Validating models of channel flow, ductile extrusion and exhumation - The Greater Himalayan Sequence, Annapurna Massif

Himalayan orogenesis is commonly explained with models of channel flow, in which the metamorphic core, referred to as the Greater Himalayan Sequence (GHS), forms a partially molten, rheologically weak mid-crustal flow. Geochronological and thermobarometric studies from the Himalaya provide support for the channel flow model, however, strain-related model predictions are unresolved and the model remains controversial. Additionally,
wedge-extrusion, underplating / thrust-stacking and tectonic wedging models are favoured by many as alternative explanations for the formation of the Himalaya.

In this thesis, strain-related predictions of the channel flow model for Himalayan orogenesis are tested with field-based structural studies together with laboratory-based microstructural and magnetic fabric analyses. Orogen-perpendicular transects along the Modi Khola and Kali Gandaki valleys in the Annapurna-Dhaulagiri Himalaya of central Nepal were chosen for study. Samples were collected from the GHS and bounding units for crystallographic preferred orientation (CPO) and anisotropy of magnetic susceptibility analyses (AMS). Both techniques were used to quantify deformation fabric strength, which provides a proxy for relative strain magnitude. These data, combined with geochronological and thermobarometric constraints, reveals the kinematic evolution of the GHS in the Annapurna-Dhaulagiri Himalaya. These data can be directly compared to predictions implied by the channel flow model in order to assess its validity.

The results support the channel flow model as a viable explanation of the mid-crustal evolution of the GHS. However, lower temperature deformation indicates that exhumation of the GHS was facilitated through wedge-extrusion and thruststacking. The development of the Himalayan orogen in the Annapurna-Dhaulagiri region is best explained by models that allow channel flow, wedge extrusion and thrust-staking to occur in a single orogen. Similar ‘composite models’ for Himalayan orogenesis have been proposed recently by other authors and reflect a growing understanding of how rheological controls on orogenesis can vary both spatially and temporarily.