Abstract:
The Main Frontal thrust (MFT) is the most active deformation front since the late-Pleistocene. However, data on spatiotemporal variations in the activity and onset-timings of the MFT are still limited. We analyzed the chronologically-constrained geomorphic markers from the Nahan Salient, NW Sub-Himalaya, India to understand spatio-temporal deformation variability along the MFT in this region.
We investigated the structural and geomorphic records of deformation to constrain the shortening rates across the MFT. The geometry of uplifted river terraces was mapped using high-precision, Real Time Kinematics Global Positioning System (RTKGPS) and chronology were established through optically stimulated luminescence (OSL) dating of the fluvial deposits overlying the strath. Field data based structural cross-sections were superimposed with uplifted terraces to estimate uplift /shortening rates.
Geomorphic indices, landform, chronology and structure data suggest that MFT is segmented and characterised by different geometry in this region. Holocene average uplift rates are significantly higher in the NW (~7 mm.a-1) and SE (~6 mm.a-1) segments but it is lower at middle segment (~1 mm.a-1). Further, our estimated average uplift rates are consistently higher in late-Holocene ~6-7 mm.a-1 (averaged over ~2-4 ka) in comparison to Early Holocene ~1 mm.a-1 (averaged over ~11-12 ka). It suggests that minimum average shortening rates range from ~1-3 mm.a-1 (averaged over ~11-12 ka) to ~10-11 mm.a-1 (averaged over ~2-4 ka) (assuming MFT fault plane dip 30) across the middle and NW & SE segment of the MFT respectively. These higher shortening rates at late-Holocene are in agreement with previous studies from adjacent areas. The local geometry of MFT is variable from NW to SE region and is reflected in local topography as well. The higher uplift regions (NW and SE segment) indicate more rugged terrain, higher local relief, and high base-level in comparison to middle segment. Therefore, we conclude that MFT in the Nahan Salient area is segmented with different fold/fault geometry and some segments are characterized by an increase in uplift rate during late Holocene. The NW and SE segments of MFT have accommodated ~70% (10 mm.a-1 out of 14 mm.a-1) of the total Sub-Himalayan shortening in the Nahan salient over the late-Holocene span.