EVALUATING NUMERICALLY MODELED GEOMORPHOLOGICAL AND SEDIMENTARY RESPONSES TO STRUCTURAL, KINEMATIC AND CLIMATIC VARIATIONS IN EASTERN BHUTAN

Numerical modeling of landscapes is often utilized to explore and quantify geomorphic responses to variations in climatic and tectonic forces. Bedrock cooling ages can identify foci of exhumation, and thus provide important constraints on landscape evolution, but may not signal differences between tectonic and climatic perturbations. Thermo-kinematic studies in Bhutan suggest that thermochronologic ages can be directly linked to an evolving sub-surface geometry derived from regional-scale 2D balanced cross sections. The evolving geometry of faults determines patterns of uplift and thus imparts a first order control on the resulting topographic evolution. This is in contrast to previous studies that have looked at the relative contributions of climate and tectonics on exhumation, which deploy simplified and static geometries (e.g. single fault ramps or static uplift fields) and do not reflect the more complex structural evolution of modern fold-thrust belts such as the Pyrenees, Andes, and Himalaya. Such complexity is likely to affect local to regional scale exhumation patterns, geomorphology (e.g. fluvial systems, terrace formation and drainage basins), and the respective signals potentially preserved in the sedimentary record.

In this study, a modified version of the TIN-based MATLAB^® landscape evolution model SIGNUM that implements the CASCADE algorithm is combined with a set of 2D thermo-kinematic models. The kinematic models predict patterns of uplift and exhumation that match those recorded by measured bedrock ages. This thermo-kinematically viable uplift pattern is used to create landscapes from which hypsometries, river steepness indices, sediment fluxes and detrital thermochronological age spectra can be sequentially extracted in order to more rigorously evaluate both tectonic and climatic parameters. We evaluate these geomorphological responses to low vs. high shortening rates, variable structural geometries, low vs. high diffusion (i.e. rain fall) and lithology dependent erodibility. Future comparisons between a predicted detrital record and the actual sedimentary record will enable new possibilities in the identification and relative contributions of major driving forces behind the structural and geomorphic evolution of fold-thrust belts.