Tectonic episodicity in the greater Himalaya, NW India

Abstract

The Himalaya is an orogenic welt within the Alpine-Tethyan mountain chain. The extant tectonic model for the Himalayan terrane stack entails continuous post-collisional convergence and persistent heating during burial and subsequent exhumation. An alternative hypothesis to this "continuous evolution" scenario involves episodic tectonic mode switching, a concept that has been documented in other orogens along the Alpine-Tethyan belt. This thesis therefore tests the possibility that there is episodic mode switching in respect to the evolution of the Greater Himalaya in its topographically high crystalline core, in Himachal Pradesh, NW India. The approach adopted employs microstructurally-focused 40Ar/39Ar geochronology, across key tectonic contacts and geological terranes of the Greater Himalaya, to constrain the timing and temperature evolution of individual deformation and metamorphic events. In the leading edge of the Greater Himalaya, geological structures in the Shimla klippe and Narkanda syncline were examined with the intent of structurally and geochronologically characterising the Main Central Thrust (MCT). However, argon geochronology of fabrics structurally above and below the tectonic contact revealed that these fabrics formed in the Eocene period or earlier, during the Mesozoic. Therefore, these shear zones, although they had previously been intimately linked with the MCT, are in fact significantly older structures. In the root zone of the Phojal fold nappe, microstructural analysis revealed the influence of two distinct thermal excursions at upper greenschist grade that occurred towards the cessation of spatially associated ductile shearing events. Mica samples from the recumbently folded fabric of the supposed 'nappe' yield complex argon apparent age spectra, from which it can be inferred that: i) there were two distinct extensional shearing events that had terminated respectively by ~35 Ma and ~24-21 Ma; ii) the Phojal fold appears to have developed some time in the Oligocene, and not in the Miocene as is now supposed; iii) the fold recumbently folded the first Eocene-Oligocene ductile shear zone before itself being overprinted and attenuated in the Miocene by extensional ductile shear zones; iv) tectonic sequence diagrams inferred from those rocks are incompatible with a structural location in the hanging wall close to a thrust. Consequently the MCT as described today was not evident as a thrust in the Kullu region during the formation of the Phojal fold. From the character and timing of tectonic sequences, it is inferred that the fold geometry developed as a result of tectonic mode switching. Argon diffusion modelling supports the conclusion based on microstructures that the M1 and M2 metamorphic events cannot have occurred as part of a single protracted heating event. There must have been cooling (and possible exhumation) in between. The research in this thesis demonstrates that the tectonic evolution of the Greater Himalaya is consistent with multiple episodes of crustal shortening followed by regional extension events. The timing of the tectonic mode switches is broadly compatible with mode switches as observed in orogens elsewhere along the Alpine-Tethyan chain.