Page 164 - Microtectonics

5.6 · Microscopic Shear Sense Indicators in Mylonite 153
nites, inter-boudin slip is antithetic to bulk shear sense;
however, if a layer or object that suffers boudinage is
highly oblique to the fabric attractor, inter-boudin slip
may be synthetic (Fig. 5.42). Since it is not always possi-
ble to determine the orientation on a train of domino
boudins, they are to be used with caution.

Fragmented Porphyroclasts

Elongate rigid porphyroclasts of amphiboles, pyroxenes
or feldspar grains can be separated into aggregates of frag-
ments with a geometry similar to asymmetric boudins,
separated by seams of cataclasite (Fig. 5.43, ×Photo 5.43;
Duebendorfer and Christensen 1998; Babaie and LaTour
1998). As in the case of boudins, there are domino type and
shear band type fragmented porphyroclasts (Fig. 5.44). The
geometry depends on the bulk shear sense and the ini-
tial orientation of microfaults in the grains, which may
be partly controlled by crystallographic directions in the
porphyroclasts (Sect. 3.12.4) and by flow type. In por-
phyroclasts, fragment geometry also depends on the
original grain shape. Simple sets of fractures tend to
dominate when the porphyroclast suffered small inter-
nal strain, but at higher strain porphyroclasts split to
form mosaic fragmented porphyroclasts (Duebendorfer
and Christensen 1998; Babaie and LaTour 1998). Domino- Fig. 5.44. Illustration of three types of fragmented porphyroclasts,
type and mosaic fragmented porphyroclasts are more and their interpretation in terms of bulk shear sense (large arrows)

Fig. 5.43. Microfaults transecting feldspar porphyroclasts in a section parallel to the aggregate lineation and normal to the foliation in
granite mylonite. The faults do not continue into the mantle of recrystallised feldspar surrounding the porphyroclasts. Dextral shear sense.
Both synthetic (bottom left) and antithetic (top right) microfaults are present. St. Barthélemy, Pyrenees, France. Width of view 9 mm. CPL