Evidence of back-folding in the Beas Valley puts Himalayan tectonic models on trial

Abstract

Field structural observations from the Himachal Himalaya, in the northwest of the mountain belt, challenge existing tectonic models and raise questions as to their validity. Microstructures and geochronological data reveal two discrete episodes of Barrovian metamorphism, the earliest during the Eocene-Oligocene transition, before an early period of recumbent folding. This metamorphic event occurred in association with a km-scale extensional ductile shear zone that is itself now recumbently folded on the km-scale, with an axial plane pressure solution cleavage. The Eocene-Oligocene gneiss complex is thus exposed in its core. The second episode of Barrovian metamorphism occurred in association with another regional-scale extensional shear zone during the Oligo-Miocene transition, thus synchronous to the South Tibetan Detachment System. This transects the recumbent fold stack. Microstructures show that the Main Central Thrust was initiated after the second phase of extension, and the associated second episode of Barrovian metamorphism had ceased operating. Further, the previously unrecognized km-scale Phojal Back-fold affects all of the above structures. Confusion caused by the misidentification of this structure led to the tectonic-wedge model, but this hypothesis can be invalidated by the structural evidence presented here. Our data support an alternative hypothesis that requires tectonic mode-switches in association with a succession of accretion events as India indents into Eurasia.