Abstract:
Balanced cross sections present valid and admissible interpretations of surface and subsurface data. Thermochronologic and foreland basin provenance data are other aspects that must be adhered to when producing balanced cross sections. If a cross section is balanced, there is an implicit kinematic sequence that will produce the illustrated geometry, allowing the production of a kinematic forward model of deformation beginning with the original pre-deformation geometry. I produced a new balanced cross section in central Nepal through the Himalayan thrust belt from hinterland to foreland and forward modeled the cross section in 2D Move. A kinematic forward model illustrates the sequence of displacement on each fault; however, it does not include the effects on the basal decollement from flexure of the Indian plate due to loading, erosion of topographic relief, or isostatic rebound due to erosion. To address these shortcomings, a multi-step reconstruction was applied to the balanced cross section in 2D move that accounts for the loading of the Indian lithosphere as thrusts faults displace topography. This thrust induced load increases the dip of the basal decollement as a flexural response of the Indian crust after loading. As deformation propagates toward the foreland, the topographic profile evolves and the foreland basin migrates forelandward of the deformation front, increasing in stratigraphic thickness over time due to subsidence and resulting deposition of sediment eroded from the hinterland. The timing of erosional unroofing and deposition implied by the flexural isostasy model is inconsistent with provenance data from the foreland basin, suggesting that the implied kinematic sequence is not viable or that new timing constraints should be chosen. Because the thrust belt experienced rapid shortening, the subsurface thermal structure is too complex to estimate cooling ages from the flexural isostasy model using a constant geothermal gradient. Cooling ages should be modeled using thermokinematic modeling software to determine the evolution of the thrust belt thermal structure.
