fault plane.

b) paired generation zone with two distinct shear planes along which melt is generated. c) Enechelon linked duplex with extensional geometry. c) Sidewall ripout showing leading andtrailing structural assemblages. After Lin (2008 ).

Microscale features and geochemistry

Pseudotachylytes consist of a microlitic to cryptocrystalline or glassy matrix which contains

clasts of the host rock which survived comminution and melting. Other features such asamygdules and flow banding show evidence of the pseudotachylyte undergoing a melt phase. Ithas been found that mafic minerals are preferentially melted, so the pseudotachylyte willcommonly have a greater mafic component than the host rock (Lin, 2008 ).

Matrix

The matrix can consists of mircolites and/or a cryptocrystalline or glassy matrix. Glass matricesare transparent or translucent under an optical microscope and consists of pure silica or chemicalcomponents of feldspar. This glass matrix is most commonly observed around irregular androunded feldspar at quartz fragments that survived fusion, indicating growth from the breakdown

of these clasts (Lin, 2008 ). Glass is rarely reported in pseudotachylytes as it is high instabilityunder geological conditions and can de-vitrify to form secondary microstructures (Maddock,1983 ; Lin, 1994 ). Crypocrystalline matrices are composed of fine-grained material that can besmall to identify with an optical microscope and may also contain small microlites. Microliticmatrices consists largely of microlites of variable size and shapes and chemistry as a function ofthe cooling rate of the pseudotachylyte.
https://structuredatabase.files.wordpress.com/2014/08/capture4.pnghttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.files.wordpress.com/2014/08/capture4.pnghttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.files.wordpress.com/2014/08/capture4.png


13/75

Microlites closely resemble microstructures found in volcanic rocks which indicate rapid coolingof a melt (Maddock, 1983 ). They are primary crystals of varying shapes and chemistry that haveformed from melt generated from frictional shear on a fault surface. It is common to observe agradation in size and increasing complexity of microlites morphology from the margins tocentre of the fault vein, largely to do with the rate of cooling across a fault vein (Swanson, 1992 ).

Microlite size can reach up to >15 m in the centre of veins and their morphology can be splitinto four groups, simple, skeletal, dentritic and spherulitic (Figure 10), which can be used to helpdetermine their chemistry (Lin, 2008 ). Microlite mainly consist of feldspar, biotite, hornblende,

pyroxene, garnet and other mafic minerals such as ilmenite and spinel. The mineral mineralogycan help determine the temperature and depth conditions the pseudotachylyte may have formedat as a result of their crystallisation temperatures (Lin, 2008 ; Di Toro, 2009 ).

Figure 10. a) a SEM-BSE image of skeletal hornblende microlites. b) Photomicrograph of biotiteand hornblende (black) dendritic microlites overgrowing plagiocalse (Pl) microlites. (from Lin,2008 ).

Clasts
https://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.files.wordpress.com/2014/08/g3952.pnghttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.files.wordpress.com/2014/08/g3952.pnghttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23m


14/75

Clasts embedded into the pseudotachylyte matrix are both single mineral or lithic clasts from thehost rock. Host rock minerals are not equally represented as clasts within the melt as preferentialconsumption of low-melting point minerals, such as phyllosilicates and amphibloes, will weightthe total clast composition to minerals with higher melting temperatures, such as plagioclase andquartz (Spray, 1992 ; Di Toro, 2009 , Di Toro & Pennacchioni, 2004 ). Clasts can show melting

embayments and commonly act as nuclei for microlites of the same species which can developradial spherulite microstructures (Di Toro & Pennacchioni, 2004 ).

Amygdules

Amygdules have been observed in pseudotachylytes (eg. Maddock et al., 1987 ; Magloughlin,1992 ) and are generally considered to form via exsolution of a vapour phase forming vesicleswhich are then infilled with fine grained material that is usually associated with hydrousalteration ( Lin, 2008 ). They vary in shape from almost perfectly circular to elliptical and can bealigned parallel or subparallel to flow bands (Figure 11). From differing mineral assemblagesand solubility functions of H2O and CO2 it has been inferred that amydules only form at shallow

crustal depths ( Maddock et al., 1987 ).

