Page 161 - Microtectonics

150 5 · Shear Zones
5.6.9 Quarter mats. Mica concentrations adjacent to a porphy-
Quarter Structures roclast in the quarters that lie in the shortening direc-
tion are known as quarter mats (Hanmer and Passchier
Porphyroclasts without mantles may show an asymmetric 1991). These probably form by preferential removal of
distribution of microstructures over the four quarters de- quartz by solution transfer at stress concentration sites
fined by the foliation and its normal; such structures have adjacent to a porphyroclast.
been named quarter structures (Fig. 5.38; Hanmer and
Passchier 1991). Quarter structures are geometrical features Asymmetric myrmekite. Myrmekite is sometimes concen-
that do not need to coincide with flow symmetry axes. Sev- trated in shortening quarters in the rim of K-feldspar crys-
eral types of quarter structures have been described and tals (Sect. 7.8.3; Simpson 1985; Simpson and Wintsch 1989).
have been empirically established as shear sense indicators. It probably forms by preferential proceeding of the K-feld-
spar breakdown reaction and associated volume loss at
Quarter folds. Microfolds in the quarters that lie in the ex- sites of high differential stress (Simpson and Wintsch
tensional direction are known as quarter folds (Fig. 5.38; 1989). The arrangement of quartz lamellae in myrmekite
Hanmer and Passchier 1991). Quarter folds probably de- may also show an internal monoclinic symmetry (Fig. 5.38
velop by rotation of layering into the extension field of inset) which can serve as an independent, internal shear
flow when passing the top of a porphyroclast during pro- sense indicator (Simpson and Wintsch 1989).
gressive deformation.
5.6.10
Lattice-Preferred Orientation

The lattice-preferred orientation (LPO) of minerals common-
ly shows a monoclinic symmetry. Crystals with elongate shape
such as mica and amphiboles can develop a monoclinic
oblique fabric with respect to another foliation such as a
layering, as described above. Moreover, the deviation in ori-
entation of such minerals around the mean can be skewed.
Such skewness is difficult to measure optically, but has been
detected in slates by X-ray goniometry (Sect. 10.3.5) and
can be used to determine sense of shear (O’Brien et al. 1987).
Minerals with equant grain shape such as quartz, cal-
cite, feldspar and olivine in mylonites commonly show a
monoclinic symmetry of LPO with the symmetry axis nor-
mal to the aggregate lineation and parallel to the main
mylonitic foliation. LPO patterns in pole diagrams for a sin-
gle crystallographic axis such as c- or a-axes in quartz can
have an internal monoclinic symmetry defined by the shape
of the pattern, and an external asymmetry with respect
to foliations in the rock. Both asymmetries are useful shear
sense indicators. Further details on LPO development and
interpretation are given in Sects. 4.4.3–4.4.5. A method to
measure quartz c-axes is presented in Sect. 10.3.
5.6.11
Vergence of Asymmetric Fold Sections

Cross sections through folds in mylonites parallel to the line-
ation commonly show a dominant vergence (Fig. 5.10). If
the folds are sheath folds generated during mylonite forma-
tion and the section is strictly parallel to the movement di-
rection, the vergence may be reliable as a shear sense indi-
Fig. 5.38. Three types of quarter structures in mylonite that can be cator. In most cases, the three-dimensional shape of the folds
used to determine sense of shear. The structures are defined by an
asymmetric distribution of fabric elements over the four quarters is unknown and it may also be unclear whether the folds
defined by the foliation and its normal. Myrmekite can also have are older or related to the mylonite formation. In those cases,
an internal asymmetry that can be used to determine shear sense asymmetric folds are unreliable as shear sense markers.