Uplift and shortening in the Nepalese and Indian Sub-himalaya determined with quantitative geomorphology

Open Access
Author:
Meghani, Nooreen Amina
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
August 18, 2014
Committee Members:
  • Eric Kirby, Thesis Advisor
Keywords:
  • geomorphology
  • stream channel incision
  • Sub-Himalaya
  • Main Frontal Thrust Fault
  • Siwalik Hills
Abstract:
The rates and distribution of active shortening throughout the Himalayan foreland have been the subject of intense study for the past several decades. Despite substantial progress, the distribution of slip on specific faults remains poorly understood. However, studies of surface process response to tectonics over the last several decades have found that responses can be quantified in relation to the magnitude of perturbation. In particular, numerous studies suggest that fluvial systems developed above growing folds exhibit strong correlations with uplift rate. Here, we build on these studies to develop a quantitative, empirical calibration of the scaling between channel steepness index (ksn – a measure of channel gradient normalized for drainage area) and uplift rate; we derive a geometric model of uplift rate using data of Lavé and Avouac (2000) and analyze channel profiles using a 30m DEM derived from SRTM data. Our analysis confirms a linear relationship between uplift rate and ksn; we apply this calibration to regions of similar lithology along the Baisahi anticline in western Nepal. At the Baisahi Anticline channel profiles exhibit smooth profiles with similar concavity, consistent with spatially uniform uplift rates above a planar ramp in the Main Frontal Thrust fault (MFT). Channel steepness indices along the western anticline are invariant with position along the anticline, suggesting a uniform uplift rate of 9 mm/yr ± 2 mm/yr. Along the eastern portion of the anticline, steepness indices increase systematically from west to east, suggesting that uplift rates increase toward the east. However, structural data suggest that this reflects an increase in the dip of the fault at depth. Our analysis suggests that the Baisahi anticline absorbs 19 mm/yr ± 3 mm/yr of active shortening. The Baisahi Anticline includes streams both within the lithology of the Bagmati calibration site and streams in the Upper Siwaliks. This allows us to create a secondary calibration that includes the scaling relationship between the Upper Siwaliks and uplift, which is distinct from the relationship see in the Lower and Middle Siwaliks. We then apply this secondary calibration to the Janauri Anticline of North-Western India. The mean shortening rate at the Janaui Anticline is ~6 mm/yr, 1/3 the shortening rate at Baisahi. This is consistent with geodetic studies that suggest a decrease in shortening rates from east to west along the Himalaya as a result of the counter-clockwise rotation of the Indian Plate with respect to the Eurasian Plate. Our results demonstrate how local calibration of channel profile adjustment to uplift rate can be used to place constraints on active deformation in regions where other geomorphic markers are sparse or absent. In addition, we find that geologic shortening rates determined with stream profile analyses nearly match geodetic rates, indicating that a majority of shortening in the Sub-Himalaya is ultimately accommodated on the MFT.