Page 41 - Microtectonics

3.3 · Dissolution-Precipitation 29
Movement on a fault can proceed along distinct slid- curs at diagenetic to low-grade metamorphic conditions.
ing planes or slickensides in or at the edge of the pro- The conditions also depend on the type of minerals in-
duced volume of cataclasite or gouge, but also by distrib- volved (Sect. 3.12) and on fluid pressure; high fluid pres-
uted cataclastic flow within the mass of fractured mate- sure promotes cataclastic flow in any metamorphic envi-
rial. Cataclastic flow operates by sliding and rotation of ronment and is responsible for the common occurrence
the fragments past each other, and further fragmentation of veins in cataclasite and breccia.
of these into smaller particles (Sibson 1977b; Evans 1988;
Blenkinsop 1991b; Rutter and Hadizadeh 1991; Lin 2001). 3.3 3.3
Rotation of fragments can be suppressed if fracturing is Dissolution-Precipitation
along crystal cleavage planes as in feldspar and amphibole,
and in such cases a crystallographic preferred orienta- An important deformation mechanism in rocks that con-
tion can result in the cataclasite or gouge (Tullis and Yund tain an intergranular fluid is pressure solution, i.e. disso-
1992; Hadizadeh and Tullis 1992; Imon et al. 2004). In the lution at grain boundaries in a grain boundary fluid phase
fractured material, cataclastic flow can occur by grain at high differential stress. Pressure solution is localised
boundary sliding with limited or no further fracturing of where stress in the grain is high, mostly where grains are
grains, or, at the other extreme, fracturing and other grain in contact along surfaces at a high angle to the instanta-
deformation processes may limit the rate at which cata- neous shortening direction (Figs. 3.6–3.8, ×Video 3.6).
clastic flow can occur (Borradaile 1981). During cataclas- Selective pressure solution at grain contacts occurs be-
tic flow, voids are created that may be filled with vein cause the solubility of a mineral in an aqueous fluid is
material precipitated from solution, which is subse- higher where a crystal lattice is under high stress than at
quently involved in the cataclasis; as a result, most localities where stress is relatively low (Robin 1978; Wheeler
cataclasite and breccia contains abundant fragments of 1987a, 1992; Knipe 1989). For example, in a sandstone
quartz or carbonate derived from these veins (Sect. 5.2). where grains are in contact (Fig. 3.6, ×Video 3.6) the grain
Fluid migration through cataclasite may also cause lattice near contact points is more strongly compressed
lithification by cementation, so that the cataclasite may than elsewhere; as a result, material will dissolve near these
be inactivated and fault propagation, fracturing and contact points and be redeposited at sites of low differen-
cataclasis migrates into another part of the rock volume. tial stress. A locally higher density of crystal defects near
Even if this does not occur, cataclastic flow is usually in- contact points may also enhance solubility (Spiers and
stable and terminates by localisation of deformation into Brzesowsky 1993). In this way, grains will change shape
slip on fault planes after which new breccia, cataclasite by local dissolution and redeposition without internal
or gouge can be produced. Cataclastic flow usually oc- deformation (Fig. 3.6b, ×Video 3.6).

Fig. 3.6.
a Oolites surrounded by a pore
fluid. At contact points, differen-
tial stresses are relatively high,
as indicated by shading.
b Pressure solution changes the
shape of the grains. Material
dissolved at the contact points
is redeposited in adjacent pore
spaces, indicated by dark shading