Direct dating of fault slip in the Himalayan orogen

In the Himalayan range, about half of the India-Eurasia convergence has been accommodated by movement of rocks along ductile shear zones and semi- to brittle thrust faults. Over the last 30 Ma, these structures rooted on the Main Himalayan thrust (MHT), the principal basal décollement soling the orogenic wedge, have developed in sequence toward the foreland. Rupturing of locked portions of the MHT on the `critical zone' along the ramp has caused the region's most devastating earthquakes, including the magnitude-7.8 Gorka earthquake. Constraining the rates of movement along the surface expressions of the MHT is critical for comprehending the temporal distribution of fault-displacements within the frontal Himalaya at geological timescales. The timing of movement along the deeper, hotter part of the MHT, exposed today as the Main Central thrust has been constrained through direct dating of accessory minerals formed during ductile deformation. However, the timing of movement on the younger, brittle frontal thrust faults has only been previously inferred by indirect methods. This project aims to directly date major movements on fault surfaces in the frontal part of the Himalayan orogen, in order to reconstruct the space-time distribution of major seismic events at geological timescales. We investigate the surface expressions of the MHT in the eastern Himalayan regions of Sikkim and Bhutan. We use petrographical, microstructural, and stable isotopic tools to characterise authigenic clays formed within fault gouges and fault-related calcite veins and slickenfibres. Ultimately, we will use K-Ar dating of clays within the fault gouge, and U-Pb dating of calcite, formed along the frontal Himalayan faults, to reconstruct an integrated displacement history along these first-order structures. Understanding of the spatial and temporal distribution of fault movement will help to inform earthquake hazard assessments in the region by identifying past locus of deformation and quantifying the long-term (>106 years) displacement rates on these structures.