I want to know the processes of development of shear zones? Could they be developed for reasons other than presence of faults in their vicinity? Is mica and quartz veins an indication of shearing in rocks? Please feel to ask for further clarification of my questions.
Dear Umer, Goodmorning and Fraternal greetings!
Thank you for your question
My answer is as follows
Shear zones involve volumes of rock deformed by shearing stress under brittle-ductile or ductile conditions, Shear zones often occur at the edges of tectonic blocks, forming discontinuities that mark distinct terranes. Shear zones can host ore bodies as a result of hydrothermal flow through orogenic belts, are commonly metasomatized, and often display some retrograde metamorphism from a peak metamorphic assemblage close to the Earth's surface, cool rocks respond to tectonic stresses with fracture and faulting.
The mechanisms of shearing depend on the pressure and temperature of the rock and on the rate of shear which the rock is subjected to. The response of the rock to these conditions determines how it accommodates the deformation.
Shear zones which occur in more brittle rheological conditions (cooler, less confining pressure) or at high rates of strain, tend to fail by brittle failure; breaking of minerals, which are ground up into a breccia. Shear zones which occur under brittle-ductile conditions can accommodate much deformation by enacting a series of mechanisms which rely less on fracture of the rock and occur within the minerals and the mineral lattices themselves. Shear zones accommodate compressive stress by movement on foliation planes.
Shearing at ductile conditions may occur by fracturing of minerals and growth of sub-grain boundaries particularly on platy minerals, especially micas.
During the initiation of shearing, a penetrative planar foliation is first formed within the rock mass. This manifests as realignment of textural features, growth and realignment of micas and growth of new minerals.
Dr. D S Subrahmanyam