Drainage Network Reveals Patterns and History of Active Deformation in the Eastern Greater Caucasus

Research Paper

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

1 1 2 GEOSPHERE; v. 11, no. 5, p. 1343–1364 Adam M. Forte , Kelin X. Whipple , and Eric Cowgill 1School of Earth and Space Exploration, Arizona State University, 781 E. Terrace Mall, Tempe, Arizona 85257, USA 2Department of Earth and Planetary Sciences, University of California–Davis, One Shields Avenue, Davis, California 95616, USA doi:10.1130/GES01121.1

11 figures; 1 table; 5 supplemental files ABSTRACT base levels, or structural regimes (e.g., Bonnet, 2009; Hoth et al., 2006; Roe et al., 2003; Willett et al., 1993); this can produce significant differences in the CORRESPONDENCE: [email protected] The Greater Caucasus Mountains are a young (~5 m.y. old) orogen within topographic form, structural architecture, sediment flux, or exhumation rates the Arabia-Eurasia collision zone that contains the highest peaks in Europe on either side of an orogen (e.g., Bonnet and Crave, 2003; Hoth et al., 2006; CITATION: Forte, A.M., Whipple, K.X., and Cowgill, and has an unusual topographic form for a doubly vergent orogen. In the Montgomery et al., 2001; Whipple, 2009; Willett, 1999). Drainage divides are E., 2015, Drainage network reveals patterns and history of active deformation in the eastern Greater east-central part (45°E–49°E), the range is nearly symmetric in terms of rarely static (e.g., Willett et al., 2014); within an active doubly vergent orogen, Caucasus: Geosphere, v. 11, no. 5, p. 1343–1364, prowedge and retrowedge widths and the drainage divide is much closer to the location of the main drainage divide is thought to represent a dynamic doi:10.1130/GES01121.1. the southern margin of the range (prowedge side) than it is to the north- balance between (1) horizontal advection of topography that drives divides ern margin (retrowedge side). Moreover, the divide does not coincide with toward the retro side of an orogen (Fig. 1B; e.g., Hovius and Stark, 2006; Stolar Received 21 August 2014 the topographic crest, but rather the crest is both shifted northward by as et al., 2006; Willett et al., 2001), (2) orographic effects that localize precipita- Revision received 4 June 2015 Accepted 23 June 2015 much as 40 km and traversed by several large north-flowing rivers. Both the tion on one side of a divide, driving it toward the dry side as catchments on Published online 5 August 2015 topographic crest and drainage divide appear to coincide with zones of active the wet side expand (Fig. 1C; e.g., Bonnet, 2009; Hoth et al., 2006; Roe et al., rock uplift, because they are characterized by bands of high local relief and 2003), and (3) local variations that affect erosion rate, such as differences in normalized channel steepness values (>300). This uplift pattern could result rock type, structural activity, or frequency of mass-wasting events that drive from a synchronous initiation of the two uplift zones or propagation of defor- the divide away from the side with faster headward erosion (e.g., Kühni and mation either northward or southward. The two propagating scenarios differ Pfiffner, 2001; Pelletier, 2004; Stark, 2010; Willett et al., 2014). fundamentally in their predictions for the relative ages of topographic fea- In most active orogens, the effects of horizontal advection and topo- tures; northward propagation predicts that the topographic crest is younger graphic enhancement of precipitation are thought to dominate, leading to than the drainage divide, and the southward scenario predicts the converse. relatively predictable divide locations (e.g., Stolar et al., 2007; Willett, 1999). Because available geologic and topographic data are consistent with both In the absence of strong orographic effects, models of doubly vergent oro- propagation directions, we use a landscape evolution model to test all three gens predict that the location of the drainage divide will mirror the asym- scenarios. Model results indicate that the current topography and drainage metric mass distribution that results from flow of material within the orogen, network is best explained by a northward propagation of deformation from leading to a prowedge that is wider than the retrowedge (e.g., Willett, 1999; the south flank into the interior of the east-central Greater Caucasus. Such Willett et al., 1993, 2001). Localization of precipitation on one side of an oro- propagation implies recent out-of-sequence deformation within the Greater gen can either increase or reduce such asymmetry, depending on whether Caucasus due to reactivation or development of new structures within the precipitation is localized on the prowedge or retrowedge, respectively, and core of the orogen. It remains unclear if such deformation is a transient re- the extent to which spatial patterns in deformation change as a result of sponse to an accretion cycle or stems from a fundamental change in the increased precipitation (e.g., Hoth et al., 2006; Willett, 1999). In all of these structural architecture of the orogen. cases, the drainage divide is predicted to be coincident with the topographic crest of the range, and thus still represents the division between the two distinctive structural regions, the prowedge and retrowedge (Fig. 1; e.g., INTRODUCTION Sanders et al., 1999). The east-central Greater Caucasus provides an interesting exception to The location of the main drainage divide in an active orogen represents a simple versions of standard wedge theory that predict that the topographic fundamental control on its structural and topographic evolution, but is also crest will coincide with the drainage divide and that both will be displaced a dynamic feature, strongly predicated on the imposed tectonic and climatic toward the retrowedge side of an orogen with minimal cross-wedge precip- For permission to copy, contact Copyright forcing (Fig. 1; e.g., Willett, 1999; Willett et al., 2001). Divides are important itation gradients (Fig. 1). Here we define the east-central Greater Caucasus Permissions, GSA, or [email protected]. because they often separate areas with different climatic conditions, effective as the section of the range between 45°E and 49°E, characterized by both the

A Drainage Divide and Topographic Crest Prowedge Retrowedge EROSION

EROSION

Deforming Material Figure 1. Schematic cross sections of doubly vergent orogens showing positions of drainage divide and topographic crest and proposed mechanisms for migration of both Non-deforming Material features. (A) Idealized case, in which divide and crest co- S incide and wedge exhibits characteristic asymmetry (after Willett and Brandon, 2002). (B) Divide and crest migrate in retrowedge direction due to flow of material within orogen from the toe of the prowedge to the face of the retro wedge, effectively widening the prowedge. (C) Divide and crest migrate toward the arid side of an orogen due to Advection Causes Divide Migration Localized Precipitation Causes Divide Migration orographic effects and localization of precipitation on one B C side of the orogen. Case here has elevated precipitation on retrowedge, causing the divide and crest to migrate in the EROSION EROSION prowedge direction, widening the retrowedge.

EROSION EROSION

S S

Dagestan thrust belt on the north (retrowedge) side of the range (Sobornov, with the northern retrowedge having a shallower average surface slope than 1994) and the Kura fold-thrust belt on the south (prowedge) side (Forte et al., the main range within the southern prowedge (e.g., Forte et al., 2014a). How- 2010, 2013), which is separated from the main range by the piggyback Alazani ever it is important to note that much of the width of the southern prowedge Basin, rendering it topographically distinct from the main range (Fig. 2C). This is represented by the Kura fold-thrust belt, which has a low topographic portion of the range is doubly vergent (Forte et al., 2014a; Mosar et al., 2010) slope of ~0.5° and in places is characterized by out-of-sequence deformation, and is the same region referred to as the “doubly vergent eastern zone” in leading to the hypothesis (Forte et al., 2013) that it may be in part subcritical. Forte et al. (2014a). North-directed subduction and/or underthrusting of Kura The prowedge and retrowedge in this portion of the orogen are nearly equal Basin lithosphere beneath the Greater Caucasus is indicated by global posi- in width (~130 km), when defined as the distance between the topographic tioning system (GPS) data (Reilinger et al., 2006; Vernant and Chery, 2006), limits of the surface expression of latest Quaternary active deformation and seismicity (Mellors et al., 2012; Mumladze et al., 2015), and tomography the topographic crest (Forte et al., 2014a). Thus, the topography across the (Skolbeltsyn et al., 2014), making the prowedge and retrowedge sides clear. east-central Greater Caucasus is decidedly different from that predicted by stan- We consider the topographic crest of the range as the division between the dard models of doubly vergent orogenic wedges (Figs. 1 and 3; e.g., Sanders prowedge and retrowedge (see Forte et al., 2014a). Although the north flank is et al., 1999; Willett et al., 1993). the retrowedge in this part of the range (Forte et al., 2014a; Mosar et al., 2010), In the case of the drainage divide position, this uncharacteristic asymmetry the drainage divide is displaced southward toward, and within, the prowedge could theoretically be driven by orographic precipitation (e.g., Bonnet, 2009; side of the orogen, and it does not coincide with the topographic crest, which Hoth et al., 2006; Roe et al., 2003) or differentials in base-level history on is 20–40 km to the north and near the center of the range (Figs. 2 and 3; Forte either side of the orogen. However, within the east-central Greater Caucasus, et al., 2014a). In addition, the topographic asymmetry in the east-central modern climatological data suggest no significant difference in mean annual Greater Caucasus departs from predictions of a doubly vergent wedge model, precipitation on either side of the drainage divide (e.g., Forte et al., 2014a).

32˚E 36˚E 40˚E 44˚E 48˚E 52˚E B

100 km N EURASIA Fig. 2C

44˚N A 20°E 40°E 60°E 80°E Caspian 50°N Black Sea GREA Sea TER C Alps AUCASUS Fig. 2B NO RTH A NA N TO LESSER LIA CAU EURASIA 0˚ N C 4 F A AU S LT U KURA S APSHERON ASIN T B 40°N AUL SILL N F LIA TO NA A 13.7 T S N A E mm/yr A E 36˚ L G Himalaya BO N 16.0 RZ RA

T M 30°N ARABI L A U IN A A mm/yr R A Mediterranean F RABIA EC FRIC A A E E Z NT A Sea S A F D G A A RO U INDIA E S 19.1 LT 500 km D mm/yr C 40° E 45° N 45° E Terek-Caspian Basin 50° E Dagestan Fig. 3,4 Terek-Sunzha Grozny Caspian Sea Thrust Belt Thrust Belt

45° N Greater Caucasus Topographic Crest

Alazani Ba Baku Sochi Main Caucasus Thrust sin Drainage i Thrust Chaladidi-Tsaish n Tbilisi Kura Fold- Divide Trialet Thrust Belt Rioni Basi a Achar Black Sea Thrust Belt Kura Basin

> 42 0–2 0 km 100 Elevation (km) Lesser Caucasus N

Figure 2. (A) Location of Arabia-Eurasia collision within the Alpine-Himalaya belt. (Shapes of boxes differ due to map projections.) Topography is shaded relief map of GTOPO30 (global 30-arc second resolution; https://lta .cr.usgs.gov/GTOPO30) digital elevation model. (B) Tectonic context of the Arabia-Eurasia collision zone highlighting the primary structural systems within the western and eastern thirds of the collision. Red arrows show motion of reference points (at tails) on Arabian plate relative to Eurasia from the GEODVEL 2010 global velocity model (Argus et al., 2010). (C) Topography and primary structures of the Greater Caucasus. Note separation between crest (dashed line) and divide (white line) in east-central Greater Caucasus. Topography for both B and C are shaded relief maps of SRTM 90 (Shuttle Radar Topography Mission, http://srtm .csi .cgiar.org/) digital elevation model.

A 46° E 47° E 48° E 49° E X′ Sulak Normalized Steepness (ksn) –0.1 5.6 θref = 0.45 Elevation (km) < 50 200–300 50–100 > 300 100–200 High ksn 21 22 Topographic Crest Drainage Divide Selected 1 River Samur 43° N

Low ksn

1 2 3 4 5 6 Alazanil Basin 7 9 Figure 3. Topography and fluvial geomorphology of 8 42° N east-central Greater Caucasus (see Fig. 2C for location). High ksn (A) Elevation draped over a hillshade digital elevation model (both from SRTM 90 data; Shuttle Radar Topography Mis- Kura Fold-Thrust Belt sion, http://srtm .csi .cgiar .org/), with normalized steepness indices (ksn) as calculated in Forte et al. (2014a). Thick white line delineates Sulak and Samur river catchments. Dark and X 25 km light blue lines mark the topographic crest and drainage divide. Numbered white squares indicate specific drainages 46° E 47° E 48° E 49° E B X Topographic Crest highlighted in Figure 5. (B) Map of local relief calculated ′ over 2.5 km circular window, with black lines showing areas 02.8 Drainage Divide Relief 2.5 km (km) in which ksn > 200. Note coincidence of bands of high local Thermochron Sample (Avdeev, 2011) relief (inferred to indicate active rock uplift) with crest and N1 divide. White polygons outline ice extent during the Last High Relief LGM Glacial Glacial Maximum (LGM; Gobejishvili et al., 2011). Red circles Exents mark locations of three low-temperature thermochronologic High ksn 200–300 analyses (from Avdeev, 2011) discussed in text. > 300 43° N

