High Strain Textures - opengeology.in

In the upper crust the pressure is less and brittle shear zone prevails whereas in the lower part pressure is enough to go for ductile shearing.

During this high strains the larger minerals often breaks into smaller reduced smaller sub-grains through the process called polygonization.
Highly strained rocks have high strain energy and recrystallize readily. If the temp. is high the resulted scattered boundaries becomes smoother and gives amoeboid apparence.
The collective process of larger grains form from sub-grains called coalescence.
Sometimes during deformation the phyllosilicates undergoes slipping causing shredded appearance.
In deformed lattice of Plagioclase and quartz the undulous extinction is present where complete grain doesn’t go extinction once rather, grain partially goes extinction on rotation.

Development of Textures

Let us consider the example of Granite undergoing the high strain environment

‘A’ is the original granite rock in hand specimen and in the thin section that is without any deformation.
‘B’ is the starting of deformation where some original clasts (porphyroclasts) are surrounded by crushed fine material/ matrix. The resulted texture is called as mortar texture.
- The Pseudotachylite may develop if there is localized rapid fragmentation and melting due to shear heating.

‘C’ Mylonization develops as we go further where there is development of foliation as shearing and recrystallization causes elongation. with the shear sence indicators. Here twinned and ductilely elongated grains are common.
‘D’ further the finer material increases and the original grains are rare. The resulted rock is called Ultramylonite and the resulted texture is Ribbon texture. Extreme fine matrix.