Page 122 - Microtectonics

5

Shear Zones

5.1 subject that has fascinated many geologists over the last decades is treated
Introduction Ain this Chapter. Concentration of deformation along shear zones, producing
mylonites or brittle fault rocks is discussed as related to depth and therefore meta-
5.2 morphic environment. The different fault rocks are treated from low to high grade
Brittle Fault Rocks or from fast to slow strain rates. The first part of the chapter deals with a descrip-
tion of brittle fault rocks that may be cohesive or incohesive, and of pseudotachy-
5.3 lytes that form by seismic events.
Mylonite Mylonites are produced predominantly by ductile deformation and usually
show significant recrystallisation of the matrix. Characteristic fabric elements are
5.4 porphyroclasts in a finer grained matrix.. Attention is given to the dynamics of
Complex Fault Rocks mylonite development and to the influence of different metamorphic conditions.
Many shear zones show evidence of repeated activity under different metamor-
5.5 phic conditions or strain rates, producing e.g. narrow cataclasites cutting mylo-
Sense of Shear nite zones.
The determination of shear sense in shear zones is of crucial importance for
5.6 the reconstruction of the tectonic evolution of e.g. colliding continents, crustal
Microscopic Shear Sense strike-slip movements or escape tectonics. A number of well established shear
Indicators in Mylonite sense indicators is presented and discussed. These include: displacement of mark-
ers, foliation curvature, shear band cleavage including C/S fabrics, mantled por-
5.7 phyroclasts, mineral fish, quarter structures and lattice preferred orientation. Other
Shear Sense Indicators more problematic shear sense indicators are also evaluated. Special attention is
in the Brittle Regime dedicated to a new topic called flanking structures, in which crosscutting elements
such as faults or veins transect the mylonitic foliation at an angle. The last section
is on shear sense indicators in the brittle regime.