Crustal structure beneath the Sub-Himalayan fold-thrust belt, Kangra recess, northwest India, from seismic reflection profiling: Implications for Late Paleoproterozoic orogenesis and modern earthquake hazard

摘要

Most compressional orogens include salients and recesses along their strike, the age and origin of which can be hard to ascertain. In the Kangra recess in the trace of the Main Boundary Thrust (MBT), the largest such recess within the active Himalayan orogen, the Sub-Himalayan sedimentary fold-thrust belt increases in width to as much as 90. km (the Kangra Basin), but narrows to as little as 10. km in the adjoining Nahan salient of the MBT (the Subathu Basin) to the southeast. New seismic reflection profiling places the Himalayan décollement at 6-8. km depth above thin but reflective Meso- to Neoproterozoic Vindhyan (Lesser Himalayan Series-equivalent) strata. These data show that the Vindhyan sedimentary rocks are thinner in the Kangra recess than further southeast, allowing the hypothesis that the width of the Lesser Himalayan thrust belt, and the existence of the Kangra recess, could be related to the pre-deformation basin thickness. This hypothesis obviates the need for control of the Kangra recess by a lateral ramp in the Main Himalayan Thrust, so making it more likely that the Kangra segment could rupture as part of an earthquake far larger than the devastating 1905. M = 7.8 Kangra earthquake. Below the Proterozoic sedimentary rocks, our reflection data show a west-southwest-dipping reflective fabric spanning a 30. km-crustal thickness, which we infer corresponds to a widespread "Ulleri-Wangtu" orogenic event at c. 1850. Ma affecting a pre-Tethyan Indian continental margin, thickening the basement by c. 20%. The deepest 10. km of this ~ 50. km-thick crust shows a more horizontal, arguably younger, reflectivity, though the Moho is not clearly marked by strong reflectors.