SOURCE CHARACTERIZATION AND GRAVITY DATA MODELING OF DELHI FOLD BELT, INDIA

Abstract

The Delhi fold belt (DFB) represents the Proterozoic Aravalli-Delhi orogeny, which lies as a prominent NE-SW trending transverse structural heterogeneity at the leading edge of the Indian peninsula. Precambrian orogenic belts help in understanding the evolution of continents and play an important role to study the tectonic history of the cratonic regions. These behave as structural heterogeneities on the underthrusting Indian plate and likely control the earthquake ruptures of the Himalayan front. The Proterozoic DFB in NW India shows a prominent NNE-SSW (~ 600 km) trending gravity high. Past studies are mainly carried out along the Nagaur-Jhalawar transect towards the south (~ 150 km) from the profile BB' of the study region. There is the abrupt absence of the Moho signature below the DFB in the seismic sections, which is although present below surrounding geological formations. Further, there is no detailed geophysical investigation addressing the DFB structure towards the north (beneath Delhi, Rajasthan and Haryana states). The interpretations in these studies are qualitative, and researchers have pointed lack of understanding in the crustal configuration of the DFB due to speculative results. In view of scanty geophysical studies, I have carried out a detailed analysis of gravity dataset of the region to understand the crustal structure below the DFB. The study addresses whether the structures modeled in the southern region of the DFB along the Nagaur-Jhalawar transect in past studies are extending towards the north in the study region. The Bouguer gravity anomaly of the study region is modeled using radially averaged power spectrum, wavelength filtering, 3D structural inversion, wavelet source edge detector (WSED) method and depth from extreme points (DEXP) method to constrain the crustal structure of the DFB. The radially averaged power spectrum of the Bouguer gravity anomaly is used to derive a priori model for an average depths of the layered interfaces, which are further used in modeling. The Bouguer gravity anomalies are separated based on the characteristics of the power spectrum to understand the deep and shallow structures. The 3D structural inversion is performed on the long-wavelength filtered gravity anomaly to derive the 3D lateral variations of the Moho interface. The derived Moho geometry is helpful in imaging the 2D density modeling. I have developed and propose a new methodology named WSED, which is efficient in deriving the source edge boundaries using gravity or magnetic data. The method uses wavelet theory to identify the directional properties of the edges. The method is found ii advantageous in comparison to conventional techniques. The method is used to identify the geological boundaries in the DFB region. Among the multiscaling methods of source characterization, the DEXP method independently estimates the homogeneity degree and depth of the sources. The DEXP transformation is tested on the synthetic gravity and magnetic data generated due to isolated and extended sources. The method is further applied to the Bouguer gravity anomaly of the DFB region to delineate the shape and depth information of the sources. In the thesis, I propose a detailed 2D crustal density model for the first time in the study region. The 3D structural inversion results show that the long-wavelength behavior of the Bouguer gravity anomaly can largely be explained with a prominent upwarp in the Moho interface. The DEXP results also corroborate this regional structure in the form of a horizontal cylinder. The misfit in the anomalies is attributed to the existence of high-density mantlederived underplated material formed by extension during Proterozoic age. The lateral Moho variations indicate the absence of a northward extension of the DFB underneath the Indo- Gangetic alluvium and deflection of the Moho towards NW direction along the Delhi- Sargodha ridge axis. This likely indicates that the plume head got deflected westward sometime after indentation of the NW corner of the Indian plate and its counter-clockwise rotation after Eocene collision.