TECTONIC EVOLUTION OF THE GREATER HIMALAYAN SEQUENCE, ANNAPURNA-DHAULAGIRI HIMALAYA, CENTRAL NEPAL

Understanding the tectonic evolution of the Himalayan orogen requires knowledge of the mid crustal deformation processes that occur during collision. We use variations in crystallographic preferred orientation (CPO) measurements and deformation microstructures observed in rocks from the metamorphic core of the orogen, the Greater Himalaya Sequence (GHS), to determine the kinematic evolution of the Annapurna-Dhaulagiri Himalaya.

Evolution of the Himalayan orogen is commonly explained with models of channel flow, which describe the GHS as a partially molten, rheologically weak mid-crustal channel. Extrusion and exhumation of the channel was facilitated via coeval movement on reverse- and normal-sense shear zones that bound the channel below and above respectively. Whilst many thermobarometric studies support the occurrence of channel flow, the spatial and temporal distribution of strain within the GHS is one aspect of the model that is yet to be fully resolved.

Quantified variations in CPO strength provide a proxy for relative strain magnitude. CPOs are measured via Electron Back Scattered Diffraction (EBSD) from samples collected along a N-S transect through the Kali Gandaki valley of central Nepal. An eigenvalue-based intensity parameter is calculated to describe the strength of each CPO fabric and stratigraphically arranged into a single plot to produce a quantified relative strain magnitude profile. Combining this strain profile with field-structural and microstructural observations and thermobarometric constraints reveals the kinematic evolution of the GHS in this region of Nepal. Importantly, low volumes of leucogranite and sillimanite bearing rocks and evidence of reverse-sense shearing overprinting normal-sense shearing at the top of the GHS suggests that the GHS was stronger here than in the eastern Himalaya (e.g. Everest-Makalu and Bhutan regions) sugggesting that channel flow was not as intense in the Annapurna-Dhaulagiri Himalaya relative to other regions.