Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Abstract

The rocks described as esmeraldites in the southern part of Centra1 Indian Bundelkhand granitic massif are suggested to the typically those formed by heterogeneous simple shear and by development of retarded runway shear instabilities in host granites.

The development of shear zones appears to have occurred under two different stress fields. The early stress field was subhorizontal and nearly equatorial with low values of ESE trending σ₃ and low ¯σ in statistically isotropic material (elastic response). This produced positive dilational shear zones marked by intrution of pegmatite, filling en echelon tensile fissures. The early shear zones were converted into regular ductile shear zone at a later stage under a subhorizontal meridional stress field within highly compressive NNE trending σ₃ and high ¯σ (viscous response within previously weakened zone). This produced a subvertical or steeply dipping ESE trending fabric. Only this two-stage model is able to explain the development of these zones. Finite strain determinations are made from deflected basic dykes, deformed quartz veins and based on the angular relationship between C and S surfaces. The dilation during early episode is indeteminate because of rotation during latershearing. The shear along fabric suggests that the foliation failed to rotate with rotating λ₁ λ₂ planes of the simple shear. The finite strains computed on the basis of angular departures between fabric and zone walls are therefore slightly underestimated.

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