A thermal event in the Dolpo region (Nepal): a consequence of the shift from orogen perpendicular to orogen parallel extension in central Himalaya?

Abstract

In the Lower Dolpo region (central Himalaya), structurally above the South Tibetan Detachment System (STDS), blastesis of static micas have been recognized. Nevertheless, until now, very little work has been done to constrain the tectonic meaning and the timing of this static mica growth. In this work we investigate samples from the STDS hanging wall, characterized by three populations of micas, defining (i) S1 and (ii) S2 foliations, and (iii) M3 static mineral growth cutting both foliations. New geochronological 40Ar/39Ar analyses on the microtexturally different micas, complemented by microstructural and compositional data, allow us to place temporal constraints on the static (re)crystallization at the STDS hanging wall. Results reveal homogeneous chemical compositions and ages of micas within the investigated samples, irrespective of the structural positions. Phlogopite and muscovite on S1 and S2 and post-kinematic biotite yielded 40Ar/39Ar ages within 14–11 Ma with decreasing ages upward. We suggest that mica (re)crystallized under static conditions during a late thermal event at low structural levels (c. 15–18 km), after cessation of the ductile activity of the shear zone. We hypothesize that this later thermal event is kinematically linked to the switch from orogen perpendicular to orogen parallel extension in central Himalaya.Supplementary material: Electron microprobe analyses of biotite and white mica are available at https://doi.org/10.6084/m9.figshare.c.5509998Thematic collection: This article is part of the Isotopic Dating of Deformation collection available at: https://www.lyellcollection.org/cc/isotopic-dating-of-deformation