Velocity modeling of a complex deep crustal structure across the Mesoproterozoic south Delhi Fold Belt, NW India, from joint interpretation of coincident seismic wide-angle and near-offset reflection data: An approach using unusual reflections in wide-angle records

Abstract

[1] Seismic wide-angle record sections from favorably placed source and limited-aperture recording spreads, particularly across collision boundaries, reveal bright unusual high-velocity contrast reflections of limited lateral coherence. The arrival times of the unusual reflections for recording updip of the reflecting plane align as high apparent velocity phases at wide-angle offsets. Coincident near-offset reflection images from similar structural features provide additional control for modeling the unusual phases on the wide-angle record sections. We propose an approach for joint interpretation of these coincident seismic data sets and modeling the unusual phases on the processed wide-angle records to delineate both the velocity stratification and the geometry of the deep crustal structures consistent with the near-offset reflection images. Processed and migrated near-offset reflection images reveal several steeply dipping isolated reflections on the western margin of the south Delhi Fold Belt (DFB), NW India, interpreted as the south Delhi thrust fault. Coincident limited-aperture, wide-angle records reveal bright unusual reflections, which are also found to be associated with the same thrust fault. Observations of similar dipping reflection patterns in various other regions lend support to consider them as genuine in-plane reflections resulting from a structural fabric imprinted on horizontally oriented geological structures. We use the reflectivity structural model from the near-offset data set that provides necessary constraints for modeling and simulating the travel times and amplitudes of the unusual reflections recognized in the wide-angle records. The unusual reflections are effectively used as part of the wide-angle data set for Gaussian beam synthetic seismogram modeling, thus delineating for the first time a complex 2-D model of the seismic velocity structure of the deep crust and uppermost mantle across the Marwar Basin (MB) and the DFB. The dipping events revealed in the wide-angle data set are successfully modeled and interpreted as reflections from the thrust fault, consistent with the near-offset reflection image. The 2-D velocity model, parameterized as a stack of isolated dipping reflectors embedded in deep crustal layers, is successively refined by including the structural features recognizable both in the near-offset reflection image and in the wide-angle records. The unusual reflections are modeled as a set of discrete reflectors representing local velocity variations imprinted on the background of a deep crustal velocity structure. The lamellar structure revealed by short, isolated dipping and the subhorizontal middle to lower crustal reflections suggests heterogeneities at scale lengths on the order of at least the Fresnel zone diameter as resolved here. The local velocity variations in the deep crust can be considered analogous, on a different scale, to the models in which velocity heterogeneities on scale lengths of the seismic wavelength constitute a medium of random distribution of scatterers. The crustal-scale south Delhi thrust fault on the western margin of the DFB, delineated as the stack of reflectors dipping 25°–30°SE, probably developed during the Mesoproterozoic collisional episode related to the Delhi orogeny, contemporaneous with the formation of Rodinia. It is concluded that complex structural features developed during collisional episodes can survive through geological periods, longer than generally expected, depending on the accretionary characteristics at the time of orogenic evolution. The 2-D velocity model further reveals significant variations of crustal seismic structure, with a relatively sharp and shallow Moho (at 39–40 km) in the MB and a deeper Moho (at 45–47 km) underlying a 12 km thick subhorizontal lamella of high-velocity (6.9–7.5 km/s) layers in the DFB regions.