EVALUATING THE TEMPORAL EVOLUTION OF TOPOGRAPHY AND EROSION IN THE HIMALAYAN FOLD-THRUST BELT USING SEQUENTIAL CROSS SECTION RESTORATION

Reconstructions of cross sections across the Himalayan fold-thrust belt (FTB) can be used to analyze the evolution of deformation, topography and foreland basin (FB) migration with time. Using a balanced cross section based on high-resolution mapping and data collection in the Bhutan Himalaya, we calculated slip amounts along individual faults and used this data in a sequential kinematic model (2D Move) to reproduce the cross section.

Known parameters include geology observed at the surface, décollement dip (4.5°), and FB thickness (6 km). Adjustable variables are topography, crustal density and effective elastic thickness (EET). By sequentially deforming the cross section with different combinations of topography, EET, and density, we can determine which combination best reproduces the known variables. Three different topographic scenarios were evaluated: (1) topography generated using Python script that increases topographic elevation in areas where structural elevation is gained, (2) simplified topography based on the modern topographic slope (2° dip from 0-5 km elevation across 140 km), and (3) no topography.

The model using topography (1) with density of 2.80 g/cm³ and 55 km EET produced a 5.0° décollement and FB thickness of ~3.35 km. Using the same density and EET, topography (2) created décollement angle 5.1° and FB thickness ~3.4 km with similar geology exposed at the final erosional surface as the topography (1) model. The main difference between the 2 models is that model (1) formed local areas of steeper topography with locally better fits to observed geology.

The observed thickness of FB strata in Bhutan is 5-6 km. Using an input density of 2.95 g/cm³ and an EET of 55 km for topography (1) and 45 km for topography (3), FB thickness increased to ~3.9 km with topography (1), and ~4.2 km with topography (3) while both décollements steepened to ~5.5°. In the model with no topography, the FB is slightly thicker, but the model using topography (1) produced a better match to surface geology, especially in central and hinterland areas of the FTB where ~2-4 km more subsurface material exists.

Modeling with higher-resolution topography (1) produces better matches to surficial geology when considering erosional surface development in cross section reconstructions, and FB thickness remains underdeveloped in all models.