Decoupling of modern shortening rates, climate, and topography in the Caucasus

Abstract

The Greater and Lesser Caucasus mountains and their associated foreland basins contain similar rock types, experience a similar two-fold, along-strike variation in mean annual precipitation, and were affected by extreme base-level drops of the neighboring Caspian Sea. However, the two Caucasus ranges are characterized by decidedly different tectonic regimes and rates of deformation that are subject to moderate (less than an order of magnitude) gradients in climate, and thus allow for a unique opportunity to isolate the effects of climate and tectonics in the evolution of topography within active orogens. There is an apparent disconnect between modern climate, shortening rates, and topography of both the Greater Caucasus and Lesser Caucasus which exhibit remarkably similar topography along-strike despite the gradients in forcing. By combining multiple datasets, we examine plausible causes for this disconnect by presenting a detailed analysis of the topography of both ranges utilizing established relationships between catchment-mean erosion rates and topography (local relief, hillslope gradients, and channel steepness) and combining it with a synthesis of previously published low-temperature thermochronologic data. Modern climate of the Caucasus region is assessed through an analysis of remotely-sensed data (TRMM and MODIS) and historical streamflow data. Because along-strike variation in either erosional efficiency or thickness of accreted material fail to explain our observations, we suggest that the topography of both the western Lesser and Greater Caucasus are partially supported by different geodynamic forces. In the western Lesser Caucasus, high relief portions of the landscape likely reflect uplift related to ongoing mantle lithosphere delamination beneath the neighboring East Anatolian Plateau. In the Greater Caucasus, maintenance of high topography in the western portion of the range despite extremely low (<2-4 mm/y) modern convergence rates may be related to dynamic topography from detachment of the north-directed Greater Caucasus slab or to a recent slowing of convergence rates. Large-scale spatial gradients in climate are not reflected in the topography of the Caucasus and do not seem to exert any significant control on the tectonics or structure of either range.

Reply to Comment by Vincent et al.

“…It is not the case that recent, rapid exhumation went “undetected” by these samples (V18); rather, these samples are incapable of recording such an event. This interpretation is consistent with the relatively robust relationship between local relief and age observed in all thermochronometric samples from the Greater Caucasus (Forte et al, ).…”

Relict basin closure and crustal shortening budgets during continental collision: An example from Caucasus sediment provenance

“…Most of the samples investigated by Vincent et al [2011] are from the low-relief southern flank of the range. The magnitudes and rates of exhumation are expected to be slow in this area, assuming that topography and long-term uplift rate are correlated, which appears to be the case in the Greater Caucasus [Forte et al, 2016]. Where Vincent et al [2011] sample high-relief areas comparable to those studied by Avdeev and Niemi [2011], the AFT ages are similarly young (e.g., an AFT age of 2.5 AE 0.6 Ma from north of the MCT).…”

“…[] are from the low‐relief southern flank of the range. The magnitudes and rates of exhumation are expected to be slow in this area, assuming that topography and long‐term uplift rate are correlated, which appears to be the case in the Greater Caucasus [ Forte et al ., ]. Where Vincent et al .…”

Supplementary material to &quot;Validation of digital elevation models (DEMs) and comparison of geomorphic metrics on the southern Central Andean Plateau&quot;

“…15 First released in 2003, the hole-filled version of the 90 m SRTM C-band (SRTMv4.1) is available for public download at http://srtm.csi.cgiar.org/ (Jarvis et al, 2008). The SRTM mission standard vertical RMSE of ≤ 16 m and sub-pixel horizontal accuracy is often surpassed (Rodriguez et al, 2006), and because of this the 90 m SRTMv4.1 continues to be applied to geomorphic studies of regional hillslope and channel morphology (e.g., Forte et al, 2016). The SRTMv4.1 was originally considered in the present study for its global coverage and widespread use, and a complete dataset covering the study area was 20 attained.…”

“…The estimation of the m/n ratio [equivalent to θ, or concavity, in the equations of Hack (1957) and Flint (1974)] is necessary 15 to normalize equation (1) across differently sized drainage areas for the mapping of regional patterns of deformation, climatic influence, and/or lithologic boundary conditions (e.g., Forte et al, 2016). As this m/n ratio falls in a restricted range (approximately 0.35-0.65), a reference value (typically 0.45) is often selected to calculate the normalized channel steepness index (ksn) that can be compared across different-sized drainage areas (Wobus et al, 2006).…”