Low N2 Relief N1 Alazani Basin V1 42° N High Relief Kura Fold-Thrust Belt

X 25 km

Instead, the major precipitation gradient is along the strike of the range, BACKGROUND driven by eastward transport of moisture-laden air from the Black Sea by the Westerlies (Borisov, 1965; Hijmans et al., 2005; Lydolph, 1977). Detailed paleo- East-Central Greater Caucasus Tectonics and Topography climatological data are not available for the late Cenozoic for either the northern or southern side of the east-central Greater Caucasus, but reconstructions for Between the Black and Caspian Seas, the Greater Caucasus Range rep- the Lesser Caucasus to the south and the northwestern Greater Caucasus both resents the modern locus of northeast-southwest shortening within the central show a trend toward more arid conditions ca. 2 Ma (e.g., Gabunia et al., 2000; portion of the Arabia-Eurasia collision zone (Fig. 2B; Allen et al., 2004; Jack- Joannin et al., 2010; Kovda et al., 2008; Kvavadze and Vekua, 1993; Messager son, 1992; Reilinger et al., 2006). The crustal architecture and kinematics of the et al., 2010). At larger scales, reconstruction of regional paleoclimate during the Greater Caucasus vary significantly along strike; the east-central Greater Cauca- late Cenozoic throughout the western Mediterranean and Central Asia suggest sus is well described as a doubly vergent orogen, while the orogen both west of that during interglacials, the current climatic regime is a reasonable approx- 45°E and east of 49°E is one sided and south directed (Forte et al., 2014a). Con- imation for the past (e.g., Dodonov and Baiguzina, 1995; Youn et al., 2014). sideration of the east-central Greater Caucasus in the context of a doubly ver- In contrast, during glacial periods, the northern part of the Westerlies, within gent orogenic wedge model with the southern and northern sides of the range the latitudinal range of the Caucasus, were partially disrupted by cold, dry air being the prowedge and retrowedge, respectively, is consistent with modeling masses flowing southward from the Siberian high pressure system and off of GPS data (Reilinger et al., 2006; Vernant and Chery, 2006), relocation of deep large continental ice sheets (e.g., Dodonov and Baiguzina, 1995), presenting earthquakes (Mellors et al., 2012), Rayleigh wave tomography (Skolbeltsyn the possibility for a more arid northern margin for the Greater Caucasus during et al., 2014), and synthesis of local earthquake catalogs (Mumladze et al., 2015), glacial periods. Thus, neither the interglacial nor glacial period climatic regime all which suggest north-directed subduction and/or significant underthrust- of the east-central Greater Caucasus is consistent with a simple orographic ing of Kura Basin lithosphere beneath the southern margin of the east-central explanation for this atypical topographic profile. Likewise, even though the Greater Caucasus to depths of at least 160 km (for a summary, see Mumladze Caspian has undergone extreme (>1 km) variations in water level (e.g., Forte et al., 2015). Convergence rates between the Kura Basin and stable Eurasia in- and Cowgill, 2013; Popov et al., 2010; Zubakov, 2001), it is unlikely that the crease eastward from ~8 mm/yr at 45°E to ~14 mm/yr near the Caspian Sea asymmetry results from greater base-level fall and headward erosion on the coast (Kadirov et al., 2012; Reilinger et al., 2006), with most convergence equally north side because base level for both sides of the east-central Greater Cauca- partitioned into shortening along the southern and northern margins of the oro- sus is set by the Caspian Sea. At present, the most reasonable explanation for gen within the east-central Greater Caucasus (Forte et al., 2014a). the locations of the drainage divide and topographic crest in the east-central Along the southern front of the main range, the locations, geometries, and Greater Caucasus is that their positions are controlled by the geometry of ac- activities of major thrusts within the east-central Greater Caucasus are not well tive faults beneath the range, similar to the proposed structural control on the constrained. A north-dipping thrust, typically referred to as the Main Caucasus drainage network in the Swiss Alps (Kühni and Pfiffner, 2001). Spatially clus- thrust and abbreviated MCT, is often cited as the primary structure within the tered zones of elevated normalized channel steepness (e.g., Kirby and Whip- southern Greater Caucasus as a whole (e.g., Khain, 1975; Philip et al., 1989); ple, 2012; Wobus et al., 2006) and local relief within the east-central Greater however there is disagreement regarding its exact location and significance Caucasus led us (Forte et al., 2014a) to conclude that the divide and crest each within the east-central Greater Caucasus. In this region, some consider the coincide with active loci of uplift in the interior of the range (Figs. 2 and 3). Main Caucasus thrust to be located near the topographic range front, and in Here we investigate the topographic and tectonic evolution of the east-cen- part buried beneath sediments of the Alazani Basin (e.g., Forte et al., 2014a; tral Greater Caucasus by integrating structural, stratigraphic and topographic Kadirov et al., 2012; Saintot et al., 2006), while others consider the Main Cauca observations with simple landscape evolution models (LEMs). The main goal sus thrust to be farther north, with its surface trace in the low-relief area be- is to determine if the two zones of active rock uplift inferred by us (Forte et al., tween the topographic crest and the drainage divide (Fig. 4; e.g., Adamia et al., 2014a) formed by southward or northward propagation. We show that current 2011; Mosar et al., 2010). For our purposes, we consider the Main Caucasus geologic observations are inconclusive because they are compatible with both thrust to be the range-front thrust system, which places Jurassic and Creta- scenarios, but that simple LEMs are most consistent with northward propaga- ceous flysch and carbonates over Jurassic and Cretaceous volcaniclastic rocks tion. Thus, it appears that the topography of the east-central Greater Cauca- (Fig. 4). Quaternary shortening along the southern margin has been localized sus records a recent northward propagation of deformation from the southern within the Kura fold-thrust belt, a series of predominantly south-vergent folds flank to within the interior of the range. The timing of this northward propa- and thrusts deforming Pliocene–Pleistocene sediments, since at least ca. 2 Ma gation of deformation appears to be broadly concurrent with that of south- in the eastern portion of the belt (Fig. 2C; e.g., Forte et al., 2010, 2013). Geomor- ward propagation of thrusting into the Kura Basin (e.g., Forte et al., 2010, 2013; phic observations along the southeastern Greater Caucasus range front (Forte Mosar et al., 2010), and we infer that the two zones of active rock uplift are et al., 2010, 2014a; Mosar et al., 2010) and similarities between average rates of above structures that are kinematically linked at depth to foreland deformation. shortening from balanced cross sections within the Kura fold-thrust belt with

46° E 47° E 48° E 49° E

Tso1 Tso2 Tso2 Topographic Crest Tso1 Ks Drainage Divide Qal Tb Ks Tso2 Tso1 Tso2 Jb Ks Ks Tso2

Tb Js Js Tb Qal

43° N Tso1

Tso1 Jb Tso2 Figure 4. Simplified tectonic map of the east-central Greater Caucasus (modified Js Ks from Forte et al., 2014a; source data and discussion of compilation therein). Solid Kb white line shows location of the Main Caucasus thrust as defined in Forte et al. Kb Alazani Basi Tb (2014a) and dashed white lines mark loca- Kvc tion of Main Caucasus thrust as defined by Kvc n Qal Mosar et al. (2010). See text for discussion Tvc of discrepancy between the locations of 42° N Tso2 Tso1 these structures. Tso1 Tso1 Tv Tso2 Tb Kura Fold-Thrust Belt Kv Qal 25 km Kv Undeformed Greater Caucasus Lesser Caucasus Qal Quaternary Alluvium Volcaniclastics Basin Sediments Tv Tertiary Volcanics Plio-Quaternary Volcanism Tvc Tertiary Tb 65– ~16 Ma Kv Cretaceous Volcanics Greater Caucasus Kvc Cretaceous Kb Cretaceous Main Caucasus Thrust Syn-Orogenic Sediments Eurasian Shelf Jb Jurassic (Forte et al., 2014a) Tso1 ~5–0 Ma Ks Cretaceous Main Caucasus Thrust Tso2~16–5 Ma Js Jurassic (Mosar et al., 2010) Fault–Kinematics Uncertain

modern geodetic rates of convergence (Forte et al., 2010, 2013) suggest that sole into a roof thrust above a triangle zone that defines the leading edge of a once deformation propagated into the southern foreland, active surface slip of north-vergent thrust system at depth (Fig. 2C; Sobornov, 1994, 1996). However, the north-dipping thrusts within the main range ceased or slowed considerably. numerous regional maps also show the Dagestan belt as a relatively simple, Along the northern margin of the range, the Dagestan thrust belt is the north-vergent thrust system (e.g., Philip et al., 1989; Ruppel and McNutt, 1990). principal active structural system (Sobornov, 1994). At the surface this sys- Unconformities and variation in stratal thicknesses suggest that the Dagestan tem is characterized by north-dipping, south-directed thrusts (e.g., Dotduyev, thrust system initiated prior to deposition of Pliocene–Quaternary sediments, 1986), inferred from analysis of seismic and well-log data to be backthrusts that although the initiation age is not well constrained (Sobornov, 1996).

) 2500 2500 Southern Streams sn Chirkey Dam Main Channel 2000 Lithologically 2000 Northern Streams Northern Controlled 1500 Knickpoints 1500 Main Channel Zone Figure 5. Profiles of topography, normal- Southern ized channel steepness index, and select Tributaries 1000 Extent of 1000 drainages across the east-central Greater Dagestan FTB Zone LGM Glaciers 500 500 Caucasus (location in Fig. 3). Top: Normal- ized channel steepness values (ksn) from

Normalized Steepness (K selected drainages in an ~30-km-wide area. 0 0 Red and blue dots are from south- and north-flowing drainages, respectively; teal Topographic Crest shows data from the Dagestan fold-thrust belt (FTB). Blue bracket outlines glacial ex- X Drainage Divide X′ tent during Last Glacial Maximum (LGM; 4 Greater Caucasus

m) 4 Dagestan Gobejishvili et al., 2011). Center: Black line Thrust Belt and gray swath show mean and maxi- 2 Kura Fold-Thrust Belt Alazani 2 mum-minimum elevation range, respec- Basin tively, for a 10-km-wide swath along the profile line. This figure is uses the same Elevation (k 0 0 013263952657891 104 117 130 143 156 169 182 195 208 221 234 247 260 underlying data as profile D in Forte et al. Distance (km) (2014a, fig. 7D therein). Bottom: Longitudi- 4 m) 4 nal profiles of selected rivers, with distance Stream 1 Western Sulak measured relative to channel heads. This Stream 2 2 2 scale indicates distance from the divide Stream 21 for all of the south-flowing rivers and most of the north-flowing ones; positive values

Elevation (k 0 0 indicate distance north of the divide, neg- –30–20 –10 050 100 150 200 ative values indicate distance south of the 4 Stream 3 m) 4 divide. Technically this distance is not from Stream 4 Eastern Sulak the divide for those portions of the north- Stream 5 2 2 ern streams that flow south from the topo- Stream 22 graphic crest before then flowing north away from the divide. This is also why the

Elevation (k 0 0 –30–20 –10 050 100 150 200 headwaters of many northern streams ap- Stream 6 4 pear significantly higher than the southern

m) 4 Stream 7 Samur streams. Stream numbers correspond to locations in Figure 3B. Stream 8 2 2 Stream 9

Stream 23 Elevation (k 0 0 –30–20 –10 050 100 150 200 Distance from Divide–Along River (km)

While active shortening in the Greater Caucasus is accommodated pre- generally scale with uplift and erosion rates (Kirby and Whipple, 2012; Wobus

dominantly along the margins of the orogen, the topography of the east-cen- et al., 2006). The two zones of high ksn in the east-central Greater Caucasus tral part of the range suggests that there are at least two range-parallel zones of likely mark zones of elevated rock uplift and erosion rates, because they do not active uplift within the interior of the mountain belt: a southern zone 30–40 km correlate to significant changes in lithology or precipitation (Figs. 3 and 4; Forte north of the northern margin of the Kura fold-thrust belt and coincident with et al., 2014a, fig. 7D therein). the drainage divide, and a northern zone roughly along the topographic crest (Figs. 3 and 4). We (Forte et al., 2014a) found two prominent zones of elevated Mechanisms for Divide Migration in the Caucasus

normalized channel steepness index values (ksn) that coincide with range-parallel bands of elevated local relief (Figs. 3 and 4), which is known to correlate In the east-central Greater Caucasus, the geometry and location of the di- well with channel steepness (e.g., DiBiase et al., 2010; Kirby and Whipple, 2012; vide and crest appear to be controlled by zones of active uplift, as indicated

Wobus et al., 2006). Accounting for variations in lithology or climate, ksn values by the separation between the divide and crest, their correspondence with

regions of elevated k and local relief, and low k and local relief in the region sn sn Present Day between the divide and crest (Fig. 3). Thus, changes in the location of active A Topographic structures within or beneath the range should have controlled both the organi m) 4 SW Drainage Crest NE zation of the drainage network and its temporal evolution. We envision two Divide Greater possible end-member relationships between the topographic crest, drainage 2 Caucasus Dagestan divide, and zones of uplift, differing in the relative timing of uplift and propa- Kura Thrust Belt Thrust Belt gation direction of deformation (Fig. 6). There is also the null hypothesis that Uplift 0 there has been no significant change in the drainage network or location of Elevation (k 100 200 active structures within the interior of the Greater Caucasus, essentially that (km) the two zones of uplift initiated simultaneously. In the first end member, deformation and uplift propagate northward rela tive to the upper plate, so that the topographic crest and its associated zone of B Option 1: Northward Propagation of Deformation uplift are younger than the divide. It is important to clarify that this scenario Topographic Crest

does not require a completely stationary drainage divide. As discussed in m) 4 SW Moves North NE

Forte et al. (2014a), the juxtaposition of high and low ksn values across the current drainage divide (Figs. 3 and 4) suggests that this feature is unstable, with 2 higher erosion rates on the southern side of the divide, thus implying northward migration over time. However, such migration should be slow and more 0 akin to scarp retreat than an abrupt shift in the divide position and topology Elevation (k 100 200 of the drainage network stemming from formation of a new structure. North- (km) ward propagation could be accomplished through several nonunique sets of structures. One option is the formation of a new structure at depth beneath the C Option 2: Southward Propagation of Deformation range, such as an out-of-sequence duplex, possibly related to propagation of Drainage Divide deformation into the Kura foreland basin. Formation of out-of-sequence hinter- m) 4 SW Moves South NE land duplexes during foreland propagation is observed in other thrust systems, and is interpreted as a deformational response to return the wedge to critical 2 taper (e.g., Gutscher et al., 1998; Konstantinovskaya and Malavieille, 2005; Mitra and Sussman, 1997). An alternative is northward expansion of the uplift zone,

