INTERPLAY BETWEEN FAULTING AND BASE LEVEL IN THE DEVELOPMENT OF HIMALAYAN FRONTAL FOLD TOPOGRAPHY

Topography preserves a potentially accessible record of the structure and evolution of an underlying fault system, provided we understand the factors that shape that topography. Studies of normal fault growth in the Basin and Range have observed that footwall topography is controlled by the fault geometry the flanking local base levels. We hypothesize that the first-order controls on thrust-fault generated fold topography are similar to those in extensional settings. We test this hypothesis by examining the morphology and geologic structure of two actively growing folds in the northwest Himalayan front.

The Chandigarh and Mohand anticlines show the following patterns: (1) most (~60-70%) growth in relief across multiple scales and catchment size is accomplished within ~5 km from the fold tips, (2) relief is partitioned unevenly between the fold flanks due to base level variations and thus catchment outlet elevation, (3) hinterland-flank catchments have relatively steeper slopes than those on the foreland flank, (4) high hillslope-scale relief is limited to high rock uplift areas and certain lithologies, and (5) existing relief represents only ~15% of the total rock eroded since faulting began implying significant erosion.

The fold topography is developed rapidly and asymmetrically as a result of the interplay between thrust fault-generated uplift (which sets the space available for the fold and the distribution of rock uplift rates) and local base level (which affects the ability of the landscape to respond by erosion). A linear rate of growth in catchment relief with range half-width correlates with fault geometry, suggesting that this is a key metric that may be used to infer fault dip at depth. In these settings, fast slip rates, weak uplifting rocks, and rapid, monsoon-driven erosion may combine to quickly limit the topographic growth of emerging folds and potentially disconnect fold morphology from the displacement field.