APPRESSED ASYMMETRIC FOLDS AND ASSOCIATED BOOKSHELF GLIDING IN A SOUTH INDIAN SHEAR ZONE

In south India, near Chitradurga (14.2°N, 76.4°E), the Precambrian Peninsular Gneiss Complex show strong development of shear zones in outcrop scale. The present shear zone, outcropped in a horizontal surface of about 20 meters x 40 meters area, trends NW-SE. In the gneiss biotite rich well foliated bands alternate with quartzo-feldspathic bands. Also, there are thicker quartzo-feldspathic veins that intruded parallel to the foliation. These veins act as markers within about 40 cm wide and up to 6 meters long NW-SE zones of intense shear-related folding. In such zones these veins show series of very steeply plunging sinistral folds. Outside these zones the veins run straight and parallel to the main NW-SE banding in the rock. These are zones of left lateral (sinistral) shear. The folds gradually become more tightly appressed with thicker hinges and thinner limbs along the length of these zones, as their axial plane traces become less oblique to the zone boundaries. Each such fold is bounded by two sharp secondary shears with right lateral (dextral) displacement antithetic to the sinistral sense of the shear zones. These secondary shears stop at the shear zone boundaries very sharply. When such a boundary coincides with any quartzo-feldspathic vein a spectacular phenomenon happens: the side of the vein towards the shear zone becomes sliced and displaced in a saw-tooth manner by the secondary shears, while the outer side away from the shear zone runs straight. This indicates a bookshelf structure involving main sinistral shear forming sinistral folds, and secondary dextral shears along planes oblique to the main shear. With increase in the amount of sinistral shear the dextral shear bounded ‘books' in the ‘bookshelf' become more stretched flattening the folds inside the ‘books', and more rotated to become less oblique to the main shear direction. Such individual bookshelf bearing zones may well be the sites of nucleation of smaller shears that coalesce to form a regional shear zone.