Page 137 - Microtectonics

126 5 · Shear Zones
5.5 5.5 movement direction is normal to a planar marker
Sense of Shear (Fig. 5.10a). Notice, however, that in other cases the ap-
parent offset on an outcrop surface or in a thin section
5.5.1 may give the wrong sense of shear (Fig. 5.13, ×Video 5.13,
Introduction ×Photo 5.13; Wheeler 1987b).

The direction of movement on a shear zone can usually 5.5.3
be shown to lie subparallel to striations, slickenfibres or Foliation Curvature
aggregate and grain lineations. Once this direction is es-
tablished, it is necessary to determine the sense of dis- Foliated ductile shear zones may show a gradient in fo-
placement (sinistral or dextral, normal or reverse) or liation development from an undeformed wall rock to-
sense of shear. Traditionally, this was mainly done using wards the core of the zone; the foliation has a character-
markers in the wall rock such as displaced layering and istic curved shape that can be used to determine sense
dykes or deflection of layering or foliation into a shear of shear (Fig. 5.10a). This structure develops if the folia-
zone (Sects. 5.5.2, 5.5.3). Additionally, the geometry of tion is ‘passive’ (Sect. 4.2.9.2) and reflects the orientation
structures in the zone can be used to determine sense of of the finite strain axes (Sect. 4.2.9.2); the curvature re-
shear. This means that it is possible to determine shear flects a gradient in finite strain from its peak in the core
sense for a shear zone in thin section, even without see- of a shear zone outwards (Sect. 9.2). It develops because
ing the zone in the field. Microstructures that can be used foliations rotate from a position between the instanta-
to determine shear sense in mylonite are described in neous extension axis and the fabric attractor towards the
Sect. 5.6. Some microstructures can also be used to de- latter with increasing strain in non-coaxial progressive
termine shear sense for brittle fault rocks in thin sec- deformation (Ramsay and Graham 1970; Ramsay 1980a).
tion. These are discussed in Sect. 5.7. Foliation curvature is a reliable shear sense indicator
if the movement direction, indicated by a lineation, is
5.5.2 normal to the axis of curvature of the foliation. However,
Displacement and Deflection of Markers care should be taken not to confuse it with deflection of
an older foliation, where the movement direction may
The simplest and best known sense of shear indicator is be oblique to the axis of curvature of the foliation, in
the displacement of markers such as dykes, veins, which case some sections through the structure may give
xenoliths and bedding along a shear zone. Commonly, the wrong shear sense (Sect. 5.5.2).
there is also a deflection of markers near the zone, due
5.6 to a strain gradient. Interpretation of both structures is 5.6
straightforward in the case of a linear marker, or if the Microscopic Shear Sense Indicators in Mylonite

5.6.1
Introduction

One of the most characteristic properties of mylonites and
some other rocks from ductile shear zones is that fabric
elements and structures show monoclinic shape symme-
try (Fig. 5.10). This effect is a result of the non-coaxial pro-
gressive deformation in shear zones due to relative displace-
ment of the wall rocks. Lineations, finite strain axes and
most foliations rotate towards the fabric attractor, which is
oblique to the extensional ISA (Sect. 2.9) and their sense of
rotation usually equals the sense of shear. Since different
fabric elements track the fabric attractor to different de-
grees, complex structures with a distinct monoclinic shape
symmetry (in the literature usually referred to as asym-
metry) develop in mylonites. This asymmetry can be used
Fig. 5.13. Schematic presentation of a shear zone illustrating how de- to deduce sense of shear. We distinguish internal and ex-
flection of a marker can give the wrong sense of shear if not used in ternal asymmetry for either single objects and fabric ele-
a section parallel to the movement direction (here marked by a
lineation L); the frontal block moves up and to the left, but the de- ments, or combinations of fabric elements with a charac-
flected layer indicates an apparent sinistral displacement on a hori- teristic monoclinic shape symmetry. A similar terminol-
zontal surface ogy is used for LPO patterns (Sect. 4.4.3). With internal