Reconciling Invariant Topography with Along-Strike Gradients in Climate and Tectonics in the Greater Caucasus Mountains

Arizona State University, Scottsdale AZ

Investigators

Abstract

The concept of plate tectonics describes the large scale structural formation of mountain belts. However, the more fundamental processes that create the topography of an active mountain range are not fully understood. In particular, does the tectonic deformation or climate control the erosion that generates the mountain topography? Despite many different types of studies, the relative importance of tectonics versus climate in controlling erosion rates and topography remains unresolved. This study takes advantage of the interesting case of the Greater Caucasus Mountains (Azerbaijan and Georgia) where the topography is constant but the mean annual precipitation (climate) and the collision rate between Arabia and Eurasia (tectonics) change along the range length. By measuring erosion rates along the range, the research team will assess the relative importance of tectonic versus climate in topographic development of this actively deforming mountain range. The project has potential to benefit society or advance desired societal outcomes by: (1) improved STEM education through development of learning materials and lab modules; (2) improved well-being of individuals in society through better understanding of natural hazards; (3) development of a competitive STEM workforce through the training of undergraduate students and a foreign graduate student plus support for an early career researcher; and (4) increased partnerships through international collaboration. The Greater Caucasus Mountains are a young predominantly east-west striking orogen that lie between the Black and Caspian Seas and represent the locus of northeast-southwest directed convergence in the central Arabia-Eurasia collision zone. Despite significant along-strike gradients in climate, convergence rate, structural geometries, crustal structure and exposed rock types, the topography of the Greater Caucasus is remarkably invariant. The western end of the range is a predominantly one-sided, south-directed orogen, with hinterland exposures of basement, that experienced slab-detachment, has a low modern convergence rate of 1-4 mm/yr and a high mean annual precipitation of 1-2 m/yr. In contrast, the eastern end of the range is a doubly-vergent orogen, is devoid of basement exposures, is underlain by a subducting slab, and experiences a convergence rate of 8-12 mm/yr and mean annual precipitation of only 0.1-0.5 m/yr. On their own, the gradients in climate and convergence predict a narrow and low elevation range in the high-precipitation, low-convergence west and a wide and high elevation orogen in the low-precipitation, high-convergence east. However, remarkably similar maximum elevations, 2.5 km scale relief, orogen width, and cross-sectional area along the strike of the range instead characterize the actual topography of the Greater Caucasus. This project tests the hypothesis that this along-strike similarity in the topography reflects a similarity in erosional flux out of the orogen along-strike. Specifically, tectonics is the primary control on the topographic form of the Greater Caucasus such that the combination of variations in convergence rate and structural style combine along-strike to produce similar rates of accretion into the orogen, and thus similar rates of rock uplift and erosion along-strike. The hypothesis will be tested by measuring basin averaged erosion rates along the southern flank of the Greater Caucasus using cosmogenic radionuclide dating of 10Be in quartz sampled from modern river sediments to measure these basin averaged erosion rates - a well-developed and time-proven method to gauge millennial-scale catchment mean erosion rates. Available historical climate and river discharge data will be used to assess the potential influence of runoff variability on modulating changes in erosional efficiency along-strike. This project is supported by the Tectonics Program, the Geomorphology and Land Use Dynamics Program, and NSF International Science and Engineering.