Page 171 - Microtectonics

160 6 · Dilatation Sites – Veins, Strain Shadows, Fringes and Boudins
6.1 6.1 are also known as pressure shadows. Since the shape of
Introduction the aggregates gives primarily information on strain dis-
tribution around an object, and not on forces, we advo-
Many deformed rocks contain sites with a deviant min- cate the use of the term strain shadow. Some veins and
eralogy and fabric, interpreted as an effect of re- strain shadows have fuzzy boundaries (Fig. 6.1). They
arrangement of material by local dilatation and pre- may form by local alteration of the wall rock along a frac-
cipitation during deformation. Such ‘dilatation sites’ ture (replacement veins) or rigid object, or by deforma-
can be isolated and elongate (veins), flanking rigid tion and recrystallisation of veins with sharp bounda-
objects (strain shadows) or occur in the neck of boudi- ries (Sect. 6.5).
naged layers or elongate crystals (Fig. 6.1). Dilatation The development of veins and strain shadows is asso-
sites can be filled by solidified melt, or by precipitation ciated with the circulation of fluids in rocks, both for
of material from solution in an aqueous fluid, and it transport of material and for propagation and opening
is the latter situation, which is discussed in this chapter. of the vein. Fluid pressure in rocks is usually between
Dilatation sites are usually filled with polycrystalline hydrostatic pressure (the pressure of a water column at a
material, which may be equigranular, but may also particular depth) and the smallest principal stress σ in
3
consist of parallel-oriented elongate or rod-shaped crys- the rock (Sect. 2.11). Due to deformation, reduction of
tals (Figs. 6.1–6.3). pore volume or metamorphic dewatering reactions, con-
Veins and strain shadows are some of the most com- nectivity to the surface may be partly or completely lost,
plex microstructures to be found in rocks, and contain and pore fluid pressure P in rocks can increase and ap-
f
much information about deformation and deformation proach a critical value P :
c
history (Figs. 6.1–6.3; Box 6.1; Taber 1918; Mügge 1928;
Pabst 1931; Zwart and Oele 1966; Choukroune 1971; Dur- P = P = σ – T s
3
f
c
ney and Ramsay 1973; Beutner and Diegel 1985; Etche-
copar and Malavieille 1987; Kanagawa 1996; Bons 2000; where T is the tensile strength of the rock on pre-exist-
s
Köhn et al. 2000; Hilgers and Urai 2002). Strain shadows ing planes of weakness. If this value is reached, cracks
filled with fluid can open at any depth and minerals can
be precipitated in such cracks. Since T is usually small
s
and σ close to the overburden pressure, except in ex-
3
tensional tectonics, the critical value can also be ex-
pressed by the pore-fluid factor λ (Sibson 1990), a ratio
v
of the pore fluid pressure to the vertical or lithostatic
stress
λ = P / σ
v f v
If λ ≈ 0.4, fluid pressure is hydrostatic, if λ =1, it
v
v
is near lithostatic. Fluid filled fractures form when
λ > 1 in a compressive regime. If differential stress
v
(σ – σ ) is small in such cases, tensional fractures
1 3
open but if differential stress exceeds a certain limit (a
commonly used rule of thumb is (σ – σ )= 4 T ; Jaeger
1
3
s
1963; Secor 1965), depending on rheological parame-
ters of the rock, the opening fractures will have a shear
component (Fig. 9.4).
The role of fluids in the formation and growth of veins
and strain shadows is relatively complex. Fluid pressure
may fluctuate in cycles of increasing pressure, leading
to fracturing and vein opening, and subsequent falling
pressure due to drainage during which mineral deposi-
tion commonly occurs (Vrolijk 1987; Cosgrove 1993;
Ohlmacher and Aydin 1997; Bons 2000; Oliver and Bons
2001). Some veins, however, show evidence of persistent
high fluid pressure during mineral growth (Henderson
Fig. 6.1. Schematic drawing of fibrous and massive veins, strain fringes
and strain shadows as treated here. Massive veins and strain shadows et al. 1990). In other vein segments, e.g. in strain shad-
are filled by a polycrystalline aggregate with a granoblastic fabric ows alongside a rigid object or in jogs along a shear