Figure 11. a) BSE micrograph of a highly vesicular pseudotachylyte with rounded vesicles (fromMaddock et al., 1987 ). b) SEM backscatter image of calcite filled amygdules showing flowalignment (from Alder et al., 2014 ).

Flow structures

Flow structures develop in some pseudotachylytes and are generally characterised by thinalternating colour bands and streaks. These coloured features are believed to be developed byviscous flow of a melt material with a non-uniform chemical composition. Curved or U-shapedflow bands can indicate the flow direction and relative flow velocities at which melt transportoccurred with the apex of the shape pointing in the flow direction. The distance between flow
https://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23Shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23ahttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23ahttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23ahttps://structuredatabase.files.wordpress.com/2014/08/g4056.pnghttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23ahttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.files.wordpress.com/2014/08/g4056.pnghttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23mhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23S


15/75

streaks increases towards the centre of the vein as a gradient of increasing flow velocity towardsthe centre is formed due to the decreasing influence of wall rock friction (Lin, 1991 ).

Figure 12. Flow banding in pseudotachylyte. Arrows indicate flow direction as shown by U-shaped banding.(after Lin, 2008).

Strain hardening

The commonly undeformed nature of pseudotachylyte suggests that post-failure strengtheningoccurs on each slip surface by fault welding accompanying solidification ( Di Toro &Pennanchoini, 2005 ). This appears to be an efficient strain hardening mechanism with themajority of pseudotachylytes being single-slip events, though examples do exist of multiplegenerations of slip ( Otsuki et al., 2005 ). Slip events that rupture repeatedly can produce a collageof layer-parallel pseudotachylyte veins ( Swanson, 1992 ).

References
https://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23lhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23ohttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23ohttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23shttps://structuredatabase.files.wordpress.com/2014/08/text3927-8.pnghttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23shttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23ohttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23dhttps://structuredatabase.files.wordpress.com/2014/08/text3927-8.pnghttps://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/%23l


16/75

Alder, S., S, S. & J, S. Structure and ancient seismicity in the Moonlight Fault Zone, MatukitukiValley, Wanaka. 47th New Zealand Branch Annual Conference on exploration, mining and NewZealand;s mineral resources, 2014 Hamilton. 1-6.

Allen, A. R. (1979) Mechanism of frictional fusion in fault zones. Journal of Structural Geology,

1, 231-243.

Andersen, T. B. & Austrheim, H. (2006) Fossil earthquakes recorded by pseudotachylytes inmantle peridotite from the Alpine subduction complex of Corsica. Earth and Planetary Science

Letters, 242 , 58-72.

Austrheim, H. & Boundy, T. M. (1994) Pseudotachylytes Generated During Seismic Faultingand Eclogitization of the Deep Crust. Science, 265 , 82-83.

Beke, B. (1964) Priciples of Comminution, Budapest, Academy Press.

Camacho, A., Vernon, R. H. & Fitz Gerald, J. D. (1995) Large volumes of anhydrous pseudotachylyte in the Woodroffe Thrust, eastern Musgrave Ranges, Australia. Journal ofStructural Geology, 17 , 371-383.

Di Toro, G. & Pennacchioni, G. (2005) Fault plane processes and mesoscopic structure of astrong-type seismogenic fault in tonalites (Adamello batholith, Southern Alps). Tectonophysics, 402 , 55-80.

Di Toro, G., Pennacchioni, G. & Nielsen, S. (2009) Chapter 5 Pseudotachylytes and EarthquakeSource Mechanics. In: EIICHI, F. (ed.) International Geophysics. Academic Press.

Lin, A. (1994) Microlite Morphology and Chemistry in Pseudotachylite, from the Fuyun FaultZone, China. The Journal of Geology, 102 , 317-329.

Lin, A. (2008) Fossil Earthquakes: The formation and preservation of pseudotachylytes, Berlin,Springer Berlin Heidelberg.