Elevation (k 0 possibly due to widening of the orogen or a change in the nature of the under- 100 200 thrusting lithosphere, e.g., a progressive transition from subduction to collision (km) with a gradual increase in the amount of material accreted into the orogen. In the second end member, the northern zone of uplift predated the south- Figure 6. Schematic cross sections showing distribution of uplift zones in the east-central ern zone and deformation migrated south relative to the upper plate. In this Greater Caucasus. (A) Present-day configuration, with zones of active elevated uplift rate beneath each major topographic feature. (B) Model of northward-propagating uplift. (C) Model of scenario, the topographic crest represents something like a paleo–drainage southward-propagating uplift. divide, with later uplift along the southern flank of the range decapitating for- merly south-flowing rivers and reversing their flow direction between the divide and crest by integration into the north-flowing river systems. Such decapi POSSIBLE GEOLOGIC EVIDENCE OF SOUTHWARD tation would produce a dramatic decrease in the length and drainage area of DIVIDE MIGRATION south-flowing rivers. From a structural perspective, southward propagation of deformation could suggest in-sequence thrust propagation and the initiation Results of landscape evolution modeling and additional geologic observa- of a relatively discrete zone of uplift near the southern range front, but would tions (discussed in the following) appear to refute southward propagation of depend on the age of this feature relative to the Kura fold-thrust belt. This struc- uplift. However, an argument for such migration can be made based on the ture could also represent reactivation or underplating in relation to a frontal extents of modern and paleo–alluvial fans on the south flank of the east-central accretion cycle (e.g., Gutscher et al., 1998; Hoth et al., 2007). We expect that this Greater Caucasus (Fig. 7). As we show here, fan length has decreased by a structure is in the form of a duplex or other blind structure, based on neotec- factor of 2–3 since ca. 2 Ma. Assuming other dominant variables were constant tonic observations indicating that there is no surface-breaking structure at the (e.g., subsidence rate), this reduction in fan length could indicate a reduction in range front along the northern margin of the Alazani Basin (Forte et al., 2010). drainage area for rivers draining the southern flank of the range, as predicted

A 46° E 47° E 48° E 25 km

Sulak –0.1 5.6 Samur Elevation (km)

Alazani Basin

42° N Catchments Sarica Alluvial Fans Vashlovani Göy-2 Drainage Kura F Xocashen Göy-1 Divide old-Thrust Belt Topographic Crest Bozdag B Step 1: Use Paleofan Length to C Step 2: Use Paleofan Area to D Step 3: Use Paleocatchment Area to Estimate Paleofan Area Estimate Paleocatchment Area Estimate Paleocatchment Length 550 105 1000 500 Estimated )

2 900 Estimated 3 Estimated Paleofan Area ) 2 Paleocatchment

450 m 10 Paleofan Areas Empirical relationship m 800 Areas 400 1 from Terrestrial and ) 10 Estimated 2 700 Martian ea (k Estimated m 350 Paleocatch- Compilation Ar –1 600 Empirical relationship from Empirical relationship an Area (k 10 ment Area Paleocatch- 300 F from modern GC fans 500 modern GC catchments ment Lengths 250 10–3 10–3 10–1 101 103 105 400 SE GC Catchments an Area (k

F 200 2 Contributing Catchment Area (km ) Catchment 300 150 Estimated 200 ~40 km di erence 100 Paleofan SE Greater Caucasus Fans Terrestrial Fans between means S. Himalayan Megafans Martian Fans 100 50 SE GC Fans Lengths Chaco Plain Megafans 0 0 010203040506070 01020304050 Catchment Length (km) Fan Length (km)

Figure 7. Analysis of alluvial fans in the southeastern Greater Caucasus (GC) indicates that they have reduced in length by ~40 km since ca. 2.5 Ma. (A) Topography along the southeastern range front; modern alluvial fans are in green and associated catchments are in blue. Thick dashed white line indicates minimum extent of paleoalluvial fans based on paleogeographic reconstructions within measured stratigraphic sections at Sarica and Vashlovani (Forte et al., 2014b). Additional control comes from stratigraphic sections at Göy (Forte et al., 2013), Xocashen (Forte, 2012; van Baak et al., 2013), and Bozdag (Forte, 2012). (B) Plot showing a linear fit through area and length values from modern fans in the southeastern Greater Caucasus used to reconstruct paleofan area from paleofan length estimated from stratigraphy in the Kura fold-thrust belt. Equation for line is y = 12.047x – 66.884 with an R2 of 0.8412. (C) Plot of relationship between fan area and catchment area was determined by Dade and Verdeyen (2007), with additional data from the Greater Caucasus (after Dade and Verdeyen, 2007). Linear fit [y = (0.5 ± 0.35)x with R2 of 0.95] is from Dade and Verdeyen (2007; full discussion of this calculation therein) and does not consider Greater Caucasus data. Combining this relationship with reconstructed paleofan area from B yields an estimate for paleocatchment area for the Caucasus. (D) Plot showing linear fit (y = 16.18x – 102.68 with an R2 of 0.6379) between area and length for modern catchments in southeastern Greater Caucasus. Combining this relationship with reconstructed paleocatchment area from C yields an estimate for paleocatchment length that is ~40 km longer than present, on average.

by southward propagation of uplift, formation of a new drainage divide, and reconstructed paleoalluvial fan areas and the general relationship between decapitation of south-flowing rivers (Fig. 6C). fan and catchment areas to estimate paleocatchment area (Fig. 7C; Dade and Modern alluvial fans in the area have an average length of ~14 km and a Verdeyen, 2007). To estimate the position of the paleo–drainage divide, we maximum of 25 km (Figs. 7A, 7B). Previous work documented an empirical use the estimated paleocatchment area and an empirical relationship between relationship between the area of an alluvial fan and the contributing drain- catchment length and catchment area to estimate paleocatchment length, the age area within the upstream catchment, and suggested that the fan area is mean of which is ~40 km longer than modern catchments (Fig. 7D). We use a approximately half the catchment area (Fig. 7C; Dade and Verdeyen, 2007; least-squares regression through lengths and areas of modern catchments in Whipple and Trayler, 1996). Using a topographic contour map derived from the southeastern Greater Caucasus that we extrapolate to the values spanned the SRTM (Shuttle Radar Topography Mission, http://srtm.csi .cgiar .org/, ~90 m by the paleocatchments. However, in this case, the calculated linear relation pixel resolution) digital elevation model, we determined areas of 22 modern between catchment length and area is not particularly robust (R2 = 0.6379). alluvial fans within the Alazani Basin and their associated contributing catch- Such a fit is not surprising, given the variability of catchment shapes, but it ment areas in the southeastern Greater Caucasus (Fig. 7A). These fan and provides an approximation of paleocatchment length. The 40 km reduction catchment areas closely follow the general empirical relationship found in di- in mean catchment length over time is similar to the distance between the verse settings (Dade and Verdeyen, 2007), indicating that the fan depositional modern drainage divide and the topographic crest of the range (Figs. 7A, 7D). and catchment accumulation areas are well equilibrated (Fig. 7C). Thus, these data are compatible with the idea that the topographic crest may Foreland-basin deposits now exposed in the Kura fold-thrust belt suggest represent a paleo–drainage divide and that the reduction in drainage area was Pliocene–Pleistocene alluvial fans extended 2–3 times farther from the Greater driven by a southward propagation of uplift that beheaded drainages in the Caucasus into the Kura Basin than today (Forte et al., 2014b). In Forte et al. southeastern Greater Caucasus. However, it is important to note that changes (2014b) exposures of Pliocene alluvial fan deposits in the Sarica and Vashlo- in precipitation, base level, local structural history, or accommodation space vani sections were described (Fig. 7A), and dated to ca. 2.5 Ma based on a could also explain this decrease in alluvial fan area, thus a southward prop- population of detrital zircons from the Sarica paleofan. The distance of these agation of uplift is not a unique solution. Assuming constant amounts of ac- sections from the range front provides minimum paleofan lengths (i.e., dis- commodation space or uplift rates within catchments since 2.5 Ma during a tance from the range front to the fan toe) of ~20–36 km (Fig. 7A). These values period in which the foreland underwent large-magnitude base-level changes are minima because they do not account for younger shortening within the (e.g., Forte and Cowgill, 2013) and active propagation of the thrust front in Kura fold-thrust belt, which initiated ca. 2 Ma in the Göy area, ~80 km to the the foreland basin (e.g., Forte et al., 2010, 2013) provides a reasonable level of east of Sarica (Fig. 7A; Forte et al., 2013). To reconstruct Pliocene fan lengths, doubt against the southward-propagation scenario. We use landscape evolu- we estimate ~12 km of total postdepositional shortening between Sarica and tion modeling to attempt to address this uncertainty. the range front, based on shortening estimates from a balanced cross section near the Sarica area (western cross section in Forte et al., 2010). Thus, LANDSCAPE EVOLUTION MODELING restored lengths of the paleofans were at least ~32–48 km (Fig. 7B); however, this assumes that the topographic range front of the Greater Caucasus has not To test the feasibility of both northward and southward propagation of up- moved significantly since that time. lift as mechanisms for producing the divergence between the crest and divide, While paleofans approximately twice the size of modern are compatible with we evaluated multiple variations of both scenarios using a landscape evolution southward divide migration, such motion is not required because the relation model (LEM). The premise behind the use of a LEM is that the distinctive fea- between fan and catchment area is fundamentally modulated by various other tures of the current cross-sectional and plan-form topography of the east-cen- factors, including sediment accumulation rate, rock uplift rate, basin geometry, tral Greater Caucasus (Figs. 3 and 5) represent a primary constraint that either drainage outlet and fan spacing, and properties of the sediment (e.g., Whipple scenario must satisfy. Thus, it may be possible to distinguish between the two and Trayler, 1996). As a preliminary test, we are interested whether the ob- mechanisms by evaluating the extent to which each can faithfully generate the served post–2.5 Ma reduction in fan length can be reasonably attributed to a observed topographic patterns. As shown here, the model results suggest that major reduction in drainage area of the scale required for southward propaga- northward propagation of uplift is the most feasible mechanism for generating tion and drainage beheading, keeping all other factors constant. Addressing the observed topography of the east-central Greater Caucasus. this question requires estimating the axial lengths of the paleocatchments. To estimate this value, we first calculate a least squares regression between fan Modeling Methodology and Parameterization length and area for modern alluvial fans in the southeastern Greater Caucasus (Fig. 7B). We then make a simplifying assumption that the modern relation- For the LEM, we use the FastScape model (Braun and Willett, 2013). This ship was true for the paleofans; this allows us to estimate paleofan areas from LEM is relatively simple in terms of the surface processes it simulates, com- the palinspastic reconstructions of paleofan length (Fig. 7B). We then use the pared to other popular LEMs (e.g., channel-hillslope integrated landscape

Forte, A.M., Whipple, K.X., and Cowgill, E., 2015, Drainage network reveals patterns and history of active deformation in the eastern Greater Caucasus: Geosphere, v. 11, doi:10.1130/GES01121.1.

SUPPLEMENTAL TEXT development model, CHILD; Tucker et al., 2001). However, FastScape is well magnitudes of uplift are largely unconstrained within the east-central Greater (http://dx.doi.org/10.1130/GES01121.S1) suited to the present problem because it allows simulation of landscapes of Caucasus; there are no published thermochronologic data for this portion of Landscape Evolution Modeling Schematic diagrams of the 13 different uplift scenarios tested with the FastScape the spatial (thousands of square kilometers) and temporal (millions of years) the range. The nearest available thermochronologic data are from the far landscape evolution model are illustrated in Figure S3 along with the final model output. Model names indicate the scenario being tested and are then numbered in the order in scale of the Greater Caucasus in model runs that take less than an hour when eastern tip of the range (Avdeev, 2011; Fig. 3B), within a south-directed, sin- which they were run. As indicated in the main text, resolutions of the models are all identical and the dimensions are all the same except for the Dagestan-1 and Dagestan-2 models. For each model, the 260 km model width was divided into 20 sections and utilizing the multithread capabilities of the code, thus permitting efficient test- gly vergent portion of the belt (Forte et al., 2014a). A pair of samples in the locations of prescribed changes in the uplift functions were given in reference to these divisions. ing of multiple scenarios. Also of particular importance is the capacity of Fast- northern half of the range (N1 and N2, Fig. 3B) record average cooling rates of Two different initial topographies were tested. One initial topography was a simple condition set at 0 meters and the other which used a pre-existing channel network developed as a restart point for the models as described in the main text. In Figure S1, we Scape to accommodate complicated uplift patterns that vary in time and space, ~10 °C/m.y. since 5 Ma, and a sample near the southern range front (V1, Fig. compare the results of the two models (North-5 and South-3) that are considered the exemplar models in the main text using these two different initial topographies. The although deformation is limited to vertical motion with no advection. 3B) yields an average cooling rate of ~15 °C/m.y. since 5 Ma (Avdeev, 2011). The overall form of the topography (e.g. location of topographic crest, divide, etc) is largely similar, but the channel network is distinctly different between the two, with the models using the dendritic initial topography having channel networks that are more realistic. The east-central Greater Caucasus strike 115°–295° in a clockwise-positive geothermal gradient in this region is also not well constrained, but choosing Within the main text and in Figure S3 we present the results using this dendritic initial topography. The initial topography for all models is shown in Figure S2. azimuthal coordinate system in which north is 000°. All models are set up standard values of between 20 and 30 °C/km suggests ranges of average uplift such that the positive x and y directions of the model domain coincide with rates of 0.3–0.5 and 0.5–0.75 mm/yr since 5 Ma for the northern and southern 1Supplemental Text. Additional details about the the across-strike (025°) and along-strike (295°) directions in the east-central samples, respectively. It is important to note that the geomorphic and geologic methods used for the paper. Please visit http://dx 1 .doi.org /10 .1130/GES01121 .S1 or the full-text article Greater Caucasus (see Supplemental Text ). For the majority of reported runs, contexts of these samples are decidedly different than the area of interest for on www.gsapubs.org to view the Supplemental Text. the model domain is 260 km (x) by 500 km (y), which generally represents the this study (Fig. 3B). These samples are from a portion of the range where the

