Differential Response of Watersheds to Tectonic Forcing along the Himalayan Frontal Thrust (Hft): Mohand Frontal Ridge, NW Himalaya
The Mohand Frontal ridge is the surface manifestation of the fault-related folding in the hanging wall of the active Himalayan Frontal Thrust (HFT). Continued convergence along the HFT has led to the emergence of the fold/ridge that resulted in the development of the watersheds on either flank of the ridge. The watersheds have also evolved through the evolution of the fold/ridge. The geometry/shape of these watersheds is marked by a distinct asymmetry across the ridge crest. The watersheds on the southern flank are large and poorly elongate whereas those on the northern flank are small and highly elongate. The number of watersheds on the southern flank is also significantly lower than those on the northern flank. This differential response of the watersheds on either flank of the ridge to the same tectonic forcing along the HFT is investigated using the morphometric approach. The analysis reveals that the watersheds on the southern flank are characterized by the relatively large number of first order streams that do not obey the law of stream number. This results in a higher bifurcation ratio for the first order streams suggesting that these streams have grown by headward erosion creating large poorly elongate watersheds. On the contrary, no such anomalies are found in the northern flank watersheds. The morphometric data clearly indicate that the two flanks are characterized by significant differences in the evolutionary processes that are related to stream network. These differences are mainly responsible for the differences in the geometry/shape of these watersheds. Based on all the dataset it seems that the watersheds on the southern flank have actively responded to the change in topographic slope caused by the slip along the HFT. On the contrary, the watersheds on the northern flank have just passively grown smaller in response to the active enlargement of watersheds on the southern flank. These processes also account for the significant difference in the number of watersheds on each of the flanks of the Mohand frontal ridge.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009