Influence of Inherited Indian Basement Cross-Strike Structures on the Evolution of the Himalayan Middle and Upper Crust

Abstract:

The Himalaya is the result of on-going convergence and collision of India and Asia. Knowledge of the configuration of the Indian plate prior to collision with Asia is often over-looked, despite its importance in controlling the subsequent evolution of the orogen. Three fault-bounded northeast-trending paleotopographic ridges of Precambrian Indian basement underlie the Ganga Basin south of the Himalaya. Analysis of spectrally filtered Bouguer gravity data and edges in its horizontal gradient at different source depths suggests that they extend as far north as the underplated Indian lithosphere beneath the Asia crust (Bagong suture), and as deep as the base of the Indian lithosphere. Along-strike diachronous deformation and metamorphism within the Himalayan metamorphic core in west-central Nepal, documented by U-Th/Pb geochronology, as well as lateral ramps in the foreland thrust belt, spatially correspond to faults in the Indian basement bounding the subsurface Faizabad ridge.

Analogue centrifuge modeling confirms that offset along such deep-seated basement faults can affect the location, orientation, and type of structures developed in the mid- and upper crust at various stages of orogenesis. Our models suggest that (1) deep-seated, reactivated basement faults can localize structures in the upper crust during different stages of orogen evolution, and (2) it is mechanically feasible for movement along a basement fault to influence the mid- and upper crust and for strain to propagate through a low-viscosity medium.

We suggest that these major orogenic cross-strike structures may have affected the ramp-flat geometry of the basal Main Himalayan thrust, in turn partition the Himalayan range into distinct zones, and ultimately contribute to lateral variability in tectonic evolution along the orogen’s strike. Our interpretation also suggests that south Tibet graben are spatially related to deep-seated lithospheric-scale faults rooted in the underplated Indian crust.