260 260 dimensions of the east-central Greater Caucasus, i.e., the orogen is ~260 km topographic crest and drainage divide are either coincident, or nearly so, and

240 240

220 220

200 200

180 180 wide from the southern to northern margins of the Kura and Dagestan fold- the orogenic width and elevation are reduced with respect to the east-central

160 160

140 140

120 lometers 120 lometers Ki Ki thrust belts, respectively (Figs. 2, 4, and 7). The length of the model (y) is ap- Greater Caucasus (Fig. 3). In addition, for the southern sample (V1), the struc- 100 100

80 80

60 60 X X 40 40 proximately half the length of the entire Greater Caucasus, but we focus our tural geometry of this portion of the southern Greater Caucasus range front 20 20 South-3: Flat Initial Topography Y South-3: Initial Dendritic Channel Network Y 0 0 0255075100 125 150 175 200225 250 275 300325 350375 400425 450 475 500 0255075100 125 150175 200225 250275 300325 350375 400425 450475 500 Kilometers Kilometers 260 260 observations and discussion on the central 100–200 km of the model results to may differ substantially from areas of the range front farther west, as argued in 240 240

220 220

200 200 180 180 avoid edge effects. All models have a spatial resolution of 100 m, similar to the Forte et al. (2013) based on a change in the topographic slope of the range and 160 160

140 140

120 lometers 120 lometers Ki Ki 100 100 SRTM topographic data, and a time step of 100 k.y. Streams are forced to drain structural styles of the Kura fold-thrust belt. Thus, while it is not appropriate to 80 80

60 60 X X 40 40

20 20 Y Y out of the northern (x = 260 km) and southern (x = 0 km) edges of the model simply translate these uplift rates westward, they constrain the magnitudes of North-5: Flat Initial Topography North-5: Initial Dendritic Channel Network 0 0 0255075100 125 150 175 200225 250 275 300325 350375 400425 450 475 500 0255075100 125 150175 200225 250275 300325 350375 400425 450475 500 Kilometers Kilometers domain, to replicate the Greater Caucasus. Base level (0 m), the constant of uplift within our modeling study. We test models where the maximum magni- 2Supplemental Figure S1. Comparing the final time- erosional efficiency (1.0E-5), and mean annual precipitation (1 m/yr) are held tude of both uplift peaks is 0.4 mm/yr (South-1, South-2, and Stationary-2), the steps of models North-5 and South-3 using a flat constant for all model runs. In addition, we do not consider flexural responses southern uplift is 0.3 mm/yr and the northern uplift is 0.4 mm/yr (Stationary-1, initial topography (left), versus a low-relief landscape to erosion in any of the presented model runs. North-1, North-2, North-3, North-4, North-5, and South-3), the southern uplift with a south-flowing dendritic channel network, shown in Supplemental Figure S2 (right). For the We modeled 13 basic uplift scenarios to evaluate sensitivity to model is 0.4 mm/yr and the northern uplift is 0.3 mm/yr (Dagestan-1 and Dagestan-2), majority of the main text and supplement, we only parameters listed in Table 1, such as the width of the uplift zones and whether and one model (South-4) where the northern uplift is 0.4 mm/yr and the south- discuss the models using the initial dendritic channel one or both of the uplift zones propagates laterally, along the strike of the ern uplift is 0.8 mm/yr, slightly greater than the maximum average uplift rate network topography. Please visit http://dx.doi .org /10.1130/GES01121 .S2 or the full-text article on www orogen. Assigned values for variables, uplift functions, and final topographic from sample V1 (see Table 1). .gsapubs.org to view Supplemental Figure S1. outputs for all models are provided in greater detail in the Supplemental Text Early tests of models with a flat initial topography produced extremely lin-

Initial Topography (see footnote 1). All scenarios have two primary peaks in uplift, which for most ear drainages, the forms of which were heavily influenced by the imposed runs are located such that in cross section they occur in the same general uplift functions (Supplemental Figure S12). To produce more realistic drainage

te Closed Boundary t Ra U = 0.01 mm/yr X 1 areas as the peaks in uplift inferred from the high ksn values in the east-central networks and to ensure that results were not biased by the initial drainage Uplif 013263952657891104 117 130 143 156 169 182 195 208 221 234 247 260 260 Greater Caucasus (Table 1; Fig. 5; Supplemental Text [see footnote 1]). Models network form, all of the models were run using a common initial topography 240 3 220 Dagestan-1 and Dagestan-2 are modified versions of the others designed to (Supplemental Text [see footnote 1] and Supplemental Figure S2 ). This ini-

200

180 determine if the observed topography can be produced by an early single peak tial topography was generated by letting a model of the same dimensions

160

140 of uplift within the core of the range, followed by initiation of structures near (x = 260 km, y = 500 km) evolve under a constant uplift rate of 0.01 mm/yr for ters

120 Kilome 100 the area corresponding to the Dagestan fold-thrust belt on the northern margin 100 m.y. (Supplemental Figure S2 [see footnote 3]). Drainages in this model

60 of the range. Models Stationary-1 and Stationary-2 explicitly test whether the were forced to exit along the southern edge (x = 0). A one-sided initial model X 40

20 topography of the east-central Greater Caucasus could be produced without geometry was chosen to avoid having a preexisting drainage divide within the Initial Topography Y 0 0255075 100 125 150 175200 225 250 275300 325350 375400 425450 475500 either northward or southward propagation of deformation. model that might influence later results. With the low uplift rate, the topogra- Kilometers Other parameters that differ between the 13 base models are the geom- phy of this initial model was subdued, with a maximum elevation of ~17 m 3Supplemental Figure S2. Final topography after run- etries of the uplift-rate profiles, the relative timing of the two uplift zones, above base level. It is important to note that this initial model topography is ning a model for 100 million years of dimensions x = 260 km and y = 500 km that drains to the south (x = 0) whether the uplifts initiate synchronously along strike or propagate laterally not meant to represent anything geologically specific to the Greater Caucasus, with an uplift rate of 0.01 mm/yr. This final topogra- in the y direction (along strike), and the difference in maximum rate of the but only to provide the ensuing models with a preexisting dendritic channel phy was used as the initial topography for all of the two zones of uplift (Table 1; Supplemental Text [see footnote 1]). Absolute network. In our analysis and discussion of model results, we consider only presented models. Please visit http://dx .doi .org /10 .1130/GES01121 .S3 or the full-text article on www .gsapubs.org to view Supplemental Figure S2.

Stationary-1 U2 U1 l Final Rate (U &U ) U < U 1 2 1 2 s Intermediate Steps te c Lateral Propagation Ra the dendritic models, but comparison of a representative set of base models South-3 tests a southward-propagation scenario. The first uplift (U1) is X Upli ft 013263952657891 104 117 130 143 156 169 182 195 208 221 234 247 260 (those without the dendritic seeds) and the presented models (those with the broad and centered in the approximate location of the topographic crest (Figs. 260 240 dendritic seeds) suggests that while the drainage networks of the presented 5 and 7B). This uplift starts at time 0 and increases in a stepwise fashion to 220 200 models appear more realistic, the general topographic forms of the two sets of one-half, three-quarters, and then full magnitude over 2 m.y., while also propa- 180 160 results are not significantly different (Supplemental Figure S1 [see footnote 2]). gating in the east-west direction (y) as described in the Supplemental Text (see 140 ters

120

Kilome In constructing and running the models, we also experimented with dif- footnote 1). At the end of 2 m.y., the uplift rate function for the northern peak 100 80 ferent erosion laws, because FastScape can be run using a unit stream power reaches its maximum and remains constant for the rest of the run. At 2.5 m.y. 60 X 40 law with a linear slope dependence (n = 1), general unit-stream power law into the run, the second uplift (U2) initiates to south of U1 and propagates in the 20 Stationary-1 Y 0 (n ≠ 1), or the ξ – q model of Davy and Lague (2009), where ξ is the ratio be- east-west direction (y) for 2.75 m.y. until it reaches the edges of the model do- 0255075 100 125 150 175 200 225 250 275300 325350 375400 425450 475500 Kilometers tween sediment river load and the depositional flux and q is river discharge. main. The uplift function then remains unchanged for another 4.75 m.y., until

4Supplemental Figure S3. Schematic representation of The latter can approximate transport-limited behavior by adjusting the alpha the end of the model run. The northern (U1) and southern (U2) uplifts have the uplift rates imposed for the various model sce- parameter within the FastScape model. In general, these different erosion laws maximum values of 0.4 mm/yr and 0.3 mm/yr, respectively. narios tested, viewed in north-south orientation with did not dramatically influence the results, so the presented models are those respect to the Greater Caucasus. Black lines indicate run with a general unit-stream power law with values of m = 0.4 (exponent on initial geometries, grey lines indicate portion of the geometry that is different after initiation of second drainage area in stream power law) and n = 0.8. We selected values of m and n Modeling Results uplift zone. Concentric circle symbol indicates lat such that the ratio of m/n = 0.5, consistent with empirical results indicating that eral propagation was implemented and dotted lines m/n ~ 0.5 is required to the match of topography of most ranges (e.g., Whipple Models with northward propagation most successfully produce the separa- indicate the partial uplift rates implemented during these lateral propagation periods. Relative differences and Tucker, 1999). We used a value of 0.8 for the slope exponent n as this was tion of the topographic crest from the drainage divide seen in the east-central between the maximum value of the two uplift rates, between values of n in which erosion rate is proportional to shear stress (n = Greater Caucasus (Figs. 8 and 9). More recent uplift in the northern zone suc- U1 and U2 are given. Geometry of uplift rate gradient 2/3) and unit stream power (n = 1). cessfully creates high peaks in the interfluves while maintaining transverse is labeled as either, “c” for a rate that decreases with the cube of distance, “s” for a rate that decreases with For simplicity, we discuss two exemplar models in detail, models South-3 drainages. In contrast, scenarios with younger uplift in the south typically fail the square of distance, or “l” for a linear decrease and North-5, but we present the final outputs of all models in the Supplemen- to decapitate drainages and force the drainage reversals required to generate with distance as described in the supplemental text. tal Text (see footnote 1) (Supplemental Figure S34). Both models were run for a jump in divide position. In addition, lateral propagation of a young northern Below each uplift function is the output topography of the final timestep of the model. Final panel shows 10 m.y. and uplift rates for the first uplift peak increase incrementally over 2 m.y. uplift is needed to establish longitudinal tributaries like those observed in the schematic of time-steps in the lateral propagation before reaching final constant values, to partially simulate the initial slow ex- Sulak and Samur drainages (Fig. 2), though significantly curved drainages like function. For this panel, the view is 90 degrees from humation phase observed in the Greater Caucasus (Avdeev, 2011; Avdeev and the Samur only occur at the very edges of the model domain in most model the other diagrams, approximately east-west in terms of the Greater Caucasus. Please visit http://dx .doi .org Niemi, 2011). However, as discussed in more detail in the following, because results. Models Stationary-1 and Stationary-2, which test a scenario with no /10.1130/GES01121 .S4 or the full-text article on www we do not have adequate constraints for absolute uplift magnitudes, erosional propagation of deformation, fail to produce a separation between the drainage .gsapubs.org to view Supplemental Figure S3. efficiency or precipitation, we do not consider times in any of the models to divide and topographic crest. After 10 m.y., the run time for the majority of be realistic with respect to timing of events within the Greater Caucasus. We other models (Table 1), these two models had produced a continuous elevated MODEL Initial Topography Initial topography therefore limit our interpretations and discussion to the relative sequences of region between the two uplift zones, so they were allowed to run for an addi Basic geometry initial_topography_n = 0 initial_topography_v1 = 0 nx = 2600 events. For both models, we use FastScape to produce digital eleva tion mod- tional 10 m.y. For Stationary-1, where the southern uplift zone is 0.3 mm/yr ny = 5000 uplift_n=0 xl = 260000. uplift_v1=0.1E-4 els for the final time step, which we then use to calculate normalized chan- and the northern is 0.4 mm/yr, at the end of the 20 m.y. run the divide and yl = 500000. nel steepness values by using the TopoToolbox-2 (Schwanghart and Scherler, crest are merged and located near the northern zone of uplift (Supplemental Time stepping Flexure flexure = 0 dt = 100000.00 2014). In this analysis, we use the same parameters of the east-central Greater Figure S3 [see footnote 4]). The final topography of Stationary-2, which has nstep = 1000 Stratigraphy nfreq = 50 Plotting options Caucasus (reference concavity of 0.45) as in Forte et al. (2014a), but with a equal magnitudes for the two uplift zones (0.4 mm/yr), also has a coincident Number of threads/cores to use plot_all = 0 larger accumulation area to limit the number of streams. divide and crest, but with the location of this topographic feature varying be- num_threads = 1 plot_topo = 1 plot_DEM = 0 North-5 tests a northward propagation of deformation. An initial southern tween the northern and southern zones of uplift (Supplemental Figure S2 [see Incision law plot_sedim = 0 plot_strati = 0 zone of uplift (U ) initiates with its peak near the location of the drainage di- footnote 3]). law = 2 plot_erosion = 0 1 m = 0.4 plot_rate = 0 n = 0.8 plot_discharge = 0 vide (Fig. 8A). This uplift starts at time 0 and increases in a stepwise fashion The two models Dagestan-1 and Dagestan-2 designed to test the influence plot_catchment = 0 kf = 0.10E-04 to one-half, three-quarters, and then full magnitude over 2 m.y., while also of the Dagestan fold-thrust belt also fail to produce topography analogous to Boundary conditions vtk = 0 vex = 5 boundary_condition = 0001 propagating in the east-west direction (y) as described in the Supplemental that of the east-central Greater Caucasus (Supplemental Text [see footnote 1]).