Lin, A., Maruyama, T., Aaron, S., Michibayashi, K., Camacho, A. & Kano, K.-I. (2005)Propagation of seismic slip from brittle to ductile crust: Evidence from pseudotachylyte of theWoodroffe thrust, central Australia. Tectonophysics, 402 , 21-35.

Maddock, R. H. (1983) Melt origin of fault-generated pseudotachylytes demonstrated by

textures. Geology, 11 , 105-108.Maddock, R. H. (1992) Effects of lithology, cataclasis and melting on the composition of fault-generated pseudotachylytes in Lewisian gneiss, Scotland. Tectonophysics, 204 , 261-278.

Maddock, R. H., Grocott, J. & Van Nes, M. (1987) Vesicles, amygdales and similar structures infault-generated pseudotachylytes. Lithos, 20 , 419-432.


17/75


18/75

Sibson, R. H. & Toy, V. G. (2006) The habitat of fault-generated pseudotachylyte: Presence vs.absence of friction-melt. Earthquakes: Radiated Energy and the Physics of Faulting. Washington, DC: AGU.

Spray, J. G. (1995) Pseudotachylyte controversy: Fact or friction? Geology, 23 , 1119-1122.

Swanson, M. T. (1992) Fault structure, wear mechanisms and rupture processes in pseudotachylyte generation. Tectonophysics, 204 , 223-242.

Wenk, H. R. (1978) Are pseudotachylites products of fracture or fusion? Geology, 6, 507-511.

White, J. C. (1996) Transient discontinuities revisited: pseudotachylyte, plastic instability andthe influence of low pore fluid pressure on deformation processes in the mid-crust. Journal ofStructural Geology, 18 , 1471-1486.

Woodcock, N. H. & Mort, K. (2008) Classification of fault breccias and related fault rocks.

Geological Magazine, 145 , 435-440.


19/75

Fault BrecciaFault breccias, or tectonic breccias, are a type of cataclastic rock formed by mechanicaldeformation during crustal movements ( Glossary of Geology, 2005 ). They are further classified

as chaotic, mosaic, or crackle breccias based on percentage of large (>2mm) clasts (Woodcockand Mort, 2008 ).

Fault Rock Classification

Breccias in Fault Zones

Generation of Fault Breccias

Fault Breccias and Fluid

References

Fault Rock Classification

Spry (1969) and Higgins (1971)

As discussed in Fault Gouge , cataclastic fault rocks are a spectrum of rock types defined bytexture including fault breccia, fault gouge , pseudotachylyte , and mylonites. Early work oncataclastic rocks used a variety of inconsistent terms to define fault rocks, Spry (1969) review ofmetamorphic deformation mechanisms and textures and Higgins (1971) review of existingterminology provided the basic fault rock nomenclature used today. Higgins defined the termsfault breccia and fault gouge as separate from the mylonite sequence. He classifiedcataclastic rocks by the presence or absence of primary coherence (Figure 1 ) and used theumbrella term cataclast ic rocks for all rocks formed by cataclasis.

Figure 1. Classification of cataclastic rocks from Higgins (1971)
https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23glossaryhttps://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23glossaryhttps://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23head1https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23head1https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23head2https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23head3https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23head3https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23head4https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23head4https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23head5https://structuredatabase.wordpress.com/fault-rocks/fault-gouge/https://structuredatabase.wordpress.com/fault-rocks/fault-gouge/https://structuredatabase.wordpress.com/fault-rocks/fault-gouge/https://structuredatabase.wordpress.com/fault-rocks/fault-gouge/https://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/https://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1969https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1969https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23h1971https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23h1971https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23h1971https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure1https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure1https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure1https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23h1971https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23h1971https://structuredatabase.files.wordpress.com/2014/08/higgins1971_highlighted.pnghttps://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23h1971https://structuredatabase.files.wordpress.com/2014/08/higgins1971_highlighted.pnghttps://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure1https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23h1971https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1969https://structuredatabase.wordpress.com/fault-rocks/faultgeneratedpseudotachylyte/https://structuredatabase.wordpress.com/fault-rocks/fault-gouge/https://structuredatabase.wordpress.com/fault-rocks/fault-gouge/https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23head5https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23head4https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23head3https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23head2https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23head1https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23glossary


