Late Quaternary deformation, strain partitioning, and growth of the fold and thrust belt of The Western Himalaya

Constraining millennial-scale fault slip rates and understanding the related structural architecture in active orogenic wedges might provide important assessment of future seismic risk evaluations. An increasing number of Holocene and Late Pleistocene deformation rates have been reported throughout the Western Himalayan frontal fold-and-thrust belt (FTB), which have illustrated that the deformation is divided along several arc-parallel fault splays rooted to the Main Himalayan Thrust décollement. Studies also report complex pattern of out of sequence faulting, as well as spatiotemporal variations in fault growth and lateral heterogeneity of the FTB. However, what drives this lateral heterogeneity is debated until now.

Late Pleistocene – Holocene deformed fluvial strath terraces across active faults in western Himalayan provide a measure of time-averaged fault slip rates along the Himalayan Frontal Thrust (HFT) and the Medlicott Wadia Thrust (MWT). While the Quaternary slip rates along the HFT varies from 3 – 12 mm/y, the slip rates from the ~700 km- long MWT is steadily high (7 – 9 mm/y).In contrast, previously published balanced cross-sections proposed slip rates of ~1 – 2 mm/y along the MWT. So from million-year to millennial timescale, there exists a significant temporal variation in fault activity. Single fault system like the MWT accommodate up to 50 - 60% of the total measured geodetic rates on Holocene/Late Quaternary timescales. These results document significant strain partitioning within the Sub-Himalaya and steady - high slip on the MWT which, beside the HFT could host the next big seismic event and damage the human-infested north India.