Age constraints on the deformation style of the South Tibetan Detachment System in Garhwal Himalaya

Abstract

In the Garhwal Himalaya, the Malari granite, a small pluton, intrudes the upper portion of the Greater Himalayan Sequence (GHS) and is deformed in the South Tibetan Detachment System (STDS) shearing. In the present work, we undertook a multidisciplinary approach involving microstructural, chemical and geochronological analyses of the samples in order to constrain the timing of shearing along the STDS. Microstructural observations of samples suggest a switch in deformation features with structural position from the structurally upper samples to the structurally lower one. X-ray maps and electron microprobe analyses reveal a quite uniform chemical composition of muscovite in the three selected samples. 40Ar/39Ar muscovite ages constrain the waning stages of ductile deformation: in rocks ductilely deformed by the STDS activity, they record recrystallization at 16.0-16.5 Ma; the age of static muscovite grown after sillimanite at 14.3 Ma post-dates cessation of movement on the STDS and pre-dates the main activity of the Main Central Thrust in the Western Himalaya.