CRUSTAL REFLECTIVITY PARAMETER FOR DECIPHERING THE EVOLUTIONARY PROCESSES ACROSS THE PROTEROZOIC ARAVALLI-DELHI FOLD BELT

Deep Continental Reflection Studies along the Nagaur-Jhalawar profile across the Aravalli-Delhi Fold Belt indicate that in most of the units comprising the seismic profile the lower crust (LC) lies towards west of their present surface exposures. Based on the reflectivity charact eristics of the LC, which are in general agreement with the globally a ccepted norms, the Nagaur-Jhalawar profile can be broadly divided into five zones: 1) the moderately reflective Marwar Basin (MB), 2) the hi ghly reflective Delhi Fold Belt (DFB), 3) the poorly reflective Bhilwa ra Granite Complex (BGC), 4) the highly reflective Hindoli Group (HG) including a thrust zone, and 5) the moderately reflective Vindhyan Bas in (VB). The most commonly accepted reasons for high lower crustal ref lectively are: 1) presence of free fluids, 2) crustal-scale ductile sh ear zones and 3) mafic intrusions and underplating with partial melts derived from the upper mantle. High reflectivity of LC in the ADFB can possibly be assigned to primary lithologic variation in the metamorph ic facies layers and Precambrian shear zones that were formed during c ompressional phase of orogeny. The poor reflectivity of LC in the BGC, comprising the Sandmata and Mangalwar Complexes, may be due to vertic ally oriented igneous intrusions which have disturbed its lamellar cha racter. The high reflectivity in the thrust zone and LC in the HG sugg ests that it is due to a totally different composition than that of BG C and may be related to metamorphic layering.