Aspects of the Syntectonic Extension and Shear Veins from the Barwaha Region, Dhar forest, Central India

Extension veins with sigmoidal crystal fibres affected by post-crystallization deformation and synkinematic with the F₂ deformational event are developed in the metamorphic rocks east of Barwaha in western Madhya Pradesh. Though the original vein textures have been modified by the effects of post-crystallization deformation such as bending, rupture and shearing, they are sufficiently well-preserved to conclude that the growth process was essentially of the' stretched crystal' or 'crack-seal' type.

The vein origin can be ascribed to a period of low grade metamorphism and associated deformation. Post-vein modifications in the matrix are not associated with any retrogressive metamorphic assemblages, suggesting that the modification process occurred under more or less the same conditions which prevailed during vein growth but under the greater influence of ductile deformation within the veins.

The veins cross-cut the regional schistosity or a statically recrystallised hornfelsic fabric and therefore they postdate the F₁ movement and the post F₁ dyke, but predate the end of regional ductile deformation F₂. Extension veins are themselves cross-cut by shear veins or the shearing is restricted to the walls of extension veins suggesting significant change in orientation of the maximum principal compressive stress towards the late stages of vein dilation. The drusy fabrics of quartz crystals in nontectonic veins developed in the Lameta sandstones are also deformed, as are those of secondary calcite in basalt flow cavities in the area to the east of Sanavad, presumably related to the rifting and volcanism of late Mesozoic-early Cenozoic period. The effect of this later tectonic activity was found in the older syntectonic veins, this being the infilling of zeolite in the late' saddle-reef' type dilation sites between the bent crystal fibres.