Deformation at the contact between the Higher Himalayan Crystalline and the Tethyan Sedimentary Sequence: Thrusting versus normal faulting conundrum

The contact between the northern edge of the Higher Himalayan Crystalline (HHC) and the overlying Tethyan Sedimentary Sequence (TSS) has long been debated as either a thrust or a normal fault. Initially thought to be a thrust contact, it was later recognized as a zone of crustal-scale normal faults dipping to the north, known as the South Tibetan Detachment System (STDS). This suggests that the overlying TSS has moved northward along the contact relative to the HHC footwall. The cause of the initiation of such a crustal-scale normal-fault system in a convergent setting remains poorly understood, which motivates the present study to re-examine the structure of the HHC-TSS contact in the Dhauliganga valley of the Garhwal Himalaya. Nevertheless, we identified a series of normal faults cutting across the regional foliation of the HHC-TSS rocks during our field investigation, characterized by intense brecciation and gouging, consistent with upper-crustal brittle deformation. Our field observations suggest that these faults primarily formed during the waning phase of Himalayan growth and are unrelated to the northward slip of the TSS over HHC, as these normal faults cut across all dominant structural elements, including the migmatitic layering of HHC at high angles.  In addition, we found a spectacular ductile shear zone within the Milam Formation of the TSS, located directly above the HHC. This zone provides strong evidence of south-vergent thrusting along the contact, as indicated by fold asymmetry, C-S structures, and low-angle Riedel shears, consistent with the Himalayan deformation. Microstructural studies of shear-zone samples reveal that quartz grains are predominantly stretched as we expect in a ductile shear zone, forming lenticular ribbons with high aspect ratios and undulose extinction, whereas the occurrence of smaller, unstrained grains along the edges of larger grains is indicative of subgrain rotation recrystallisation typical of high temperatures (~400°-500°C). XRD analysis further confirmed the presence of graphite in the mylonitized samples, and the alignment of graphite along shear fabrics suggests the influence of shear heating during their formation. Our new findings of deformation structures along the HHC-TSS contact recognize the importance of reevaluating and expanding our understanding of the structural evolution in this area, particularly in the context of the India-Eurasia collision. Based on field and microstructural observations, we suggest that the collision between India and Eurasia caused the TSS to thrust over the HHC, while steeply dipping normal faults that have affected all previous structural features in the HHC and TSS are a later phenomenon that helped the Himalayan mountain belt attain stability of the Himalayan wedge from a supercritical stage.