Page 143 - Microtectonics

132 5 · Shear Zones
thetic with that of the main shear zone. The asymmetry
of C'-type shear band cleavage is opposite to that of ob-
lique foliation (Fig. 5.14).

5.6.4
Porphyroclast Systems in Mylonites – Introduction

Porphyroclasts are large single crystals in a more fine-
grained matrix of different, usually polymineralic
composition, which are common in mylonitic rocks
(Fig. 5.20). If they are equidimensional and have a sharp
boundary with the matrix, they are known as naked clasts
or, if the porphyroclasts have an elongate shape with
monoclinic symmetry, mineral fish (Fig. 5.20). In many
cases, porphyroclasts have attached polycrystalline rims
that differ in structure or composition from the matrix;
such assemblages are known as porphyroclast systems.
Porphyroclast systems can have a number of character-
istic shapes that can be used to determine shear sense
and other kinematic parameters. In many cases, the sur-
rounding rim has a tapering shape on opposite sides of
the porphyroclast. If the material in the rim is of the same
composition as the porphyroclast, the rim is called a
mantle and the structure is known as a mantled porphy-
roclast or mantled clast (Figs. 5.10b, 5.20). If the rim has
a composition different from the porphyroclast, the ad-
jacent tapering domains are known as strain shadows and
the entire structure as a porphyroclast with strain shad-
ows (Fig. 5.20; Sect. 6.3–6.5 and treated separately below).
Such stain shadows are commonly composed of carbon-
ate, quartz, mica or opaque minerals, which were appar-
ently not formed by reaction with the porphyroclast, but
by precipitation from solution. If it can be shown that
the material in the rim is formed by transformation of
Fig. 5.19a,b. Schematic diagram showing the development of C'-type the porphyroclast, the rim is known as a reaction rim
shear band cleavage with relatively rigid microlithons. The C'-type (Sect. 7.8, cf. Lafrance and Vernon 1998).
shear bands develop at a late stage of the shear zone activity after a
foliation is well established. Shear bands can theoretically develop Sigmoids are aggregates of grains of a mineral A in a
in two orientations, but only one, at a small angle to the foliation, is matrix of another mineral, lacking a clear porphyroclast
realised. Notice that the shortening direction must be relatively steep core (Fig. 5.20). They can have a similar shape to σ-type
with respect to the shear zone and that the shear zone must be ex- mantled clasts or mineral fish.
tending In geological practice, there is a tendency to call all
structures in Fig. 5.20 “sigma-objects” indiscriminately.
Shear band cleavages have both internal and external We discourage this attitude, since each of the structures
asymmetry that can be used as shear sense indicators in Fig. 5.20 forms by a different mechanism, as explained
(e.g. Malavieille and Cobb 1986; Davis et al. 1987; Saltzer below.
and Hodges 1988). The internal asymmetry is a sigmoi- Porphyroclasts, fish, strain shadows and sigmoids are
dal shape of the older foliation between shear bands observed in many geological environments including vol-
(Fig. 5.14(1)). The external asymmetry is the angle be- canic flows (Benn and Allard 1989; Luneau and Cruden
tween the enveloping surface of the older foliation and 1998), cataclasite (Cladouhos 1999a,b) and mylonite
the shear bands (Fig. 5.14(2)). C'-type shear band cleav- (Passchier 1987; Hanmer 1990; Shelley 1995; Masuda et al.
age has an additional external asymmetry element since 1995c).
the shear bands are inclined to the shear zone in a char- In the following sections, we first describe the shape
acteristic way (Figs. 5.14(3), 5.15, 5.16, ×Photo 5.15). of different types of porphyroclast systems (Sects. 5.6.5,
Shear sense in both C- and C'-type shear bands is syn- 5.6.6), followed by a review of the present understanding