20/75

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 theformation of Higgens cataclastic rocks and instead uses the collective term fault rocks for rocks

found in zones of shear dislocation. Sibsons classification scheme is divided by fabric and primary cohesion; here fault breccia is defined as incohesive with a random fabric and visiblerock fragments >30% of the rock mass. Sibson also recognised that there are cohesive brecciasand included a series of crush breccias defined as a range of fragment size with 0 10% matrix.

Figure 2. Classificat ion of fault rocks from Sibson (1977)

Fault breccias underwent a series of further classification since Sibson (1977) . Some keyschemes include Sibson (1986) which subdivides fault breccia into attrition, distributed crush,
https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure2https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure2https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1969https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1969https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23h1971https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23h1971https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.files.wordpress.com/2014/08/sibson1977_highlighted.pnghttps://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.files.wordpress.com/2014/08/sibson1977_highlighted.pnghttps://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23h1971https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1969https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure2https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977


21/75

and implosion breccias based on mechanical processes during brecciation. Using hydrothermal breccias Jebrak (1997) classified breccias by brecciation mechanisms based on particle sizedistribution. 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 Woodcockand Mort (2008) . Here fault breccia definition approximates usage for sedimentary rocks, relyingon clast size as the primary criterion and clast proportion as a secondary criterion rather thandifferentiating 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 isoften difficult to determine proportions in the field. Clasts are defined as > 2mm in diameter. TheWoodcock and Mort (2008) classification can therefore be used for fault breccias which haveany combination of cohesion, cementation, foliation, and/or variable clast size.
https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23j1997https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23k2003https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23k2003https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure3https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure3https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23bfh2007https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23bfh2007https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23bfh2007https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23bfh2007https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23h1971https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23h1971https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23h1971https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23k2003https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23k2003https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.files.wordpress.com/2014/08/wm2008_highlighted.pnghttps://structuredatabase.files.wordpress.com/2014/08/wm2008_highlighted.pnghttps://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23k2003https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23h1971https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23bfh2007https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23bfh2007https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure3https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23k2003https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23j1997


22/75

Figure 3. Classificat ion of fault rocks from Woodcock and Mort (2008)

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 4 a). Crackle breccias have clasts with little to no rotationrelative to each other and thin cement or matrix seams between clasts. Mosaic breccias showincreased separation and rotation of clasts. Chaotic breccia clasts have been strongly rotated andshow little to no geometric fit with adjacent clast. Mort and Woodcock (2008) show that clastarea as a percentage of total area can be used as a proxy for clast tesselation (Figure 4 b) althoughit is important to remember that this textural spectrum is variable. Therefore, using Woodcockand Mort (2008) classification scheme for fault rocks, fault breccia is defined as having atleast 30% of its volume being clasts larger than 2 mm in diameter and can be furthersubdivided into the crackle-mosaic-chaotic breccia spectrum approximately correspondingto large (>2 mm) clast percentages of 75% and 60% .

Figure 4. From Woodcock and Mort (2008) . a) Thin section examples of crackle breccia, mosaic breccia, and chaotic breccia from the Dent Fault Zone, NW England. b) Subdivision of fault breccias by percentage of large clasts

Breccias in Fault Zones
https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23k2003https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23k2003https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23l1999https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23l1999https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure4https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure4https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23mw2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23mw2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure4https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure4https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.files.wordpress.com/2014/08/wm2008c_combined.pnghttps://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.files.wordpress.com/2014/08/wm2008c_combined.pnghttps://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure4https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23mw2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure4https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23l1999https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23k2003https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008


23/75

Sibson (1977) describes a bimodal deformation model for fault zones (Figure 5 ) wheredeformation can be friction dominated, elastico-frictional, or quasi-plastic where rocks canreadily deform by crystal plasticity. Fault breccias exist purely in the elastico-frictional domainwith 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 faultzone 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.

