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

Chiara Montemagni (1), Salvatore Iaccarino (2), Chiara Montomoli (3), Rodolfo Carosi (2), Arvind K. Jain (4) & Igor M. Villa (1,5,6)
(1) Dipartimento di Scienze dell’Ambiente e della Terra, Università degli Studi di Milano Bicocca, Piazza della Scienza 4, 20126 Milano, Italy. (2) Dipartimento di Scienze della Terra, Università di Torino, v. Valperga Caluso 35, 10125, Torino, Italy. (3) Dipartimento di Scienze della Terra, Università di Pisa, v. S. Maria 53, 56126 Pisa, Italy. (4) CSIR-Central Building Research Institute, Roorkee -247667, Uttarakhand, India. (5) Institut für Geologie, Universität Bern, Baltzerstrasse 3, 3012 Bern, Switzerland. (6) Centro Universitario Datazioni e Archeometria, Università degli Studi di Milano Bicocca, 20126 Milano, Italy. Corresponding author e-mail: c.montemagni@campus.unimib.it.

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.

South Tibetan Detachment System, Garhwal Himalaya, leucogranite, ⁴⁰Ar/³⁹Ar geochronology.