Crustal thickening under the palaeo-meso-Proterozoic Delhi Fold Belt in northwestern India: evidence from deep reflection profiling

Abstract

The results of deep reflection profiling studies carried out across the palaeo-meso-Proterozoic Delhi Fold Belt (DFB) and the Archaean Bhilwara Gneissic Complex (BGC) in the northwest Indian platform are discussed in this paper. This region is a zone of Proterozoic collision. The collision appears to be responsible for listric faults in the upper crust, which represent the boundaries of the Delhi exposures. In these blocks the lower crust appears to lie NW of the respective surface exposures and the reflectivity pattern does not correspond to the exposed blocks. A fairly reflective lower crust northwest of the DFB exposures appears to be the downward continuation of the DFB upper crust. The poorly reflective lower crust under the exposed DFB may be the westward extension of the BGC upper crust at depth. Thus, the lower crust in this region can be divided into the fairly reflective Marwar Basin (MB)-DFB crust and a poorly reflective BGC crust. Vertically oriented igneous intrusions may have disturbed the lamellar lower-crustal structure of the BGC, resulting in a dome-shaped poorly reflective lower crust whose base, not traceable in the reflection data, may have a maximum depth of about 50 km, as indicated by the gravity modelling. The DFB appears to be a zone of thick (45-50 km) crust where the lower crust has doubled in width. This has resulted in three Moho reflection bands, two of which are dipping SE from 12.5 to 15.0 s two-way time (TWT) and from 14.5 to 16.0 s TWT. Another band of subhorizontal Moho reflections, at ≈ 12.5 s TWT, may have developed during the crustal perturbations related to a post-Delhi tectonic orogeny. The signatures of the Proterozoic collision, in the form of strong SE-dipping reflections in the lower crust and Moho, have been preserved in the DFB, indicating that the crust here has not undergone any significant ductile deformation since at least after the Delhi rifting event.