An integrated geological and geophysical model of the Rampur Tectonic Window: implications for seismic hazard.

Abstract

The Rampur-Bushahr region in Himachal Pradesh hosts the Rampur Tectonic Window, where Lesser Himalayan metasediments are exposed within the Greater Himalayan Crystalline sequence. Although such windows are attributed to processes like duplexing, erosion, and ramp-related denudation, the subsurface geometry and its relationship with the Main Himalayan Thrust (MHT) remain poorly constrained. Understanding this structure is essential for evaluating strain partitioning and seismic hazard in the north western Himalayas. This study integrates geoid, gravity, seismic tomography, seismicity, and topography with surface geology using geophysical-petrological modelling (LitMod2D_2.0) to constrain the crustal architecture beneath the window. The results suggests a Ramp–Flat–Ramp geometry of the MHT, consistent with regional seismic observations. Alternative models with plausible deviation from flat-ramp-flat geometry yield similar misfits in gravity data but share a common first-order structure of a frontal ramp and flat segment. The Rampur Tectonic Window is interpreted as an exhumed culmination above a crustal ramp, where the flat segment acts as a major zone of strain accumulation, while adjacent ramps and bounding thrusts accommodate strain release. Despite its shallow sensitivity, available Bouguer gravity anomaly shows limited ability to distinguish between alternative geometries, highlighting non-uniqueness and the need for focused geophysical investigations. Seismicity and focal mechanisms indicate active back-thrusting and splay faulting, suggesting complex inter-seismic strain partitioning. The inferred MHT geometry has important implications for earthquake processes, where stress accumulation in the flat segment and transfer from deeper events may control bimodal rupture behaviour. These results emphasize the need for physics-based rupture simulations to better understand partial versus full rupture and improve seismic hazard assessment in the region.