Evolution of the southern part of the Aravalli-Delhi orogen western India.

This thesis is concerned with the structural evolution of the early-mid Proterozoic Aravalli-Delhi orogenic belt. Emphasis is placed on elucidation of movement directions as deduced from strain and kinematic data, which are ultimately incorporated into a plate tectonic model involving oblique convergence of a continental margin and island arc. The orogen is separated into a series of predominantly NNE-trending terranes by a number of fundamental lineaments; the Great Boundary Fault, the Rakhabdev-Kashangar lineament, and the newly recognised Kumbhalgarh Steep Zone and Ranakpur Shear Zone. The stratigraphy, mineralisation and structure of the intervening belts; Aravalli, Jharol and Delhi are considered in detail. The Aravalli Belt is interpreted as a passive continental margin; volcanics and clastics associated with rifting of the Archaean Bundhelkhand-BGC continent at ca 2.2 Ga, are unconformably covered by carbonates. The deep-water sediments of the Jharol Belt are interpreted as distal-fan deposits of the Aravalli shelf-rise. The Delhi belt is as an (1.7-1.5 Ga) island arc complex; passive sedimentation on the Aravalli margin was terminated by arc accretion ca 1.5 Ga ago. Thick sequences of continent-derived clastics (the Debari Formation) lying unconformably on carbonates of the Aravalli System may be related to down-warping of the margin during early arc-continent convergence. East-west trending stretching directions and reclined folds in the Aravalli Belt are a product of restacking attenuated crust of the passive margin. Strain data from conglomerates at the base of the thrust stack indicate a predominantly constrictional strain field. L and LS fabrics, shear criteria, ramp structures and the general increase in strain toward the base of the system indicate that wrench and thrust strains were dominant components during eastward translation. A (gravity induced.) stretch in the direction of flow was an additional strain component. Thrusting of the Aravalli margin was contemporaneous with recumbent folding of back or fore-arc sediments of the Delhi arc complex. With continued convergence, early recumbent folds and thrusts in all three belts were progressively buckled and rotated to a sub-vertical attitude. Ultimately, the entire collision evolved into a zone of dextral transpression and successive generations of folds in the Delhi 'flower-structure' were transected. Complex non-coaxial strain patterns and shear criteria in the Ranakpur Shear Zone are interpreted as a product of strain incompatibility on the edge of the flower structure. Slip-line theory is advocated to explain the complete transposition of the Aravalli and Jharol belts in the narrow 'neck' of the orogen. It is concluded that the bulk deformation history; early thrusting, later flattening and ultimate wrenching parallel to the orogen, is compatible with models of plate collision in the Phanerozoic.