Thesis Defence: Structural Evolution of the South Tibetan Detachment System in the Tsum Valley, Central Himalaya
December 5 at 9:30 am - 1:30 pm
Danny Richard, supervised by Dr. Kyle Larson, will defend their thesis titled “Structural Evolution of the South Tibetan Detachment System in the Tsum Valley, Central Himalaya” in partial fulfillment of the requirements for the degree of Master of Science in Earth and Environmental Sciences.
An abstract for Danny Richard’s thesis is included below.
Defences are open to all members of the campus community as well as the general public. Registration is not required for in-person defences.
ABSTRACT
The South Tibetan Detachment System (STDS) is a network of orogen-parallel, low-angle normal faults and shear zones traced along the entire length of the Himalayan orogen. It played a critical role in facilitating large-scale, normal-sense movement between the upper and middle crust during convergence. Despite its significance, key controls on the deformation processes of the STDS remain poorly understood. This study presents a time-resolved deformation history of the STDS in the Tsum Valley (Manaslu region, central Nepalese Himalaya), detailing its kinematics, deformation
mechanisms, and stress history.
In the Tsum Valley, the STDS is a ~2500 m wide zone of ductile deformation that includes Greater Himalayan Sequence (GHS) and Tethyan Sedimentary Sequence (TSS) rocks, without a discrete brittle detachment at higher levels. In-situ Rb-Sr geochronology indicates that deformation and cooling within the STDS occurred between 17-13 Ma. Deformation occurred under monoclinic-dominant general shear flow. Distinct flow geometries between the rocks within the STDS and those at its boundaries imply that the boundary rocks record a different strain path, potentially linked to the onset of E-W extension in the region. STDS deformation was coupled with exhumation-driven cooling, resulting in the telescoping of isotherms from 500-600°C in the GHS to <400°C and then to <200-170°C in the TSS over ~1200m.
Together with the kinematic data, deformation mechanisms indicate that the actively deforming portion of the STDS migrated to higher structural levels – from the silicate-rich GHS to the carbonate-rich TSS – during progressive shearing and cooling. In the GHS, high-temperature ductile shearing became more localized with increasing structural level, with deformation style transitioning from penetrative, involving all major minerals, to localized within quartz and mica layers/shear bands. In the TSS, marbles underwent dynamic recrystallization of calcite at high temperatures, transitioning to mechanical twinning during later stages of exhumation and cooling. The presence of carbonate rocks enabled deformation to persist in a ductile regime at relatively low temperatures and shallow crustal levels, without the formation of a discrete brittle structure. This study emphasizes that while structural level, depth, and temperature were significant factors, lithology was crucial in controlling the structural evolution of the Tsum Valley STDS.
