Page 39 - Microtectonics

3.2 · Brittle Fracturing – Cataclasis 27
Fig. 3.3.
Wing-cracks form at the tip of a non-
propagating microfracture

Fig. 3.2. a An isolated crack in an isotropic material propagates
radially. b If cracks are close together they may obtain a curved shape
because the stress fields at the tips of the cracks influence each other

is a shear component along the microfracture but it can-
not propagate laterally for some reason, e.g. when the frac-
ture lies along a short grain boundary, horn-shaped wing
cracks may form (Horii and Nemat-Nasser 1985; Fig. 3.3).
Microfractures are called intragranular if they only affect a
single grain. Fractures that transect several grains are known
as intergranular or transgranular fractures (Fig. 5.1).
Fracture propagation as described above is valid for
continuous media such as single grain interiors of non-
porous polycrystalline rocks. In porous rocks, the situa-
tion is slightly different. Fractures mostly form and propa-
gate at sites where grains touch (Fig. 3.4). In poorly or
unconsolidated porous material, compression normal to
the contact of impinging grains leads to fractures which
radiate out from the edge of contact sites, known as im-
pingement microcracks. These are either straight and diago-
nal or occur in a cone-shaped pattern known as Hertzian
fracture (Figs. 3.4, 5.1) (Dunn et al. 1973; Gallagher et al.
1974; McEwen 1981; Zhang et al. 1990; Menéndez et al. 1996). Fig. 3.4. In porous rocks, impingement microcracks can form at con-
Impingement microcracking may induce splitting of tact points. Two examples are given, Hertzian- and diagonal intra-
grains or shedding of fragments from the sides of grains. granular microcracks
When a critical differential stress is reached in the frac-
ture tip, fractures can grow laterally with a velocity that is a Microfractures described above can form by stress
significant fraction of the velocity of elastic waves in solids, enhancement at their nucleation sites in response to high
as anyone will recognise who has seen glass shatter. Alter- bulk differential stress or, in the case of porous rocks, due
natively, stress at the fracture tip can induce slow growth of to lithostatic pressure and pore collapse in the absence of
a microfracture known as subcritical microcrack growth bulk differential compressive stress. Other possible causes
(Atkinson 1982; Darot et al. 1985). The speed of subcritical for microfracture nucleation and propagation are elastic
microcrack growth does not only depend on stress, but also or plastic mismatch, where two mineral phases have dif-
on temperature and chemical environment, especially of the ferent rheological properties and local stress concentra-
fluid in the crack. Subcritical microcrack growth can hap- tion builds up (Tapponier and Brace 1976; Wong and
pen by volume change due to phase change (Blenkinsop Biegel 1985; Hippertt 1994). Common examples are cracks
and Sibson 1991) but most commonly by stress corrosion at corners of mica grains in quartz and fractured feld-
cracking due to breaking of bonds in the crystal at the crack spar grains in ductile quartz mylonite (Chap. 5). Cracks
tip by chemical reaction (Atkinson 1984; Kerrich 1986). may also form as accommodation features related to other
Subcritical micro crack growth is probably faster than proc- structures such as twins or kinks (Carter and Kirby 1978;
esses like dissolution-precipitation (Sect. 3.3). Sect. 3.5), by different thermal expansion or contraction