Smith Lecture: Tectonics, Climate, and Topography: A View from the Greater Caucasus
Adam M. Forte, Louisiana State University
The potential for interactions and feedbacks between climatically mediated surface processes and active tectonics has been a motivating question within large swaths of the Earth Sciences for years. Conflicting results have been presented at both local and global scales arguing for either clear coupling between climate and tectonics or a complete dominance of tectonics. Ultimately, careful analysis of the details of the nebulously defined ‘climate’, structural geometries, and topography (as this serves as the interface between tectonic and surface processes) are required to resolve these issues. Here I present a case study of an active collisional orogen, the Greater Caucasus, where gradients in both climate and tectonics do not appear to be reflected in the topography, suggesting that this area may have a lot to teach us about the more general question. In detail, the Greater Caucasus mountains are a young (~5 Ma), active orogenic system that is the current locus of NE-SW convergence within the central Arabia-Eurasia collision zone. Importantly, the orogen is characterized by a variety of NW-SE, along-strike gradients including an order of magnitude eastward decrease in mean annual precipitation from ~2 to ~0.5 m/yr and an order of magnitude eastward increase in geodetic shortening velocity from ~2 to ~12 mm/yr. Despite these gradients, the topography of the range is surprisingly similar along-strike which suggests: (1) broadly similar rates of rock uplift, and (2) very limited influence of precipitation on the topography of the range. However, this hypothesis is predicated on the existence of a single relationship between topography and uplift/erosion rate. Here we test this assumption with a new suite of erosion rates estimated from catchment averaged 10Be inventories along the southern range front of the GC. Erosion rates range from 30-5600 m/Myr with the majority of rates being below 2000 m/Myr. Our results are consistent with a single relationship between erosion rate and topography as quantified by normalized channel steepness (ksn). These data also indicate a strongly non-linear relationship between ksn and erosion rate such that topography seem insensitive to increases in erosion rate beyond ~500 m/Myr. There is limited evidence of any influence of mean precipitation on either the topography or the erosion rates of the GC. We hypothesize that this lack of sensitivity to mean precipitation and the related non-linearity in the ksn - erosion rate relationship may be linked to the extremely low variability in runoff observed in gauged basins throughout the region, a hypothesis consistent with theoretical expectations of a stochastic threshold incision model. Spatial patterns in 10Be erosion rates largely mirror those observed in a suite of available bedrock low-temperature thermochronologic cooling ages and new detrital zircon (U-Th)/He ages, i.e. areas with high erosion rates generally have young cooling ages and total amounts of exhumation seem largely constant along-strike within the range, suggesting that this pattern is long-lived. More broadly, the results from the Caucasus reflect that local nuances in both the climate and tectonics are essential for understanding the potential for (or lack of) coupling between surface processes and tectonics.
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