Precipitation Text (see footnote 1). The northern uplift zone (U2) initiates 2.5 m.y. into the run The only southward-propagation models that partially resemble the east-cen- precipitation_n = 0 and propagates in the east-west direction (y) for 2.75 m.y. until it reaches the tral Greater Caucasus are ones in which the magnitude of the southern uplift precipitation_v1 = 1.00000 edges of the model domain. The model then runs for another 4.75 m.y. The zone is at least twice that of the northern zone of uplift, e.g., South-4 (Supple-

5 5 Supplemental File. Zipped file containing FastScape northern (U2) and southern (U1) uplifts have maximum values of 0.4 mm/yr and mental File ). In this specific case, the later development of a new, southern model input files for models discussed in the text and 0.3 mm/yr, respectively. zone of uplift causes a drainage reversal, but also causes the new drainage the Supplemental Text. Please visit http://dx.doi .org /10.1130/GES01121 .S5 or the full-text article on www .gsapubs.org to view the Supplemental File.

TTABLEABLE 1. SUMMASUMMARYRY OF PPARAMETERSARAMETERS USED IN LANDSCAPE EVOLUTION MODELMODELSS TTimeime of Relation between Model Continuous

Propagation propagation Lateral maximum uplift U11 U22 duration or discrete †††† Model* direction (Ma)**(Ma)** propagation ratesrates (mm/yr)(mm/yr) (mm/yr)(mm/yr) U11/U/U22 (Ma)(Ma) uplifts Parameters being testetestedd

†† Stationary-1 NA NA Both (S)(S) U11 << U22 (N)(N) 0.0.3300.4.4 0.75 20 Discrete Whether propagation of uplift is requirerequiredd Stationary-2 NA NA Both (S)(S) U11 = U22 (N)(N) 0.0.4400.4.4 121 200 Discrete

South-1 South 5.5 Both (N)(N) U11 = U22 (S)(S) 0.0.4400.4.4 111 100 Discrete Geometry of southern uplift, comparing a narrow (1) versus South-2 South 5.5 Both (N)(N) U11 = U22 (S)(S) 0.0.4400.4.4 111 100 Discrete wide (2) uplifting zone §§ South-3 South 2.5 Both (N)(N) U11 >> U22 (S)(S) 0.0.4400.3.3 1.33 10 DiscretDiscreteeNNarrowarrow uplift peaks— southward migration exemplaexemplarr

South-4 South 2.5 Both (N)(N) U11 <<<< U22 (S)(S) 0.0.4400.8.8 0.0.552200 Discrete InitiationInitiation ofof aa veryvery largelarge (rate)(rate) southern uplift zone

North-1 North 5.5 South Only (S)(S) U11 << U22 (N)(N) 0.0.3300.4.4 0.75 10 Continuous InﬂuenceInfluence of of lateral lateral propagation propagation on continuous uplifting zone North-2 North 5.5 North Only (S)(S) U11 << U22 (N)(N) 0.0.3300.4.4 0.75 10 Continuous model

North-3 North 5.5 North Only (S)(S) U11 << U22 (N)(N) 0.0.3300.4.4 0.75 10 Discrete ImportanceImportance ofof laterallateral propagationpropagation occurring only in later uplift (3) North-4 North 5.5 Both (S)(S) U11 << U22 (N)(N) 0.0.3300.4.4 0.75 10 Discrete or in both (4(4))

North-5 North 2.5 Both (S)(S) U11 << U22 (N)(N) 0.0.3300.4.4 0.75 10 DiscretDiscreteeNNarrowarrow uplift peaks— northward migration exemplaexemplarr

Dagestan-1 North 0.5 North Only (S)(S) U11 >> U22 (N)(N) 0.0.4400.3.3 1.33 20 Discrete InﬂuenceInfluence of of Dagestan Dagestan thrust thrust belt, belt, without (1), and with (2), lateral Dagestan-2 North 1.0 Both (S)(S) U11 >> U22 (N)(N) 0.0.4400.3.3 1.33 20 Discrete propagation of main uplift Note: NA—notNA—not applicable.applicable. *Model*Model dimensions:dimensions: x=x = 260 km (across strike) and y=y = 500 km (along strike), except for Dagestan-1 and Dagestan-2, in which y=y = 250 kkm.m. †† There is no propagation in Stationary-1 or Stationary-2, but the southern and northern uplift is adopted as U11 and U22,, respectivelyrespectively,, forfor consistencyconsistency withwith otherother models.models. §§Exemplar models shown in bold. **T**Timeime relativerelative toto initiationinitiation ofof model.model. ††††Continuous uplifts lack a local minimum between the two uplift rate peaks.

divide to quickly become the topographic crest, with peak elevations signifi- file oriented across strike through the North-5 model results (Fig. 9B) further cantly higher than those above the northern uplift. Such topography is incon- illustrates the similarity between the topography produced in this model and sistent with that observed in the east-central Greater Caucasus. the topographic form of the east-central Greater Caucasus (Fig. 5). In addition, The exemplar northward propagation model, North-5, produces patterns longitudinal profiles of selected drainages (Fig. 9B) appear similar in form to of normalized channel steepness, topography, and longitudinal profiles that south- and north-flowing drainages within the east-central Greater Caucasus are generally similar to those seen in the east-central Greater Caucasus (Figs. (Fig. 5). In contrast, the exemplar southward propagation model, South-3, has

8 and 9). It is important to note that the absolute values of ksn and topogra- high ksn values on either side of the drainage divide, which is also the topo-

phy in the east-central Greater Caucasus and the model outputs are different, graphic crest, and a third zone of high ksn values in the vicinity of the south-

thus we only compare the spatial patterns between high and low ksn values ern uplift. The topography of the South-3 results in both map (Fig. 8E) and

and topographic form (Fig. 8). A difference between absolute values of ksn is cross section (Fig. 9B) view differ significantly from the observations within not unexpected, as the models are not tuned to match the Greater Caucasus the east-central Greater Caucasus, including a significantly diminished topo- because the information required to do so is either unknown (in the case of graphic expression of the southern peak and more symmetrical river longitu- the erosional efficiency) or lacks the spatial and temporal certainty required dinal profiles between north and south flowing drainages. for accurate implementation in the model (in the case of the uplift rate and Two other models, North-3 and North-4, have similar uplift geometries precipitation). as in North-5, but the northern uplift initiates later in the model run, starting Comparing the synthetic topography in North-5 and the real range, both 5.5 m.y. after the start as opposed to 2.5 m.y. (Table 1). In addition, the geome-

have high ksn values south of the drainage divide and north of the topographic try of the southern uplift was wider in North-3 and North-4 (Supplemental Text

crest, with lower ksn values in the intervening region. A swath topographic pro- [see footnote 1]). These two models also successfully produce a separation

Final Rate (U ) A North-5 B South-3 U 1 U2 1 Final Rate (U2) U U1 Rate Replaced by U2 U1 2 U1 Intermediate Steps max U < max U Lateral Propagation te 1 2 te max U1 > max U2 t Ra t Ra Uplif Uplif 013263952657891 104 117 130 143 156 169 182 195 208 221 234 247 260 013263952657891 104 117 130 143 156 169 182 195 208 221 234 247 260 Across-Strike Distance (km) Across-Strike Distance (km)

X′ Y′ 260 C D ksn E Topographic < 20 20–40 240 Crest > 40

220 Figure 8. Setup and results for two exemplar landscape evolution models. (A) Setup for model North-5, testing northward propagation. (B) Same as A, but for South-3, N1 N2 N3 200 testing southward propagation of uplift. Diagrams show the imposed uplift function for the model. Bullseye indicates lateral propagation was used; dotted lines indicate the partial uplift rates implemented during lateral propaga- 180 m) tion. See main text and Supplemental Text (see footnote 1) N1 for further discussion. (C) Representative strip through the N2 east-central Greater Caucasus (see Fig. 2 for location) high- N3 N4 160 lighting key topographic features. (D) Result from model North-5 showing that northward propagation successfully reproduces first-order topography of the east-central 2 1 140 Greater Caucasus. (E) Result of model South-3 showing that southward propagation fails to reproduce the observed topography. For C, D, and E, black lines show calculated normalized channel steepness values. Light and dark blue lines 120 are the drainage divide and topographic crest, respectively. oss-Strike Distance (k 1 2 r

100 Ac S1 S2 S1 S2 S3 Drainage 80 Divide Model North-5 Model South-3 60 Option 1—Northward Option 2—Southward ksn 40 < 100 Propagation of Uplift Propagation of Uplift 100–300 Result—Separates divide Result—Fails to separate > 300 20 and crest divide and crest

Eastern Greater Caucasus 0 X Y

Topographic Crest A North-5 east-central Greater Caucasus topography. It may also indicate that for drain- 1.4 Drainage Divide 1.2 ages like Samur and its tributaries to form, initiation of this northern uplift peak m) X X 1.0 ′ after the southern is required, but still relatively early within the history of the 0.8 0.6 range. However, the time scales of the models are not well calibrated to the 0.4 0.2 actual Greater Caucasus. 0 Elevation (k We also attempt to use the LEM results to distinguish between two tectonic 0 20 40 60 80 100 120 140 160 180 200 220 240 260 models for the northward-propagation scenario (Fig. 10). The progressive wid- Across-Strike Distance (km) ening of the orogen could produce an expansion of the southern zone uplift, 0.6 forming a continuous zone of uplift roughly between the drainage divide and 0.4

m) the topographic crest (e.g., North-1 or North-2, Fig. 10; Supplemental Text [see 0.2 Stream S1 0 Stream S2 footnote 1]), whereas the formation of a hinterland duplex or reactivation of Stream S3 –40 –20 0 20 40 60 80 structures might produce a more discrete zone of uplift (e.g., North-3, North-4, 0.6 Stream N1 0.4 Stream N2 Stream N3 North-5, Dagestan-1, Dagestan-2; Supplemental Text [see footnote 1]). Models 0.2

Elevation (k Stream N4 0 with a continuous zone of uplift fail to provide a separation between the topographic crest and the drainage divide, and generally produce topography incon- –40 –20 0 20 40 60 80 Distance From Divide (km) sistent with that of the east-central Greater Caucasus. Therefore, the scenario modeled in North-5 best reproduces the modern topography (Figs. 8 and 10).

Drainage Divide and B South-3 Topographic Crest 1.4 DISCUSSION 1.2

m) 1.0 Y Y′ 0.8 Reconciling Conflicting Results 0.6 Mean Elevation 0.4 Elevation Range 0.2 Results of the landscape evolution modeling suggest that northward prop- 0

Elevation (k agation of uplift within the interior of the east-central Greater Caucasus is 0 20 40 60 80 100 120 140 160 180 200 220 240 260 most consistent with the current topography of the range (Figs. 8 and 9). How- Across-Strike Distance (km) ever, apparent reductions in size of paleoalluvial fans based on exposures 0.6 0.4 within the Kura fold-thrust belt stratigraphy are consistent with decapitation of

m) 0.2 Stream S1 south-flowing drainages and a reduction in drainage area that could be driven 0 Stream S2 by a southward propagation of uplift (Fig. 7). We consider the results of the Stream N1 –80––60 40 –20 020406080 0.6 Stream N2 landscape evolution modeling, and thus the northward propagation of uplift, 0.4 Stream N3 0.2 to be more robust because several alternate solutions can explain the reduc- Elevation (k 0 tion in alluvial fan size along the southeastern Greater Caucasus. –80––60 40 –20 020406080 While the approximate twofold decrease in alluvial fan length since 2.5 Ma Distance from Divide (km) could result from a structurally controlled reduction in drainage area, it could also be explained by an increase in subsidence rates during growth of the Figure 9. (A) Top: 20-km-wide swath topographic profile across strike of model North-5; see Greater Caucasus, an increase in aridity leading to decreased sediment flux, Figure 8D for swath locations. Bottom: Selected drainages from the north and south side or a dramatic rise in base-level (e.g., Clevis et al., 2003; Dade and Verdeyen, of the model. See Figure 8D for location of rivers. (B) Top: 20-km-wide swath topographic profile, same setup as for model South-3; see Figure 8E for location of swaths. Bottom: 2007; Whipple and Trayler, 1996). Natural examples and numerical models of Selected drainages from model outputs; see Figure 8E for locations of rivers. alluvial fans indicate that the amount of accommodation space exerts a primary control on alluvial fan size (e.g., Clevis et al., 2003; Dade and Verdeyen, 2007; Whipple and Trayler, 1996). The amount of available accommodation between the drainage divide and crest, but the geometry of the northern drain- space (independent of sediment flux into a basin) will be a combination of sub age network did not mirror the northern east-central Greater Caucasus as well sidence rate and regional base level. In general, there is an inverse correlation as North-5, i.e., longitudinal rivers like the Samur did not form (Supplemental between alluvial fan size and the amount of accommodation space, because Text [see footnote 1]). This further indicates that relatively discrete uplift peaks an increase in accommodation causes the same volume of material delivered with a lower uplift zone between them are required to faithfully reproduce the to a basin to be distributed over a smaller area, thereby reducing fan size.

