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Authors
Affiliations
1 Department of Applied Geology, The University, Sagar 470003, IN
2 Department of Applied Geology, Indian School of Mines, Dhanbad 826004, Bihar, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 33, No 5 (1989), Pagination: 387-418
Abstract
The morphology of ductile shear zones in the granitic basement of Bijawar rocks is described. The dextral shear zones are more ubiquitous than their sinistral counterparts and the obtuse angle between the two sets faces the direction of maximum shortening in rocks, the exact value of the angle presumably controlled by the mean stress. The maximum displacement vector of shear zones is generally subhorizontal. The negative dilatation in some shear zones has caused the overall defoi marion in them to be of the flattening type. The orientations of the principal stresses are calculated. The cleavage trajectories do not sometimes exactly bisect the acute angle between the conjugate zones, presumably due to rotation of principal strains with progressive shear or to the more accentuated development of one set of zones.
The sinusoidal and conjugate folds of monochnic or triclinic symmetry (D-1) in the Bijawar rocks are shown to be related genetically and from the point of view of orientation of principal compressive stresses to the ductile shear zone development in basement granite. The brittle-ductile shear zones in the basal carbonate unit associated at places with pressure solution or tectonic stylolitic structures whose formation was accompanied by about 14 to 24 per cent volume reduction formed at the same time.
There was a renewed activity during post-Vindhyan time along pre-existing shear zones in the older rocks but in the opposite sense under very Jow P-T conditions which produced ductile D-2 structures in Bijawar rocks, culminating in small or moderate scale strike-slip faults. The youngest movements along these faults were, however, with a vertical component under a subvertically oriented maximum principal compression. It appears that most shear zones have an minal period of strain softening, followed by rather sudden strain hardening or loss of material contmuity which causes either the termination of such zones or their passing into model ate scale wrench faults.
