Inherited Cross‐Strike Faults and Oligocene‐Early Miocene Segmentation of the Main Himalayan Thrust, West Nepal

Abstract

AbstractThe geometry of orogenic basal detachments controls the location and partitioning of seismicity, yet how it influences past episodes of orogenesis is often overlooked. We integrate recently published microstructural, phase equilibria modeling, monazite U‐Th/Pb petrochronology, and white mica 40Ar/39Ar geochronology data from the Greater Himalayan sequence to characterize the geometry of the Himalayan basal detachment, the Main Himalayan thrust (MHT), during the Oligocene and early Miocene. The Karnali and Jajarkot klippen, located at equivalent along‐strike positions in the direction of tectonic transport in the foreland of west Nepal, record contrasting pressure‐temperature‐time‐deformation paths. Units in both klippen reached similar peak pressures with a 5‐ to 10‐Myr time difference: Units in the Karnali klippe were exhumed while units in the Jajarkot klippe were buried. We interpret the contrasting metamorphic pressures between 35 and 25 Ma to reflect the influence of a vertical or steeply east dipping lateral ramp (or tear fault) in the MHT between the two klippen. The location of this putative Oligocene MHT ramp coincides with other evidence for a MHT lateral ramp since the Pleistocene to the present. Segmentation of the MHT in west Nepal is therefore an active and protracted phenomenon that started in the Oligocene or earlier. We interpret this offsetting of the MHT to be linked to the Lucknow basement fault, a spatially coinciding inherited cross‐strike structure in the subducted Indian lithosphere. Our study highlights the important applications of metamorphic petrology and petrochronology to seismotectonics by illustrating the timescales of lateral segmentation of an orogenic basal detachment.