Orogen-parallel deformation of the Himalayan mid-crust: Insights from structural and magnetic fabric analyses of the Greater Himalayan Sequence, Annapurna-Dhaulagiri Himalaya, central Nepal

The metamorphic core of the Himalaya (Greater Himalayan Sequence, GHS), in the Annapurna-Dhaulagiri region, central Nepal, recorded orogen-parallel stretching during midcrustal evolution. Anisotropy of magnetic susceptibility and field-based structural analyses suggest that midcrustal deformation of the amphibolite facies core of the GHS occurred under an oblate/suboblate strain regime with associated formation of low-angle northward dipping foliation. Magnetic and mineral stretching lineations lying within this foliation from the top of the GHS record right-lateral orogen-parallel stretching. We propose that oblate strain within a midcrustal flow accommodated oblique convergence between India and the arcuate orogenic front without the need for strain partitioning in the upper crust. Oblate flattening may have also promoted orogen-parallel melt migration and development of melt-depleted regions between km3 scale leucogranite culminations at ~50–100 km intervals along orogen strike. Following the cessation of flow, continued oblique convergence led to upper crustal strain partitioning between orogen-perpendicular convergence on thrust faults and orogen-parallel extension on normal and strike-slip faults. In the Annapurna-Dhaulagiri Himalaya, orogen-parallel stretching lineations are interpreted as a record of transition from midcrustal orogen-perpendicular extrusion to upper crustal orogen-parallel stretching. Our findings suggest that midcrustal flow and upper crustal extension could not be maintained simultaneously and support other studies from across the Himalaya, which propose an orogen-wide transition from midcrustal orogen-perpendicular extrusion to upper crustal orogen-parallel extension during the mid-Miocene. The 3-D nature of oblate strain and orogen-parallel stretching cannot be replicated by 2-D numerical simulations of the Himalayan orogen