Reconciling Invariant Topography with Significant Along-Strike Gradients in Climate and Tectonics in the Greater Caucasus
Adam Forte, Arizona State University
Kelin Whipple, Arizona State University
The Greater Caucasus Mountains (GC) are a predominantly east-west striking orogen, that lie between the Black and Caspian Seas and represent the locus of NE-SW directed convergence in the central Arabia-Eurasia collision zone. Despite significant along-strike gradients in climate, convergence rate, structural geometries, crustal structure and exposed lithologies, the topography of the GC is remarkably invariant. In detail, the western end of the range is a predominantly singly-sided, south-directed orogen lacking a well developed fold-thrust belt, with hinterland exposures of basement rocks, may have experienced a slab-detachment event, has a low modern convergence rate of 1-4 mm/yr and a high mean annual precipitation (MAP) of 1-2 m/yr. Contrastingly, the eastern end of the range is a doubly-vergent orogen with active fold-thrust belts in both the northern and southern forelands, is devoid of basement exposures, is underlain by a subducting slab, and experiences a convergence rate of 8-12 mm/yr and MAP 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, the actual topography of the GC is instead characterized by remarkably similar maximum elevations, 2.5 km scale relief, orogen width, and cross-sectional area along the strike of the range. This disconnect between the topography and the various forcing mechanisms could reflect either (1) a dynamic balance between competing along-strike gradients, or (2) one or more of the driving mechanisms have changed recently and the topography of the range has not yet responded. Importantly, these two alternatives make fundamentally different predictions regarding along-strike variation in millennial-scale erosion rates. Although no published erosion rates exist for the GC to date, we can utilize established relationships between catchment scale erosion rates and topography (hillslope gradients and channel steepness) developed elsewhere to assess whether an along-strike gradient in erosion rates is likely in the GC. The relationship between channel steepness and mean hillslope gradient is invariant along strike and suggestive of a moderate erosional efficiency, implying no significant climatic control on topography and erosion rates. This finding suggests that there is no significant along-strike gradient in either erosion or rock uplift rates, implying that gradients in MAP and convergence are compensated for by other forcing mechanisms. Ultimately, measurements of catchment-scale erosion rates along-strike are essential to test these ideas.
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