GEOSPHERE | Volume 11 | Number 5 Forte et al. | Active deformation in the eastern Greater Caucasus Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/5/1343/3333710/1343.pdf 1357 by guest on 25 September 2021 on 25 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/5/1343/3333710/1343.pdf Research Paper C 2 1 Uplift Uplift and crest R uplif Option 1— Discrete A Rate B Rate 01 01

Elevation (km) Elevation (km) esult max U max U 0. 2 0. 4 0. 6 0. 8 1. 0 1. 2 1. 4 0. 2 0. 4 0. 6 0. 8 1. 0 1. 2 1. 4 Nor Nor 0 0 32 32 ting zones E F Model 0 0 63 63 — th-5 X Z th-2 X 1 Nor Nor 95 1 95 < max U < max U 20 20 Separat 26 26 Elevation R Mean Elevation th-2 th-5 X X Nor 40 40 57 57 ′ 89 89 Ac 2 60 60 2 th-5 es divide ross-Strike Distance (k 11 1 ange Dr U 104 04 U k 1 80 80 ainage Divide Ac

Dr 11 71 117 sn 1 To > 40 20–4 0 < 20 r ainage Divide and oss-Strike Distance (k 130 pogr 100 100 30 U 14 31 143 2 aphic Crest U 120 120 156 56 2 D 16 91 2 169 1 divide and cres uplif Option 2— Expansion of R 140 140 m) esult 182 82 To 195 195 160 160 pogr t nor Model 20 82 208 — 180 180 aphic Crest 221 21 m) Fa th 23 42 234 200 200 ils to separate ward Z Z

247 Nor 47 ′ 26 0 260 220 220 t th-2 240 240 F Steps Replaced by U Pr Lateral U F inal Ra inal Ra X 1 1 opagation Intermediate Ra Z ′ 260 260 te ′

te (U te (U

2 1 U ) ) 2

100 120 140 160 180 200 220 240 260

40 60 80 20 0 Across-Strike Distance (km) ness (k from model North-5, with channel steep - for model North-2. (C) Map of final output model North-5. (B) Imposed uplift function (North-2). (A) Imposed uplift function for one with a single broad zone of uplift two discrete zones of uplift (North-5) and ward-propagation model results, one with Figure 10. Comparison of two north - location shown in D). graphic crest and drainage divide (swath that this model fails to separate the topo - through model North-2, demonstrating in C). (F) Swath profile (20 km wide) topographic crest (swath location shown separation between drainage divide and through model North-5, highlighting clear (E) Swath topographic profile (20 km wide) (D) Map of final output for model North-2. Setup and symbols same as in Figure 8. sn ) and main physiographic features.

GEOSPHERE | Volume 11 | Number 5 Forte et al. | Active deformation in the eastern Greater Caucasus 1358 Research Paper

In the case of the southeastern Greater Caucasus, an increase in average ward expansion of the zone of uplift does not reproduce the relief structure or subsidence rate within the foreland basin likely occurred coincident with, or drainage patterns of the region between the topographic crest and drainage shortly after, the onset of rapid exhumation of the Greater Caucasus and pre- divide; we therefore favor a model with two discrete zones of uplift (Fig. 10). sumed growth of topography. An important additional consideration is the for- However, the caveats and assumptions of the modeling approach must be mation of the Kura fold-thrust belt as both a barrier to sediment dispersal from considered. One potentially limiting factor is the lack of data to constrain in- the Greater Caucasus and a driver of local subsidence in the Alazani Basin. put variables. While the models were designed to represent the east-central Timing of fold-thrust belt formation in this region is not well constrained, but Greater Caucasus, they are not direct simulations because we have no mea- the initiation ages of 2 and 1.5 Ma in the Göy area (Forte et al., 2013) suggest surements of actual uplift rates or field calibration for variables used in the ero- that preliminary development of the Kura fold-thrust belt could also influence sion law (i.e., erosional coefficient, exponents m and n for drainage area and the extent of alluvial fans in the southeastern Greater Caucasus. The initiation slope terms). Such uncertainties should have secondary impact on formation of the Kura fold-thrust belt may have also led to some amount of range front of the primary topographic patterns, such as the separation between the drain- retreat, further contributing to an apparent reduction in alluvial fan length. The age divide and topographic crest, although they are likely essential for under- southeastern Greater Caucasus range front on the north side of the Alazani standing the rates and time scales over which these features form. A second Basin is extremely embayed; this is interpreted to indicate cessation or signif- restriction is the lack of advection in FastScape, because horizontal motion and icant slowing of activity on the surface trace of the Main Caucasus thrust (Fig. resulting advection of topography are important in simulations of both individ- 6A; Forte et al., 2010). When the surface trace of the Main Caucasus thrust was ual folds (e.g., Hardy, 1995, 1997; Waltham and Hardy, 1995) and orogen-scale active, the range front was likely coincident with the trace of this thrust, which deformation (e.g., Braun and Yamato, 2010; Willett et al., 2001). Although we throughout much of the east-central Greater Caucasus is 5–10 km south of the attempted to approximate transport-limited erosion by performing some runs modern range front and buried under sediments of the Alazani Basin (Fig. 2C). with the FastScape implementation of the Davy and Lague (2009) erosion Decreases in precipitation can also decrease alluvial fan size during tran- model, no sediment is deposited between the two uplift zones in runs using sient response of a landscape, assuming that rates of subsidence or creation this erosion law, which we speculate is due to elevated uplift rates within this of accommodation space remain constant (e.g., Clevis et al., 2003; Whipple zone. A true transport-limited erosion law might not exhibit the same behavior and Trayler, 1996). Within the Caucasus, there was a regional shift to more arid and could be important in allowing models with southward-propagation to conditions ca. 2 Ma (e.g., Gabunia et al., 2000; Joannin et al., 2010; Kovda et al., produce the observed topography. Transport-limited erosion laws specifically 2008; Kvavadze and Vekua, 1993; Messager et al., 2010); however, the scale of include sediment flux, and consider the role of sediment in both protecting this aridification is unclear. Also, in general, climatic variations probably do the river bed from incision and enhancing incision through abrasion or salta- not exert enough control to effect such a significant reduction in fan size once tion; such laws may better capture the behavior of bedrock rivers under certain a landscape has returned to steady state (e.g., Clevis et al., 2003; Dade and conditions (e.g., Gasparini et al., 2007; Sklar and Dietrich, 2006, 2008). Trans- Verdeyen, 2007; Whipple and Trayler, 1996), thus the importance of climate in port-limited erosion in south-flowing rivers between the two zones of uplift reducing fan size in this case is uncertain, but still potentially relevant. could help facilitate drainage reversal. In this case, younger initiation of the Finally, an increase in regional base level could drive this apparent reduction southern uplift could cause deposition and a reduction in gradient in channel in alluvial fan size by effectively increasing the accommodation space closer to segments between the two zones of uplift, thereby facilitating headward ero- the range front. In Sarica and Vashlovani, the deposits immediately overlying sion by north-flowing drainages and eventual capture and drainage reversal. this alluvial fan complex are interpreted to have been deposited in a lacustrine However, if this occurred in the east-central Greater Caucasus we would ex- setting that correlates with the Akchagyl regional stage (Forte et al., 2014b), pect remnants of alluvial deposits in the low-relief area between the drainage which represents an extreme sea-level highstand in the Caspian that submerged divide and topographic crest. No such deposits are shown on geologic maps of much of the Kura Basin (e.g., Forte and Cowgill, 2013; Popov et al., 2006). this region, although the maps are large-scale overviews (e.g., Nalivkin, 1976).

Interpreting the LEM Results Implications for East-Central Greater Caucasus Tectonics

The LEM results provide good evidence that the current topography of the The initiation of an uplift zone coincident with the topographic crest after east-central Greater Caucasus results from northward propagation of uplift. the formation of the more southern uplift zone along the drainage divide im- The simplest interpretation of this finding is consistent with previous work that plies out-of-sequence deformation within the east-central Greater Caucasus. suggests it is difficult to modify an entrenched, antecedent drainage network This younger zone of uplift roughly corresponds with the area highlighted by once it is established (e.g., Castelltort and Simpson, 2006; Kühni and Pfiffner, Mosar et al. (2010) as a zone of increased uplift within the core of the east-cen- 2001; Oberlander, 1985). In addition, the modeling suggests that a simple north- tral Greater Caucasus; however, we do not consider this zone to be bounded

by active thrust as interpreted by Mosar et al. (2010). The gradual, decreasing or ramp is causing the southern uplift zone, the evidence for the current inac-

gradients in ksn on either side of the high ksn and local relief areas near the tivity of the range-front thrust systems suggests that these structures may be topographic crest are not consistent with predictions for a discrete, surface in the process of being deformed by movement on this blind structure (Fig. breaking fault (e.g., Kirby and Whipple, 2012). This strongly suggests that nei- 11). The geometry of the northern uplift is less clear and could be above either ther the southern range front fault, e.g., our Main Caucasus thrust, nor faults (1) an additional south-vergent duplex or ramp within the north-dipping thrust within the interior of the range near the topographic crest or drainage divide, system that appears to link the Main Caucasus thrust and the Kura fold-thrust e.g., the Main Caucasus thrust of Mosar et al. (2010), actively break the surface. belt, or (2) a north-vergent structure within with south-dipping décollement of Instead, we hypothesize that motion on the structures associated with both the Dagestan thrust belt. From the available data, we can also not eliminate the zones of uplift are blind and transfer slip into the foreland fold-thrust belts possibility of a high-angle structure within the interior of the range, similar to

along the margins of the range (Fig. 2C). Thus, these zones of uplift appear that depicted by Mosar et al. (2010), but if such a structure exists, the ksn data to illuminate the subsurface locations of geometric complexities (e.g., ramps, suggest that it does not break the surface. duplexes, or blind splay faults) along the main thrusts or shear zones that At this point we favor a south-vergent geometry for the northern uplift, as underlie the east-central Greater Caucasus. However, the topographic obser- geologic data attest to recent expansion of the south-directed Kura fold-thrust vations and LEM results do not provide a clear indication of the subsurface belt (Forte et al., 2010, 2013, 2014a; Mosar et al., 2010), and the Dagestan thrust geometries or vergence directions of the structures producing the uplift. In the belt along the northern margin of the range appears less active in comparison. case of the southern uplift zone coincident with the drainage divide (as noted Although we suggested (Forte et al., 2014a) that ~50% of the geodetically mea-

in Forte et al., 2014a), the prominent northward increasing ksn values along the sured convergence across this portion of the Greater Caucasus is accommo-

southern range front (Figs. 3 and 4) are most consistent with a similar north- dated on the north flank of the range, we also found relatively low snk values ward-increasing gradient in uplift rate (e.g., Gasparini and Whipple, 2014). This for rivers in this area (Figs. 3 and 4); this may suggest minimal activity in the gradient in uplift rate could be consistent with growth of a duplex underneath Dagestan thrust belt, but would also be consistent with movement on shal- the southern range front and would most likely be a south-vergent set of struc- low-dipping thrusts, consistent with geometries illustrated by Sobornov (1996;

tures, given the structural vergence within the Kura fold-thrust belt and struc- Fig. 11). Mainstem channels have high ksn where they cross the Dagestan thrust tures along strike within the southeastern Greater Caucasus. If either a duplex belt, but these are generally correlated with either lithologically supported

Sobornov, 1996 5 (k S Adamia et al., 2011 m) N 0 Lesser Caucasus KFTB Greater Caucasus DTB 0 Figure 11. Schematic crustal-scale cross section across the east-central Greater Caucasus modified from profile 3 in Forte et al., 2014a, fig. 8 therein). Structures at m) the southern and northern ends are from 50 Moho published cross sections (Adamia et al., Earthquake Magnitude 2011; Sobornov, 1996). Colored bars in- (Mumladze et al., 2015) dicate patterns of normalized channel Depth (k steepness. Earthquake locations are from Mumladze et al. (2015). Geometries within Inferred Slab the central portion of the range are uncon- 100 246 strained, but are consistent with surface 0 100 200 300 observations and results of the landscape Across Strike Distance (km) evolution modeling. KFTB—Kura fold- Channel thrust belt; DTB—Dagestan thrust belt. Steepness Structure from Published Cross Section Inferred Slab Boundary High Hypothetical Structure Topography Moho (Zor, 2008) 5x Vertically Exaggerated Topography Low

knickpoints (e.g., knickzones occurring at transition between overlying car- 2006; Kühni and Pfiffner, 2001). Similar to the findings presented by Kühni bonate and underlying clastics) or anthropogenic features (e.g., Chirkey Dam, and Pfiffner (2001), our results suggest that the drainage network of orogens Fig. 3; Forte et al., 2014a). More work focused on the neotectonics of the (e.g., divide positions) is strongly influenced by the location of active struc- Dagestan thrust belt is needed to resolve this question. tures during early orogenesis. Considering this example in the context of the Reactivation of structures within the hinterland of an orogen can also occur expectations of models of doubly vergent orogenic wedges, the east-central during an accretion cycle, which is when a discrete thrust sheet is added to the Greater Caucasus demonstrate that local variations in structural geometry can orogen, causing the deformation front to expand (e.g., Gutscher et al., 1998; strongly influence the topographic form of a doubly vergent orogen. Hoth et al., 2007). The formation of the Kura fold-thrust belt may reflect such We suggested (Forte et al., 2014a) that the Greater Caucasus may represent an accretion cycle. To estimate the time scale of a full accretion cycle we use an orogen in the early stages of transition from single to double vergence. the following expression from Hoth et al. (2007): The form of the drainage network may be similarly transitional, and evolv- ing toward a topology that is more consistent with a doubly vergent orogen. t = 4D/v, (1) As highlighted in Forte et al. (2014a), the juxtaposition between high and low

