With a long and complex deformational history since the initiation of the collision between India and Eurasia in the early Cenozoic, the Himalaya has undergone multiple deformational and thermal events. The effects of these events are preserved in the microstructural patterns defining temporal sequences that are consistent in the rocks. These microstructural patterns appear to be indicative of broader-scale processes. Therefore, an effective way of studying the orogeny is to characterise and sequence the microstructures using Tectonic Sequence Diagrams (TSDs) and various geochronology methods.
This method has been applied to the crystalline core of the Himalaya in the Kullu Valley, NW India, with the aim of gaining a better understanding of how and when the deformational and thermal events took place. This, on a larger-scale, has important implications for the evolution and exhumation of the crystalline series and its km-scale SW-verging recumbent folds. These recumbent folds are bounded below by major crustal-scale shear zones of the Main Central Thrust (MCT), and above by the South Tibetan Detachment (STD). These folds and the MCT and STD shear zones have been the core of many tectonic models, which have attributed the exhumation of the folded series to the operation of the MCT and STD. Little evidence has been provided, however, regarding the temporal and genetic connections between these structures.
The findings of this PhD project reveal a fundamental disagreement with tectonic models involving channel flow, fold-nappe formation and tectonic wedging, which assume a pure compressional tectonic regime has been in effect since the collision. The tectonic history proposed here suggests that the evolution of the crystalline series, at least in the north-western part of the orogen, did not take place during a single tectonic mode. In contrast to previous models, exhumation in the NW Himalaya was a complex process that best can be explained by “tectonic mode switches” involving several switches between crustal shortening and extension.