Page 126 - Microtectonics

5.2 · Brittle Fault Rocks 115

Fig. 5.3. Cohesive fault breccia in quartzite. Angular fragments of variable size are present. Orobic Alps, Italy. Width of view 8 mm. PPL

commonly show evidence for abundant pressure solution 5.2.5
and precipitation effects. Rock fragments are transected Pseudotachylyte
by healed cracks aligned with fluid inclusions. Veins of
quartz, calcite, epidote or chlorite, and in ultramafic rocks, Pseudotachylyte is a cohesive glassy or very fine-grained
serpentine, are common. These veins form during and fault-rock with a very distinct fabric (Magloughlin and
after brittle deformation, since they have commonly been Spray 1992). Its curious name derives from its resem-
fractured. Although most cataclasites have random fab- blance to tachylyte, a mafic volcanic glass, while the ma-
rics, foliated cataclasite does occur, especially where the terial is obviously not of volcanic origin (Shand 1916).
host rock is rich in micas (Chester et al. 1993; Evans and Pseudotachylyte has a number of characteristic geomet-
Chester 1995; Lin 1996, 1997, 1999; Lin et al. 1998; Chester ric features that usually allow its distinction from other
and Chester 1998; Evans 1998; Mitra 1998). Such rocks brittle fault rock types. It is composed of a dark matrix
may contain a compositional layering (Kanaori et al. 1991) material with minor inclusions of mineral or wall rock
and a preferred orientation of mica fragments, elongate fragments (Figs. 5.4–5.6). It usually occurs in a charac-
grains or new-grown micas (Evans 1988) wrapping around teristic geometrical setting of a planar main fault vein
large grains of resistant minerals, e.g. quartz and feldspar (Sibson 1975; Spray 1992) or generation surface, up to a
in granitic rocks. The foliation can also be spaced and few mm thick and irregular injection veins which branch
result from parallel alignment of minor shear fractures from the main fault vein into the wall rock (Figs. 5.2, 5.4,
(Chester et al. 1985) or of dissolution planes filled with 5.5). Main fault veins are usually planar, up to a few cen-
opaque material (Mitra 1998). Shear band cleavage struc- timetres wide and occur as straight bands in outcrop.
tures (Sect. 5.6.3) are common in such foliated cataclasites Occasionally, they occur as pairs of subparallel surfaces,
(Lin 1999, 2001). Deformation mechanisms in cataclasite connected by injection veins. The volume of rock con-
are mainly cataclastic flow in and between grains, grain taining injection veins is known as the reservoir zone
boundary sliding and pressure solution. Cohesive breccia (Magloughlin and Spray 1992). Main fault veins can be
and cataclasite are thought to develop at greater crustal difficult to spot in the field, especially if they lie parallel
depth than incohesive ones. Unless stated otherwise, cata- to older layering or foliation; injection veins are more
clasite and breccia are understood to mean the cohesive conspicuous and allow recognition of pseudotachylyte
form in the following sections. in outcrop. Pseudotachylyte veins have distinct, sharp and