in which accretion time scale (t) is a function of the décollement depth (D) ksn values to the south and north of the drainage divide, respectively, is not and convergence velocity (v). Using a detachment depth of 10 km (Fig. 11) stable (e.g., Willett et al., 2014). The drainage divide will only maintain its po- and a convergence velocity for this portion of the orogen of between 6 and 8 sition as long as the structure beneath it is active. If activity diminished on this mm/yr (Forte et al., 2014a; Reilinger et al., 2006) suggests that the time scale structure, we speculate that the drainage divide would migrate northward, and of a full accretion cycle is 5–6 m.y. This time scale is similar to the age of the assuming no further change in structural geometries, will eventually be more Kura fold-thrust belt (ca. 3–2 Ma; Forte et al., 2010, 2013), suggesting that active closely associated with the topographic crest of the range. The rate of this uplift within the interior of the range could result from formation of the Kura scarp retreat would depend on the rate of structurally driven uplift, the erosive fold-thrust belt during an accretion cycle. Analogue models indicate that such power of the rivers draining the southeastern Greater Caucasus, and the erod- hinterland activity can be expressed as either reactivation of older structures ibility of the different rock types in these catchments. This would also poten- (e.g., Hoth et al., 2007) or formation of new hinterland duplexes during under- tially bring the overall topographic profile of the east-central Greater Caucasus thrusting (Gutscher et al., 1998). The latter case is more consistent with obser- closer to the predictions of a doubly vergent wedge model, with a wider, more vations noted here that indicate a lack of evidence for surface-breaking struc- shallowly sloping prowedge in the south and, narrower, steeper retrowedge tures in the interior of the range or along the northern margin of the Alazani in the north and a coincident drainage divide and topographic crest. However, Basin, in the east-central Greater Caucasus. the reasons for the similar widths and uncharacteristic topographic form of An alternative option, given the location of the northern uplift zone near the the prowedge and retrowedge within the modern Greater Caucasus remain an center of the range and in the vicinity of deep (>50 km) earthquakes interpreted active area of research. as being related to subduction and/or underthrusting of Kura Basin lithosphere (e.g., Forte et al., 2014a; Mumladze et al., 2015), is that the formation of this uplifting zone is related to changes in the subsurface geometry of the sub- CONCLUSION ducting and/or underthrusting lithosphere, possibly as increasingly thick, and more buoyant lithosphere is underthrust (Fig. 11). Distinguishing whether the Coupling topographic, structural, and stratigraphic observations from the activity of these hinterland structures is related to a structural change within east-central Greater Caucasus with results of landscape evolution modeling the orogen or simply part of an accretion cycle requires better documentation suggests recent structural reorganization or reactivation within the interior of both the geometries and histories of these zones and the responsible struc- of this portion of the orogen. While the loci of active shortening structures tures. Regardless of the exact cause of these zones, active uplift in the core of have progressively stepped southward, largely abandoning the Main Cauca- the range is consistent with results from low-temperature thermochronome- sus thrust and other north-dipping thrusts within the main range to form the try, indicating that the youngest thermochronometric ages, and thus the most Kura fold-thrust belt, results presented here suggest that active uplift within rapid exhumation, occur in the center of the range (e.g., Avdeev, 2011; Avdeev the interior of the range also stepped northward. LEMs that best reproduce and Niemi, 2011). the relief and drainage structure of the east-central Greater Caucasus involve two discrete zones of uplift, with a southern zone near the modern-day range Drainage Networks and Active Deformation front initiating first, followed by the initiation of a northern zone of uplift, near the modern-day topographic crest. The available data do not allow us to de- The LEM results presented here are consistent with previous work on the fine subsurface structural geometries beneath these zones of elevated uplift interaction between developing orogens and rivers that suggest it is very dif- rate, but the simplest explanation is formation of south-vergent duplexes or ficult to modify antecedent drainage networks (e.g., Castelltort and Simpson, developments of ramps at depth that feed slip to active shortening structures

in the Kura fold-thrust belt. While we consider the LEM results robust, they re- Dodonov, A.E., and Baiguzina, L.L., 1995, Loess stratigraphy of central Asia: Palaeoclimatic and quire field verification, especially considering the stratigraphic evidence from palaeoenvironmental aspects: Quaternary Science Reviews, v. 14, p. 707–720, doi:10 .1016 /0277-3791(95)00054-2. the Kura fold-thrust belt that could indicate a southward propagation of uplift. Dotduyev, S.I., 1986, Nappe structure of the Greater Caucasus Range: Geotectonics, v. 20, More generally, our results also demonstrate that local structural geometry p. 420–430. can dramatically influence the topographic evolution of an orogen, specifically Forte, A.M., 2012, Late Cenozoic evolution of the Greater Caucasus Mountains and Kura foreland basin: Implications for early orogenesis: Davis, University of California, 348 p. related to maintaining the location of drainage divides. Forte, A.M., and Cowgill, E., 2013, Late Cenozoic base-level variations of the Caspian Sea: A new review of its history and proposed driving mechanisms: Palaeogeography, Palaeoclimatol- ogy, Palaeoecology, v. 386, p. 392–407, doi:10.1016/j.palaeo.2013.05.035. ACKNOWLEDGMENTS Forte, A.M., Cowgill, E., Bernardin, T., Kreylos, O., and Hamann, B., 2010, Late Cenozoic deformation of the Kura fold-thrust belt, southern Greater Caucasus: Geological Society of America We thank Jean Braun for providing the source code for the FastScape landscape evolution model Bulletin, v. 122, p. 465–486, doi:10.1130/B26464.1. and Byron Adams for discussions regarding modeling methodology. We also thank editor Ray- Forte, A.M., Cowgill, E., Murtuzayev, I., Kangarli, T., and Stoica, M., 2013, Structural geometries mond Russo and reviewer Silvan Hoth for helpful comments that improved this manuscript. and magnitude of shortening in the eastern Kura fold-thrust belt, Azerbaijan: Implications This material is partially based upon work supported by National Science Foundation grant EAR- for the development of the Greater Caucasus Mountains: Tectonics, v. 32, doi:10 .1002 /tect 0810285 (Cowgill) and a School of Earth and Space Exploration Postdoctoral Fellowship at Arizona .20032. State University (Forte). Forte, A.M., Cowgill, E., and Whipple, K.X., 2014a, Transition from a singly vergent to doubly vergent wedge in a young orogen: The Greater Caucasus: Tectonics, v. 33, p. 2077–2101, doi: 10.1002/2014TC003651. REFERENCES CITED Forte, A.M., Sumner, D.Y., Cowgill, E., Stoica, M., Murtuzayev, I., Kangarli, T., Elashvili, M., Adamia, S., Zakariadze, G.S., Chkhotua, T., Sadradze, N., Tsereteli, N., Chabukiani, A., and Godoladze, T., and Javakishvirli, Z., 2014b, Late Miocene to Pliocene stratigraphy of the Kura Gventsade, A., 2011, Geology of the Caucasus: A review: Turkish Journal of Earth Sciences, Basin, a subbasin of the South Caspian Basin: Implications for the diachroneity of stage v. 20, p. 489–544. boundaries: Basin Research, v. 27, p. 247–271, doi:10.1111 /bre.12069. Allen, M.B., Jackson, J., and Walker, R., 2004, Late Cenozoic reorganization of the Arabia-Eur- Gabunia, L., Vekua, A., and Lordkipanidze, D., 2000, The environmental contexts of early human asia collision and the comparison of short-term and long-term deformation rates: Tectonics, occupation of Georgia (Transcaucasia): Journal of Human Evolution, v. 38, p. 785–802, doi: v. 23, TC2008, doi:10.1029/2003TC001530. 10.1006/jhev.1999.0383. Argus, D.F., Gordon, R.G., Heflin, M.B., Ma, C., Eanes, R.J., Willis, P., Peltier, W.R., and Owen, Gasparini, N.M., and Whipple, K.X., 2014, Diagnosing climatic and tectonic controls on topog- S.E., 2010, The angular velocities of the plates and the velocity of Earth’s centre from space raphy: Eastern flank of the northern Bolivian Andes: Lithosphere v. 6, p. 230–250, doi:10 geodesy: Geophysical Journal International, v. 180, p. 913–960, doi:10 .1111 /j.1365-246X .2009 .1130/L322.1. .04463.x. Gasparini, N.M., Whipple, K.X., and Bras, R.L., 2007, Predictions of steady state and transient Avdeev, B., 2011, Tectonics of the Greater Caucasus and the Arabia-Eurasia orogen [Ph.D. thesis]: landscape morphology using sediment-flux-dependent river incision models: Journal of Ann Arbor, University of Michigan, 137 p. Geophysical Research, v. 112, F03S09, doi:10 .1029 /2006JF000567 . Avdeev, B., and Niemi, N.A., 2011, Rapid Pliocene exhumation of the central Greater Caucasus Gobejishvili, R., Lomidze, N., and Tielidze, L., 2011, Late Pleistocene (Würmian) glaciations of the constrained by low-temperature thermochronometry: Tectonics, v. 30, TC2009, doi:10 .1029 Caucasus, in Ehlers, J., et al., eds., Quaternary glaciations—Extent and chronology: A closer /2010TC002808. look: Developments in Quaternary Science 15: Amsterdam, Elsevier, p. 141–147, doi:10 .1016 Bonnet, S., 2009, Shrinking and splitting of drainage basins in orogenic landscapes from the /B978-0-444-53447-7.00012-X. migration of the main drainage divide: Nature Geoscience, v. 2, p. 766–771, doi:10 .1038 Gutscher, M.-A., Kukowski, N., Malavieille, J., and Lallemand, S.E., 1998, Episodic imbricate /ngeo666. thrusting and underthrusting: Analog experiments and mechanical analysis applied to the Bonnet, S., and Crave, A., 2003, Landscape response to climate changes: Insights from experi- Alaskan accretionary wedge: Journal of Geophysical Research, v. 103, p. 10,161–10,176, doi: mental modeling and implications for tectonic versus climatic uplift of topography: Geology, 10.1029/97JB03541. v. 31, p. 123–126, doi:10.1130/0091-7613(2003)031<0123:LRTCCI>2.0.CO;2. Hardy, S., 1995, A method for quantifying the kinematics of fault-bend folding: Journal of Struc- Borisov, A.A., 1965, Climates of the U.S.S.R.: Edinburgh and London, Oliver & Boyd, 255 p. tural Geology, v. 17, p. 1785–1788, doi:10.1016/0191-8141(95)00077-Q. Braun, J., and Willett, S.D., 2013, A very efficient 0(n), implicit and parallel method to solve the Hardy, S., 1997, A velocity description of constant-thickness fault-propagation folding: Journal of stream power equation governing fluvial incision and landscape evolution: Geomorphol- Structural Geology, v. 19, p. 893–896, doi:10.1016/S0191-8141(97)00013-8. ogy, v. 180–181, p. 170–179, doi:10.1016/j.geomorph.2012.10.008. Hijmans, R.J., Cameron, S.E., Parra, J.L., Jones, P.G., and Jarvis, A., 2005, Very high resolution in- Braun, J., and Yamato, P., 2010, Structural evolution of a three-dimensional, finite-width crustal terpolated climate surface for global land areas: International Journal of Climatology, v. 25, wedge: Tectonophysics, v. 484, p. 181–192, doi:10.1016/j.tecto.2009.08.032. p. 1965–1978, doi:10.1002/joc.1276. Castelltort, S., and Simpson, G., 2006, River spacing and drainage network growth in widening Hoth, S., Adam, J., Kukowski, N., and Oncken, O., 2006, Influence of erosion on the kinematics mountain ranges: Basin Research, v. 18, p. 267–276, doi:10.1111/j.1365-2117.2006.00293.x. of bivergent orogens: Results from scaled sandbox simulations, in Willett, S.D., et al., eds., Clevis, Q., De Boer, P.L., and Wachter, M., 2003, Numerical modeling of drainage basin evolution Tectonics, climate, and landscape evolution:: Geological Society of America Special Paper and three-dimensional alluvial fan stratigraphy: Sedimentary Geology, v. 163, p. 85–110, doi: 398, p. 201–225, doi:10.1130/2006.2398(12). 10.1016/S0037-0738(03)00174-X. Hoth, S., Hoffmann-Rothe, A., and Kukowski, N., 2007, Frontal accretion: An internal clock for Dade, W.B., and Verdeyen, M.E., 2007, Tectonic and climatic controls of alluvial-fan size and bivergent wedge deformation and surface uplift: Journal of Geophysical Research, v. 112, source-catchment relief: Journal of the Geological Society (London), v. 164, p. 353–358, doi: B06408, doi:10.1029/2006JB004357. 10.1144/0016-76492006-039. Hovius, N., and Stark, C.P., 2006, Landslide-driven erosion and topographic evolution of active Davy, P., and Lague, D., 2009, Fluvial erosion/transport equation of landscape evolution models mountain belts, in Evans, S.G., et al., eds., Landslides from massive rock slope failure: NATO revisited: Journal of Geophysical Research, v. 114, F03007, doi:10 .1029 /2008JF001146 . Science Series Volume 49, p. 573–590, doi:10.1007/978-1-4020-4037-5_30. DiBiase, R.A., Whipple, K.X., Heimsath, A.M., and Ouimet, W.B., 2010, Landscape form and mil- Jackson, J., 1992, Partitioning of strike-slip convergent motion between Eurasia and Arabia in lenial erosion rates in the San Gabriel Mountains, CA: Earth and Planetary Science Letters, eastern Turkey and the Caucasus: Journal of Geophysical Research, v. 97, p. 12,471–12,479, v. 289, p. 134–144, doi:10.1016/j.epsl.2009.10.036. doi:10.1029/92JB00944.

