Page 123 - Microtectonics

112 5 · Shear Zones
5.1 5.1 formation of a volume of brittle fault rock in a fault zone
Introduction along the active fault. Many processes influence the ability
of a fault to propagate and slip such as the regional stress
In general, deformation in rocks is not homogeneously dis- field and geometry of rock units, fluid pressure in the fault
tributed. One of the most common patterns of heterogene- and wall rock and the interaction of the brittle fault rock
ous deformation is the concentration of deformation in pla- with fluids. Faults can show velocity weakening or velocity
nar zones that accommodate movement of relatively rigid strengthening behaviour (Tse and Rice 1986; Chester et al.
wall-rock blocks. Deformation in such high-strain zones 1993). In the first case, resistance to sliding decreases with
usually contains a rotation component, reflecting lateral dis- increasing velocity and the fault can produce earthquakes.
placement of wall rock segments with respect to each other; In the second case, faults decelerate and sliding is asysmic
this type of high-strain zone is known as a shear zone. De- and stable. A higher temperature seems to promote veloc-
formation in a shear zone causes development of charac- ity strengthening behaviour (Shimamoto 1989; Chester et al.
teristic fabrics and mineral assemblages that reflect 1993). Presently, there are no ways to distinguish brittle fault
P-T conditions, flow type, movement sense and deforma- rocks that form in velocity weakening and strengthening
tion history in the shear zone. As such, shear zones are an segments of faults, except for pseudotachylyte which forms
important source of geological information. exclusively on rapidly moving faults (Sect. 5.2.5). Fluids that
Shear zones can be subdivided into brittle zones or faults, infiltrate a fault strongly influence its mechanical behav-
and ductile zones (Chap. 3). Ductile shear zones are usually iour, but in a complex way. An increased fluid pressure de-
active at higher metamorphic conditions than brittle shear creases the strength of the fault by decreasing the effective
zones (Figs. 3.44, 5.2). Major shear zones which transect the normal stress over the fault (Sect. 3.2). Fluids may also cause
crust or upper mantle have both brittle and ductile segments. weakening by reaction of stronger phases to weaker miner-
The depth of the transition between dominantly brittle and als in fault rocks, or by stress corrosion in the fault process
ductile behaviour depends on many factors such as bulk zone (Chap. 3). Fluids may cause fault rock strengthening
strain rate, geothermal gradient, grain size, lithotype, fluid by precipitation of vein material such as quartz, calcite or
pressure, orientation of the stress field and pre-existing fab- even K-feldspar, cementing fault rock fragments together
rics (Sect. 3.14). Ductile shear zones may develop in mar- (Fredrich and Evans 1992; Wintsch et al. 1995; Wintsch 1998).
bles at metamorphic conditions where quartzites would Precipitation of vein material may even cause a decrease in
deform by brittle fracturing, and different minerals in a permeability of the fault zone and thereby an increase in
small volume of rock can show contemporaneous brittle fluid pressure and fault rupture, after which permeability is
and ductile deformation (Fig. 3.42). increased and fluid pressure falls until renewed precipita-
Major shear zones can be active for considerable periods tion causes the next cycle of fault activity (Sibson 1990; Cox
of time, and material in the shear zone may be transported et al. 1991; Cox 1995).
upwards or downwards in the crust. Consequently, rocks in
major shear zones commonly show evidence of several over- 5.2.2
printing stages of activity at different metamorphic condi- Incohesive Fault Rocks
tions. Minor shear zones may also show several overprinting
stages since shear zones, once formed, are easily reactivated. Brittle fault rocks can be subdivided into incohesive and
A special terminology is used for rocks that have been cohesive types. Incohesive brittle fault rocks are usually
deformed in shear zones, partly independent of their lithol- found in faults, which have been active at shallow crustal
ogy (Sibson 1977b). They are usually referred to as fault rocks levels. They occur in fault zones of variable thickness and
or deformation zone rocks (Sibson 1977b; Scholz 2002; Schmid can be subdivided into incohesive breccia, incohesive cata-
and Handy 1991; Blenkinsop 2000), even if deformed in duc- clasite and fault gouge. Incohesive breccia consists for
tile shear zones. The most common types are brittle fault rocks, more than 30 vol-% of angular fragments of the wall rock
mylonites and striped gneiss. An excellent detailed treatment or of fractured veins, separated by a fine-grained matrix.
of fault rocks and structures in shear zones is given in the In cataclasite, less than 30 vol-% fragments are present in
fault related rocks atlas edited by Snoke et al. (1998). the fine-grained matrix. In fault gouge, few large frag-
ments occur isolated in the matrix. This matrix may be
5.2 5.2 foliated, and fragments commonly have a lensoid shape
Brittle Fault Rocks (Chester et al. 1985; Chester and Logan 1987; Evans 1988;
Kano and Sato 1988; Lin 1996, 1997; Takagi 1998). The wall
5.2.1 rock and included fragments in incohesive cataclasite and
Introduction fault gouge commonly show polished surfaces (slicken-
sides) with striations or fibres (slickenfibres – Sect. 6.2.5)
Brittle fault rocks form by fault propagation through intact that can be used to determine movement direction and
rock, commonly along some older plane of weakness, and shear sense (Sect. 5.7.2) along the fault zone.