Active Tectonics of Himalayan Frontal Fault Zone in the Sub-Himalaya

Abstract

The Himalaya grew progressively with the development of fold-thrust systems towards foreland from north to south. The Cenozoic foreland basin of the Sub-Himalaya zone was developed in front of the rising Himalaya. It is bounded between the Main Boundary Thrust (MBT) and the Himalayan Frontal Thrust (HFT). The Himalayan Frontal Fault zone encompasses the frontal anticlines, dun (intermontane basins) and hinterland fold-thrusts. The tectonic geomorphological mapping and optical simulation luminescence (OSL) dating of geomorphic surfaces/sediments indicate Late Pleistocene-Holocene deformation in the fold-thrust system of the Himalayan Frontal Fault zone. The frontal anticlines are developed as fault-bend folds over the HFT. The backlimbs of the anticlines are largely characterised with formation of intermontane basins of duns. The surface ruptures of past earthquakes are recorded in paleoseismological-investigated trenches over the Himalayan front along the HFT trace, whereas epicentres of the earthquakes are in the Lesser Himalayan hinterland. The HFT represents the surface expression of the Main Himalayan Thrust (MHT). The slip generated over the earthquake rupture is transferred to the MHT-HFT. This suggests coseismic slip contributes incrementally to the growth of the frontal anticlines. Using strath surfaces over the hanging walls of reverse faults, the uplift and shortening rates are computed. The estimated shortening rates are 18–20 mm/year in the central Himalaya and 13–15 mm/year in northwest Himalaya during Late Pleistocene-Holocene. The long-term shortening rates are nearly consistent with the current rates derived through geodetic (GPS) method. In the Sub-Himalayan hinterland, there are out-of-sequence faults which show Late Pleistocene-Holocene activation. Late Pleistocene activation is observed along certain segments of the MBT. Normal faulting is ascribed largely to activation of the MBT, whereas left-lateral sense of motion along fault scarp in the alluvial fan covering the MBT is also observed.