Dating tectono-thermal events within the crystalline series of the Himalaya, the Kullu valley, NW India

Abstract

The Greater Himalayan series preserve valuable geological information revealing a complex deformational and thermal history since the collision between India and Eurasia. This research employed field structural geology, microstructural analysis and geochronometers such as 40Ar/39Ar and U–Pb to reconstruct the deformational and thermal history of the crystalline series within the Kullu Valley, NW India. Key localities featuring geologically important structures within and in the vicinity of the Phojal recumbent fold were studied: an S-type gneiss (top-gneiss-series) at the structurally-highest part of the fold, deformed and undeformed leucogranites intruded into the crystalline series, and the phyllonites deformed by the Main Central Thrust. The sequencing of the thermal and deformational events that have been preserved within the microstructure of the Greater Himalaya series in the Kullu Valley indicates a complex tectonic history that can be summarised as: i) a leucogranite formation event in the mid-Eocene; ii) a deformation event in the late-Eocene; iii) regional Barrovian metamorphism in the early- Oligocene; iv) SW-verging recumbent folding in the Oligocene; v) extensional South Tibetan Detachment-related shearing in the Oligo-Miocene; vi) an early-Miocene static thermal event; and vii) mid-Miocene thrusting over the Main Central Thrust, followed by regional uplift in the Pliocene that resulted in the surface exhumation of the series. This study shows that the tectonic models that have been proposed for the evolution and exhumation of the Greater Himalaya might not be applicable to the NW Himalaya. The tectonic history revealed by the present study is inconsistent with the tectonic models involving: i) channel flow—because the Main Central Thrust and the South Tibetan Detachment were not coeval in the NW Himalaya; ii) fold-nappes—because the Main Central Thrust operated at least 10 Myr after the recumbent folding; and iii) wedge tectonics—because the South Tibetan Detachment was not folded by the Phojal fold. These models assume that the Himalaya evolved in a purely compressional tectonic regime since the continental collision. It is shown in this thesis that the model of tectonic mode switches is more applicable to the NW Himalaya—a compressional mode was in effect until the Oligocene, then from the late-Oligocene to mid-Miocene a switch to extension occurred and finally the mode switched back to compression from the mid-Miocene until the present day. It appears that the tectonic history of the crystalline series of the Himalaya has been controlled mainly by the kinematics of the subducting Indian plate, whereby a more dominant convergence induced crustal shortening, and a more accentuated roll-back stimulated crustal extension. Show more