Figure 5. Conceptual model of a fault zone showing fault rock location at depth from Sibson(1977)
https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure5https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure5https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure5https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure6https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure6https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure6https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.files.wordpress.com/2014/08/sibson1977model_highlighted.pnghttps://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.files.wordpress.com/2014/08/sibson1977model_highlighted.pnghttps://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure6https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure5https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977


24/75

Figure 6. Model of principle slip zone architecture and associated breccia formation from Sibson(1986)

Generation of Fault Breccias

Fault breccias occur on faults with increasing dilation through three mechanical processes(Figure 7 ) according to Sibson (1986) .

1) Attritional wear . Attritional wear is a combination of frictional wear and grain cataclasisassociated 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 primarycontrols 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 andmicrofaulting over a broad area in the vicinity of antidilational jogs to produce a structurelessmicrobreccia. 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 wallrock breccia in psuedotachylyte matrix provides evidence that cavity opening can be co-seismicand can occur at several kilometers depth. Texturally breccias formed by implosion are the mostrecognisable showing good fit with adjacent clasts and little sign of frictional attrition, whatwould be considered crackle to mosaic breccias by Woodcock and Mort (2008) . Texture is
https://structuredatabase.files.wordpress.com/2014/08/sibson1986_updated.pnghttps://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure7https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure7https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure7https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23k1989https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23k1989https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23sh1990https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23sh1990https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23k1989https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23k1989https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23k1989https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23k1989https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23h1971https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23h1971https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23h1971https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23sch1987https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23sch1987https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23sh1990https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23sh1990https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23sh1990https://structuredatabase.wordpress.com/fault-rocks/fault-gouge/https://structuredatabase.wordpress.com/fault-rocks/fault-gouge/https://structuredatabase.wordpress.com/fault-rocks/fault-gouge/https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.files.wordpress.com/2014/08/sibson1986_updated.pnghttps://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23wm2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-gouge/https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23sh1990https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23sch1987https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23h1971https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23k1989https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23k1989https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23sh1990https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23k1989https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1977https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure7https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.files.wordpress.com/2014/08/sibson1986_updated.png


25/75

particularly important in implosion breccias as it acts as a record of incremental or multi-episodeformation. This, combined with the common presence of hydrothermal minerals in veins ormatrix, indicates that the common mechanism is hydraulic implosion due to rapid generation offluid 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) ]

Figure 7. Fault brecciation mechanisms and associated distinguishing characteristics andstructural associations from 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 atshallow depths with low confining pressure ( Loucks, 1999 ). Fault void collapse creates volumesof chaotic, hanging wall clast supported, breccias larger than void volume ( Loucks, 1999 ) andmay display bedding defined by clast shape and size at the angle of repose for scree (Woodcocket al., 2006 ).
https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23w2006https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23w2006https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure8https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure8https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23l1999https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23l1999https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23l1999https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23l1999https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23w2006https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23w2006https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23w2006https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23w2006https://structuredatabase.files.wordpress.com/2014/08/sibson1986breccias.pnghttps://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23w2006https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23w2006https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23l1999https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23l1999https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure8https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23w2006https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986https://structuredatabase.files.wordpress.com/2014/08/sibson1986breccias.pnghttps://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1986


26/75


27/75
https://structuredatabase.files.wordpress.com/2014/08/smith2008.pnghttps://structuredatabase.files.wordpress.com/2014/08/smith2008.png


28/75

Figure 9. Conceptual model of breccia generation on Zuccale Fault by fluidizat ion from Smith etal., (2008) . (a) Precursors to fluidizat ion: the Zuccale fault possesses a strongly foliated faultcore which acts as a low-permeability seal to CO2-bearing fluids migrating within the footwall.The fault core is underlain by a high-angle footwall fault. Fluids infiltrate pre-existing frictional

breccias, leading to dissolution and a loss of cohesion. (b) Fluidization: periodic slip along high-

angle footwall faults leads to focused and rapid fluid flow, causing fluidizat ion of clasts withinthe frictional breccias. The fluid pulse spreads laterally as it encounters the fault core. Ponding offluids, and deformation of the boundary, may occur during continued input of fluids. (c)Hydrofracturing: critical fluid overpressure leads to embrittlement within the core of the Zuccalefault, allowing fluids to drain from footwall to hanging wall. The fractures undergo healing

processes returning to a low-permeability nature, allowing the fault-valve cycle to repeat.

