STRUCTURAL ANALYSIS OF EXHUMATION OF THE HIMALAYAN METAMORPHIC CORE, GARHWAL HIMALAYA

Structure associated with the transition from ductile to brittle deformation of the Greater Himalayan Sequence (GHS) is key to understanding exhumational mechanisms of the metamorphic core of the Himalayan Orogen and other mountain systems. We are investigating the structure of exhumation in the GHS of the Garhwal region, which exhibits three deformation events. D1 is represented by layer parallel foliation (S1) and general shear with flow mostly to the SW as manifest by rotated porphryoclasts, isoclinal folds with axes trending WNW-ESE (F1), and crenulation axes (L1). D2 is partitioned between the upper and lower GHS. D2 of the lower GHS is expressed as asymmetrical folds with wavelengths up to 5 km, and crenulation axes trending NE-SW (F2a). Much of the GHS is duplicated by these large folds. D2 in the upper GHS is primarily pure-shear, layer-parallel extension expressed as oblate-shaped boudins up to 300 meters in length. Less prevalent simple shear is expressed as asymmetrical folds (F2b) with axes trending NNW-SSE and axial planes dipping west, which indicate top down to the ENE. D3 is a brittle phase of exhumation-related deformation involving both shortening and extension. At the base of the GHS, SW verging thrust faults crosscut ductile features of the Main Central Thrust Zone. The extensional phase of D3 is expressed throughout the GHS and into the Lesser Himalayan Sequence as conjugate fractures, normal faults, and joints. The frequency of these features increases up structural section. Conjugate fractures and en echelon fracture arrays exhibit a vertical ó1 and ó3 predominately toward the NE. Normal faults are mostly top down to the NE. The top of the GHS is bounded by the Malari Detachment, which is part of the STDZ and juxtaposes unmetamorphosed Tethyan Himalayan Sequence (THS) with migmatitic grade GHS. The Malari leucogranite, which intrudes both the GHS and THS, is cut by this detachment. This implies a syn- to post-D2 emplacement for the Malari leucogranite. Our study indicates a partitioning of strain during D2 that has implications for testing the channel flow model. It also shows that the zone of hinterland extension in the Garhwal Himalaya is much broader than previously reported. This implies that the amount of extension across the range may be underestimated.