Karnali and Jajarkot Klippen in Western Nepal Himalaya Inconsistent with Tectonic Wedging Model Predictions

Abstract:

The Himalayan metamorphic core, exposed between two opposite sense shear zones, is locally preserved in a series of foreland klippen. The upper shear zone, the South Tibetan Detachment (STD), is a key element in many competing tectonic models. One of these models, tectonic wedging, requires that the STD merges with the reverse-sense basal shear zone towards the foreland. We tested this hypothesis in two foreland klippen in western Nepal. The Karnali klippe is a doubly-plunging synform underlain by a folded reverse-sense shear zone. It comprises amphibolite metamorphic facies rocks overlain by greenschist to subgreenschist facies sedimentary rocks. The contact is marked by a folded ca. 1 km thick normal-sense shear zone, which we correlate with the STD. Quartz and calcite recrystallization textures and quartz crystallographic preferred orientations suggest an abrupt decrease in temperature of deformation from ~750 °C in the footwall to 580 and 475 °C at the base and top of the shear zone, respectively, and to 150-200 °C in the hanging wall. In-situ monazite petrochronology indicates prograde metamorphism between 36 and 30 Ma in the immediate footwall of the STD, followed by tectonic exhumation from 28 to <24 Ma, possibly starting as early as 30 Ma. Preliminary muscovite ⁴⁰Ar/³⁹Ar ages suggest that deformation along the STD ceased by ca. 18 Ma. Field data from the adjacent Jajarkot klippe indicate a similar first order structural architecture, although protoliths, metamorphic grade and deformation temperature differ significantly. Transport-parallel exposure of the STD in this area implies a minimum slip of 165 km. The presence of the STD on both flanks of the Karnali and the Jajarkot klippen is inconsistent with predictions that the STD merges at depth with the basal shear zone in the Karnali klippe and north of the Jajarkot klippe. Our observations are consequently not compatible with the tectonic wedging model proposed for western Nepal.