Page 40 - Microtectonics

28 3 · Deformation Mechanisms
of adjacent minerals (Vollbrecht et al. 1991) or by phase Box 3.1 Evidence for fractures and cataclastic flow
transformation with volume increase such as coesite to
quartz and aragonite to calcite (Wang et al. 1989; Wang Fractures are easy to recognise by their sharp, narrow and usu-
ally straight nature and displacement of markers. More diffi-
and Liou 1991; Kirby and Stern 1993).
cult is the recognition of such structures when healed and over-
Microfractures are commonly healed and filled with a printed. Healed fractures can be recognised as arrays of fluid
secondary mineral phase, commonly the same phase as or solid inclusions in a plane. Zones of cataclastic flow in thin
the host crystals in optical continuity. This makes espe- section may be confused with shear zones that consist of dy-
cially tensional microcracks difficult to see, except in namically recrystallised material (Figs. 3.29, 3.37). A cataclasite
differs from a ductile deformed and recrystallised rock by (1) a
cathodoluminescence (Stel 1981; Chap. 10.2.1; Fig. 10.9a).
larger range in grain size, in many cases fractal (Blenkinsop
In many cases, trails of fluid inclusions prove the former 1991a); (2) the presence of grains that have angular outlines
presence of healed microcracks (Figs. 3.22, 10.17c,e,f). and straight sharp boundaries, and (3) the presence of poly-
Healed microcracks aligned with inclusions have been crystalline rock fragments (Fig. 3.5; however, a ductile deformed
named Tuttle lamellae by Groshong (1988). sandstone or sedimentary breccia may contain polycrystalline
rock fragments too, so care is needed). The constituent grains
Displacement on microfractures as described above
show no grain shape preferred orientation if the host material
will be in the order of microns, and not geologically sig- consists of equant minerals such as quartz and feldspars. In
nificant. However, microfractures can multiply and grow some cases, cataclastic material is recrystallised after deforma-
until their stress fields start interfering, after which they tion, and distinction may then be impossible. Optical criteria
can impinge by changes in the propagation direction of are often insufficient for unequivocal identification of catacla-
site; only transmission electron microscope (TEM) investiga-
the tips, or by creation of bridging secondary microfrac-
tion is conclusive in such cases (Sect. 10.2.5).
tures (Kranz and Scholz 1977; Costin 1983; Blenkinsop
and Rutter 1986; Menéndez et al. 1996). As a result, larger
microfaults form which can accommodate displacements plane. Fracturing can operate fast, approaching seismic
that are geologically significant. Such frictional sliding velocity, or slow by fracturing of individual grains. Frac-
occurs on rough fault surfaces and asperities on the fault turing can be transgranular, breaking grains into ever-
surface must be smoothened or fractured before sliding finer fragments in a process called constrained commu-
can take place (Wang and Scholz 1995). Therefore, the nition (Sammis et al. 1987; Antonellini et al. 1994; Menén-
minimum differential stress needed for movement along dez et al. 1996). In this case the final particle size distribu-
a fault depends on the normal stress that keeps the sides tion (PSD) can be fractal (Sammis et al. 1987; Blenkinsop
of the fault together. Although its magnitude depends on 1991b). In sediments, however, especially poorly lithified
the orientation of the principal stress to the fault plane, ones deformed at shallow depth, fracturing can also oc-
the normal stress σ increases proportional to the mean cur by rupture in grain contact cement, or by flaking of
n
stress in the rock while its effect decreases if the fluid grains, in which case grains show conchoidal fracture sur-
pressure P in the fault increases. Therefore the effective faces and intermediate size particles are underrepresented
f
normal stress (σ = σ – P ) is usually quoted for analyses. (Rawling and Goodwin 2003). Commonly, slow trans-
e n f
A higher effective normal stress means that a higher dif- granular fracturing is aided by processes such as pres-
ferential stress is needed for fault motion. sure solution, intracrystalline deformation (Lloyd 2000;
Sliding on faults and fracturing of wall rock forms a Hadizadeh and Tullis 1992), chemical reactions and min-
volume of brittle fault rock such as gouge, cataclasite and eral transformation (Atkinson 1982; Blenkinsop and
breccia (Figs. 3.5, 5.3; Box 3.1; Sect. 5.2) along the fault Sibson 1991).
Fig. 3.5.
a Cataclasite fabric – angular
fragments of all sizes, some
transecting grain boundaries,
are embedded in a fine-grained
matrix. Many larger fragments
are crossed by healed fractures,
aligned with fluid- and solid in-
clusions. b Recrystallised fabric
of small new grains that grew at
the expense of old grains. The
new grains show little variety in
grain size