Joannin, S., et al., 2010, Early Pleistocene climate cycles in continental deposits of the Lesser Reilinger, R., et al., 2006, GPS constraints on continental deformation in the Africa-Arabia-Eur- Caucasus of Armenia inferred from palynology, magnetostratigraphy, and 40Ar/39Ar: Earth asia continental collision zone and implications for the dynamics of plate interactions: Jour- and Planetary Science Letters, v. 291, p. 149–158, doi:10 .1016 /j .epsl .2010 .01 .007 . nal of Geophysical Research, v. 111, B05411, doi:10 .1029 /2005JB004051 . Kadirov, F., Floyd, M., Alizadeh, A., Guliev, I., Reilinger, R., Kuleli, S., King, R., and Toksoz, M.N., Roe, G.H., Montgomery, D.R., and Hallet, B., 2003, Orographic precipitation and the re- 2012, Kinematics of the eastern Caucasus near Baku, Azerbaijan: Natural Hazards, v. 63, lief of mountain ranges: Journal of Geophysical Research, v. 108, 2315, doi:10.1029 p. 997–1006, doi:10 .1007 /s11069 -012 -0199 -0 . /2001JB001521. Khain, V.Y., 1975, Structure and main stages in the tectono-magmatic development of the Cauca Ruppel, C., and McNutt, M., 1990, Regional compensation of the Greater Caucasus mountains sus: An attempt at geodynamic interpretation: American Journal of Science, v. 275 A, based on an analysis of Bouguer gravity data: Earth and Planetary Science Letters, v. 98, p. 131–156. p. 360–379, doi:10.1016/0012-821X(90)90037-X. Kirby, E., and Whipple, K.X., 2012, Expression of active tectonics in erosional landscapes: Journal Saintot, A., Brunet, M.-F., Yakovlev, F., Sebrier, M., Stephenson, R.A., Ershov, A.V., Chalot-Prat, F., of Structural Geology, v. 44, p. 54–75, doi:10 .1016 /j .jsg .2012 .07 .009 . and McCann, T., 2006, The Mesozoic–Cenozoic tectonic evolution of the Greater Caucasus, in Konstantinovskaya, E., and Malavieille, J., 2005, Erosion and exhumation in accretionary oro- Gee, D.G., and Stephenson, R.A., eds., European lithosphere dynamics: Geological Society gens: Experimental and geological approaches: Geochemistry, Geophysics, Geosystems, of London Memoir 32, p. 277–289, doi:10.1144/GSL.MEM.2006.032.01.16. v. 6, Q02006, doi:10 .1029 /2004GC000794 . Sanders, C.A.E., Andriessen, P.A.M., and Cloetingh, S.A.P.L., 1999, Life cycle of the East Car Kovda, I., Mora, C.I., and Wilding, L.P., 2008, PaleoVertisols of the northwest Caucasus: (Micro)mor- pathian orogen: Erosion history of a doubly vergent critical wedge assessed by fission track phological, physical, chemical, and isotopic constraints on early to late Pleistocene climate: thermochronology: Journal of Geophysical Research, v. 104, p. 29,095–29,112, doi:10 .1029 Journal of Plant Nutrition and Soil Science, v. 171, p. 498–508, doi:10.1002 /jpln.200700037 . /1998JB900046. Kühni, A., and Pfiffner, O.A., 2001, Drainage patterns and tectonic forcing: A model study for the Schwanghart, W., and Scherler, D., 2014, Short communication: TopoToolbox 2—MATLAB based Swiss Alps: Basin Research, v. 13, p. 169–197, doi:10 .1046 /j .1365 -2117 .2001 .00146 .x . software for topographic analysis and modeling in Earth surface sciences: Earth Surface Kvavadze, E., and Vekua, A., 1993, Vegetation and climate of the Dmanisi man period (East Geor- Dynamics, v. 2, p. 1–7, doi:10.5194/esurf-2-1-2014. gia) from palynological data: Acta Palaeobotanica, v. 23, p. 343–355. Sklar, L., and Dietrich, W.E., 2006, The role of sediment in controlling steady-state bedrock chan- Lydolph, P.E., 1977, Climates of the Soviet Union: World Survey of Climatology Volume 7: Amster- nel slope: Implications of the saltation-abrasion incision model: Geomorphology, v. 82, dam, Elsevier Scientific Publishing Company, 435 p. p. 58–83, doi:10.1016/j.geomorph.2005.08.019. Mellors, R.J., Jackson, J., Myers, S.C., Gok, R., Priestly, K., Yetirmishli, G., Turkelli, N., and Sklar, L., and Dietrich, W.E., 2008, Implications of the saltation-abrasion bedrock incision model Gololadze, T., 2012, Deep earthquakes beneath the Northern Caucasus: Evidence of active or for steady-state river longitudinal profile relief and concavity: Earth Surface Processes and recent subduction in western Asia: Seismological Society of America Bulletin, v. 102, p. 862– Landforms, v. 33, p. 1129–1151, doi:10.1002/esp.1689. 866, doi:10 .1785 /0120110184 . Skolbeltsyn, G., Mellors, R., Gök, R., Türkelli, N., Yetirmishli, G., and Sandvol, E., 2014, Upper Messager, E., Lordkipanidze, D., Kvavadze, E., Ferring, C.R., and Voinchet, P., 2010, Palaeoenviron- mantle S wave velocity structure of the East Anatolian–Caucasus region: Tectonics, v. 33, mental reconstruction of Dmanisi site (Georgia) based on palaeobotanical data: Quaternary p. 207–221, doi:10.1002/2013TC003334. International, v. 223–224, p. 20–27, doi:10 .1016 /j .quaint .2009 .12 .016 . Sobornov, K.O., 1994, Structure and petroleum potential of the Dagestan thrust belt, northeast- Mitra, G., and Sussman, A.J., 1997, Structural evolution of connecting splay duplexes and their ern Caucasus, Russia: Bulletin of Canadian Petroleum Geology, v. 42, p. 352–364. implications for critical taper: An example based on geometry and kinematics of the Canyon Sobornov, K.O., 1996, Lateral variations in structural styles of tectonic wedging in the northeast- Range culmination, Sevier Belt, central Utah: Journal of Structural Geology, v. 19, p. 503–521, ern Caucasus: Bulletin of Canadian Petroleum Geology, v. 44, p. 385–399. doi:10 .1016 /S0191 -8141 (96)00108 -3 . Stark, C.P., 2010, Oscillatory motion of drainage divides: Geophysical Research Letters, v. 37, Montgomery, D.R., Balco, G., and Willett, S.D., 2001, Climate, tectonics, and the morphology of L04401, doi:10.1029/2009GL040851. the Andes: Geology, v. 29, p. 579–582, doi:10 .1130 /0091 -7613 (2001)029 <0579: CTATMO>2 .0 Stolar, D.B., Willett, S.D., and Roe, G.H., 2006, Climatic and tectonic forcing of a critical orogen, .CO;2. in Willett, S.D., et al., eds., Tectonics, climate, and landscape evolution: Geological Society Mosar, J., Kangarli, T., Bochud, M., Glasmacher, U.A., Rast, A., Brunet, M.-F., and Sosson, M., 2010, of America Special Paper 398, p. 241–250, doi:10 .1130 /2006 .2398(14). Cenozoic–recent tectonics and uplift in the Greater Caucasus: A perspective from Azerbaijan, Stolar, D.B., Willett, S.D., and Montgomery, D.R., 2007, Characterization of topographic steady in Sosson, M., et al., eds., Sedimentary basin tectonics from the Black Sea and Caucasus to state in Taiwan: Earth and Planetary Science Letters, v. 261, p. 421–431, doi:10.1016/j .epsl the Arabian Platform: Geological Society of London Special Publication 340, p. 261–280, doi: .2007.07.045. 10.1144 /SP340 .12 . Tucker, G.E., Lancaster, S.T., Gasparini, N.M., and Bras, R.L., 2001, The channel-hillslope inte- Mumladze, T., Forte, A.M., Cowgill, E., Trexler, C.C., Niemi, N.A., Yikilmaz, M.B., and Kellogg, grated landscape development model (CHILD), in Harmon, R.S., and Doe, W.W., eds., Land- L.H., 2015, Subducted, detached, and torn slabs beneath the Greater Caucasus: Journal of scape erosion and evolution modeling: New York, Kluwer AcademicPlenum Publishers, Geophysical Research, v. 5, p. 36–46, doi:10 .1016 /j .grj .2014 .09 .004 . p. 349–388. Nalivkin, D.V., 1976, Geologic map of the Caucasus (in Russian): Moscow, U.S.S.R., Ministry of van Baak, C.G.C., Vasiliev, I., Stoica, M., Kuiper, K.F., Forte, A.M., Aliyeva, E., and Krijgsman, W., Geology, scale 1:500,000. 2013, A magnetostratigraphic time frame for Plio-Pleistocene transgressions in the South Oberlander, T.M., 1985, Origin of drainage transverse to structures in orogens, in Morisawa, M., Caspian Basin, Azerbaijan: Global and Planetary Change, v. 103, p. 119–134, doi:10 .1016 /j and Hacker, J.T., eds., Tectonic geomorphology: Binghamton Symposia in Geomorphology .gloplacha.2012.05.004. 15: Boston, Allen & Unwin, p. 155–182. Vernant, P., and Chery, J., 2006, Low fault friction in Iran implies localized deformation for the Pelletier, J.D., 2004, Persistent drainage migration in a numerical landscape evolution model: Arabia-Eurasia collision zone: Earth and Planetary Science Letters, v. 246, p. 197–206, doi:10 Geophysical Research Letters, v. 31, L20501, doi:10.1029 /2004GL020802 . .1016/j.epsl.2006.04.021. Philip, H., Cisternas, A., Gvishiani, A., and Gorshkov, A., 1989, The Caucasus: An actual example Waltham, D., and Hardy, S., 1995, The velocity description of deformation. Paper 1: Theory: of the initial stages of continental collision: Tectonophysics, v. 161, p. 1–21, doi:10 .1016 /0040 Marine and Petroleum Geology, v. 12, p. 153–163, doi:10.1016/0264-8172(95)92836-L. -1951(89)90297 -7 . Whipple, K.X., 2009, The influence of climate on the tectonic evolution of mountain belts: Nature Popov, S.V., Shcherba, I.G., Ilyina, L.B., Nevesskaya, L.A., Paramonova, N.P., Khondkarian, S.O., Geoscience, v. 2, p. 97–104, doi:10 .1038 /ngeo413 . and Magyar, I., 2006, Late Miocene to Pliocene palaeogeography of the Paratethys and its Whipple, K.X., and Trayler, C.R., 1996, Tectonic control of fan size: The importance of spatially relation to the Mediterranean: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 238, variable subsidence rates: Basin Research, v. 8, p. 351–366, doi:10 .1046 /j.1365-2117 .1996 p. 91–106, doi:10 .1016 /j .palaeo .2006 .03 .020 . .00129.x. Popov, S.V., Antipov, M.P., Zastroshnov, A.S., Kurina, E.E., and Pinchuk, T.N., 2010, Sea-level fluc- Whipple, K.X., and Tucker, G.E., 1999, Dynamics of the stream-power river incision model: Impli- tuations on the northern shelf of the eastern Paratethys in the Oligocene–Neogene: Stratig- cations for height limits of mountain ranges, landscape response timescales, and research raphy and Geological Correlation, v. 18, p. 200–224, doi:10 .1134 /S0869593810020073 . needs: Journal of Geophysical Research, v. 104, p. 17,661–17,674, doi:10.1029 /1999JB900120 .

Willett, S.D., 1999, Orogeny and orography: The effects of erosion on the structure of mountain Wobus, C.W., Whipple, K.X., Kirby, E., Snyder, N.P., Johnson, J., Spyropolou, K., Crosby, B.T., and belts: Journal of Geophysical Research, v. 104, p. 28,957–28,981, doi:10 .1029 /1999JB900248 . Sheehan, D., 2006, Tectonics from topography: Procedures, promise, and pitfalls, in Willett, Willett, S.D., and Brandon, M.T., 2002, On steady states in mountain belts: Geology, v. 30, p. 175– S.D., et al., eds., Tectonics, climate, and landscape evolution: Geological Society of America 178, doi:10.1130/0091-7613(2002)030<0175:OSSIMB>2.0.CO;2. Special Paper 398, p. 55–74, doi:10.1130/2006.2398(04). Willett, S.D., Beaumont, C., and Fullsack, P., 1993, Mechanical model for the tectonics of doubly Youn, J.H., Seong, Y.B., Choi, J.H., Abdrakhmatov, K., and Ormukov, C., 2014, Loess deposits in vergent compressional orogens: Geology, v. 21, p. 371–374, doi:10.1130 /0091 -7613(1993)021 the northern Kyrgyz Tien Shan: Implications for the paleoclimate reconstruction during the <0371:MMFTTO>2.3.CO;2. late Quaternary: Catena, v. 117, p. 81–93, doi:10.1016/j.catena.2013.09.007. Willett, S.D., Slingerland, R.L., and Hovius, N., 2001, Uplift, shortening, and steady state topogra- Zor, E., 2008, Tomographic evidence of slab detachment beneath eastern Turkey and the Cauca- phy in active mountain belts: American Journal of Science, v. 301, p. 455–485. sus: Geophysical Journal International, v. 175, p. 1273–1282. Willett, S.D., McCoy, S.W., Perron, J.T., Goren, L., and Chen, C.-Y., 2014, Dynamic reorganization Zubakov, V.A., 2001, History and causes of variations in the Caspian Sea level: The Miopliocene, of river basins: Science, v. 343, 6175, doi:10 .1126 /science .1248765. 7.1–1.95 million years ago: Water Resources, v. 28, p. 249–256, doi:10.1023 /A: 1010440421772.