Fault Breccias and Fluid

Secondary Mineralisation

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 replacementof clasts or matrix (Woodcock and Mort, 200 8), reducing permeability or dissolution andincreased permeability ( Jebrak, 1997 ). Episodic fluid migration and associated mineralisation infault zones is intrinsically linked to rapid changes in fluid pressure at dilational jogs where fluidinflux 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) describesthe fault-valve model ( Figure 10 ) as impermeable faults which act as fluid barriers except for ashort period post-failure when the faults become fluid conduits and, on intersection with highfluid pressure differentials, become fluid-pressure-activated valves. Changes in fluid pressureare therefore intimately linked to the earthquake cycle as fluid pressure cycling affects fault

behaviour and strength (Sibson, 1990 ; 1992 ; Smith et al., 2008 ).
https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1987https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1987https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23w2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23w2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23j1997https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23m2004https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23m2004https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23m2004https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1987https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1987https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1987https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1987https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23t2005https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23t2005https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23t2005https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23w2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23w2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23w2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1990https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1990https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure10https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure10https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1990https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1990https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1990https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1992https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1992https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1992https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1992https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1990https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure10https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1990https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23w2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23t2005https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1987https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1987https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23m2004https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23j1997https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23w2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1987https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s2008https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s2008


29/75

Figure 10. Fluid migration by fault valve model ( Sibson, 1990 ) with (a) large fluid pressuredifferentia ls in the inter-phase, (b) post-seismic upwards discharge of fluids

Caine et al. (1996) showed that the percentage of fault rocks, fault breccias included, in totalfault width relative to the percentage of damage structures (e.g. faults, fractures, veins) can beused to qualitatively model fluid flow in and around fault zones. Caines scheme indicates thatwell developed fault rocks without well-developed damage structures act as a barrier to fluidflow 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 smallfaults with breccia cores and demonstrates that the fault breccias act as barriers to verticalgroundwater flow. Furthermore, as breccia thickness varies along fault it acts to channel fluidalong the fault and into the footwall at thin breccia zones allowing deeper crustal flow (Figure12) or it can locally trap upward migrating fluids increasing fluid pressure gradients across thefault. Breccia thickness, and any fluctuations in breccia thickness, will therefore significantlyimpact the groundwater system around the fault.
https://structuredatabase.files.wordpress.com/2014/08/sibson1990.pnghttps://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1990https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1990https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23c1996https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23c1996https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure11https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure11https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure11https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23g2001https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23g2001https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure12https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure12https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure12https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure12https://structuredatabase.files.wordpress.com/2014/08/sibson1990.pnghttps://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure12https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure12https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23g2001https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23figure11https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23c1996https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23s1990https://structuredatabase.files.wordpress.com/2014/08/sibson1990.png


30/75

Figure 11. Fault zone permeability structures and associated attributes from Caine et al. (1996)

Figure 12. Schematic model of the interaction between fault breccia and groundwater flow in thevicinity of the fault from Gudmundsson (2001)

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 faultrupture followed by a decrease in porosity on the main rupture surface as fluid pathways are
https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23c1996https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23c1996https://structuredatabase.wordpress.com/fault-rocks/fault-breccia/%23c1996https://structuredatabase.wordpress.com/fault-rocks/fault-br

Date post:	01-Jun-2018
Category:	Documents
Upload:	amey-dashputre
View:	216 times
Download:	0 times

Fault Generated Pseudotachylyte

Documents