Drainage network reveals patterns and history of active deformation in the eastern Greater Caucasus

Abstract

The Greater Caucasus Mountains are a young (~5 m.y. old) orogen within the Arabia-Eurasia collision zone that contains the highest peaks in Europe and has an unusual topographic form for a doubly vergent orogen. In the east-central part (45°E-49°E), the range is nearly symmetric in terms of prowedge and retrowedge widths and the drainage divide is much closer to the southern margin of the range (prowedge side) than it is to the northern margin (retrowedge side). Moreover, the divide does not coincide with the topographic crest, but rather the crest is both shifted northward by as much as 40 km and traversed by several large north-flowing rivers. Both the topographic crest and drainage divide appear to coincide with zones of active rock uplift, because they are characterized by bands of high local relief and normalized channel steepness values (>300). This uplift pattern could result from a synchronous initiation of the two uplift zones or propagation of deformation either northward or southward. The two propagating scenarios differ fundamentally in their predictions for the relative ages of topographic features; northward propagation predicts that the topographic crest is younger than the drainage divide, and the southward scenario predicts the converse. Because available geologic and topographic data are consistent with both propagation directions, we use a landscape evolution model to test all three scenarios. Model results indicate that the current topography and drainage network is best explained by a northward propagation of deformation from the south flank into the interior of the east-central Greater Caucasus. Such propagation implies recent out-of-sequence deformation within the Greater Caucasus due to reactivation or development of new structures within the core of the orogen. It remains unclear if such deformation is a transient response to an accretion cycle or stems from a fundamental change in the structural architecture of the orogen. GEOSPHERE

Landscape Response to Lateral Advection in Convergent Orogens Over Geologic Time Scales

“…However, because we model the midcrustal ramp with the adjacent décollement, our results show the emergence of legacy landscapes with high-relief, tapered topography composed of advectionparallel interfluves, and elevated but gradually declining K sn over tens of kilometers in the direction of lateral advection over geologic time. These results are consistent with geomorphic observations in convergent orogens where lateral advection over a midcrustal ramp was proposed (e.g., McQuarrie et al, 2008;Whipple et al, 2016;Yue et al, 2005) and suggest that a complex array of fault geometries (e.g., Forte et al, 2014Forte et al, , 2015 or a gradual decline in rock uplift rates over large distances (e.g., Gasparini & Whipple, 2014) is not a necessity. Systematic differences in K sn in trunk relative to tributary streams allude to the importance of increased sediment flux, changes in drainage area, and the transient nature of legacy landscapes.…”

“…Previous studies attributed the distribution of gradually declining K sn to a gradual decline in the rock uplift field in the direction of advection (Bolivia and Nepal, e.g., Gasparini & Whipple, 2014;Whipple et al, 2016;Wobus, Whipple, & Hodges, 2006) or a complex suite of structural geometries (eastern Greater Caucasus, e.g., Forte et al, 2015). However, our simulations qualitatively reproduce the suite of first-order geomorphic observation from natural settings (i.e., high relief, advection-parallel interfluves, tapered topography, and a gradual decline of K sn in the direction of advection).…”

“…For example, the distribution of elevated K sn led Wobus et al () to propose out‐of‐sequence faulting in Central Nepal, Adams et al () to suggest active duplex growth beneath eastern Bhutan, and Yanites et al () to identify the location of active faults and/or ramp structures along the Peikang River in Taiwan. Instead of an abrupt change in K sn , a gradual decrease over tens of kilometers in the direction of advection to the northwest of the Beni Escarpment in the Bolivian Andes (Gasparini & Whipple, ), to the southwest of the eastern Greater Caucasus (Forte et al, ) and to the south in Central Nepal (Whipple et al, ), has been interpreted as a gradual change in rock uplift rates or result of complex fault geometries over these areas. While zones of high K sn within convergent orogens strongly support the presence of high rock uplift rates, they rarely show the abrupt change in K sn magnitude that is characteristic for regions of high differential rock uplift rates with limited lateral advection.…”

Reply to Comment by Vincent et al.

“…However, that study provided no primary data bearing upon the location of this important structure. As noted in Forte et al (), the location of the MCT is disputed, and we follow that study in placing the MCT along the Zangi thrust. Near Lagich, Azerbaijan, the Zangi thrust juxtaposes markedly different sections of Aptian‐Albian strata, with turbidite facies deposits of the relict basin in the hanging wall thrust over volcaniclastic deposits of Lesser Caucasus affinity in the footwall to the south.…”

Evolution of the Greater Caucasus Basement and Formation of the Main Caucasus Thrust, Georgia

“…In the Kazbegi region of Georgia (Figure ), several early studies place the MCT along the Adaykom‐Kazbek (or Adaykomskiy) fault, which broadly juxtaposes the crystalline Gveleti and Dariali massifs to the north against Caucasus Basin strata to the south (Leonov, ; Shempelev, ), whereas other studies place the MCT on the Tiba fault ~20 km to the south (e.g., Rogozhin et al, ; Vincent et al, ). Likewise, the location of the MCT is also disputed within the Caucasus Basin strata in eastern Greater Caucasus, with some authors placing it within the main range (e.g., Mosar et al, ; Vincent et al, ) and others on the Zangi fault farther to the south, at a major structural juxtaposition of differing Cretaceous facies (Cowgill et al, ; Forte et al, ; Khain et al, ; Kopp & Shcherba, ). In the western Greater Caucasus, the MCT footwall comprises either the Dizi series or Caucasus Basin strata, depending on location.…”