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

Abstract:

The Greater Caucasus mountains (GC), which represent the locus of NE-SW directed convergence in the central Arabia-Eurasia collision, are an E-W striking orogen characterized by significant gradients in climate and tectonics. West of 45ºE the GC are a singly-sided, south-directed orogen with a low modern shortening rate of 1-4 mm/yr and a high mean annual precipitation (MAP) of 1-2 m/yr. Contrastingly, east of 45ºE, the GC are doubly-vergent, underlain by a subducting slab, shortening at 8-12 mm/yr and have a MAP of 0.1-0.5 m/yr. Despite these significant gradients, the topography of the GC is remarkably similar along-strike, suggesting (1) an eastward increase in erosion rate to balance the shortening gradient, (2) a westward increase in uplift rate to balance the MAP gradient, or (3) similar uplift and erosion rates along-strike independent of the MAP and shortening gradients. We 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. We combine this with an analysis of the modern GC climate from satellite (TRMM and MODIS) and river discharge data. The relationship between channel steepness and mean hillslope gradient is invariant along-strike. Precipitation variability is positively correlated with elevation and constant along-strike, but runoff variability is low throughout the range and does not vary systematically along-strike. Together these results suggest no significant gradient in erosional efficiency along-strike. Invariant topography and erosional efficiency along-strike could imply similar uplift and erosion rates along-strike, but ultimately require quantitative measures of both to confirm. If erosion and uplift rates are similar along-strike, this requires an additional mechanism to explain the invariant topography despite the strong gradient in modern shortening rate.