THE GEOMORPHIC RESPONSE TO ACTIVE FOLDING OF THE SIWALIK HILLS IN NORTHWEST INDIA

Studies of normal fault growth in the Basin and Range have observed that footwall topography is (a) limited by the width of the range and hence (b) ultimately controlled by fault geometry and the flanking base levels. Here we hypothesize that the first-order controls on fold topography are similar to those in extensional settings. We test this hypothesis by describing correlations between topography and tectonics as well as the erosion processes of the Mohand and Chandigarh anticlines, actively growing folds resulting from shortening along the Himalayan Frontal Thrust (HFT) in northwest India.

In general, fold flank catchment relief and along-strike range width increase to their maximum values ~8-12 km from the fault tips and remain relatively uniform thereafter. However, northern flank catchment relief in the Mohand subsequently decreases towards the fold center concurrent with rising local base level. The southern fold flanks are characterized by high relief, dendritic drainage, moderate vegetation cover, wide channels incised into bedrock and lower relative mean precipitation. Northern flanks possess similar-to-less relief, parallel-to-dendritic drainage, but substantial vegetation cover, less incised channels, and higher precipitation. Within the fold topography we observe thin regolith (~10-50 cm), abundant landslide scars and deposits, and wide, shallow channels filled with gravel. Comparison of across-strike topography with HFT subsurface geometry shows that relief correlates positively with uplift magnitude. Drainage divides are offset into the ranges ~2-10 km relative to the fold axes and southern flank base levels are ~50-150 m lower than northern flanks. Erosional mass balance estimates suggest that current mean topography represents ~15-30% of the total rock uplifted since faulting began.

We conclude that, similar to normal faults, the across-strike topographic relief is a reflection of the combined effects of (a) the displacement field determined by the fault geometry and (b) a significant contrast in base level across the folds resulting from intermontane sediment storage on the hinterland flanks relative to the foreland flanks.