Tracking sediment provenance and erosionalevolution of the western Greater CaucasusGiovanni Vezzoli,1* Eduardo Garzanti,1 Stephen J. Vincent,2 Sergio Andò,1 Andrew Carter3 and Alberto Resentini11 Laboratory for Provenance Studies, Department of Earth and Environmental Sciences, Università Milano-Bicocca, Milano, Italy2 CASP, Department of Earth Sciences, Cambridge University, Cambridge, UK3 London Thermochronological Research Group, Research School of Earth Sciences, UCL – Birkbeck College, London, UK

Received 17 January 2013; Revised 3 March 2014; Accepted 6 March 2014

*Correspondence to: Giovanni Vezzoli, Laboratory for Provenance Studies, Department of Earth and Environmental Sciences, Università Milano-Bicocca, 20126 Milano,Italy. E-mail: giovanni.vezzoli@unimib.it

ABSTRACT: This article investigates landscape characteristics and sediment composition in the western Greater Caucasus by usingmultiple methods at different timescales. Our ultimate goal is to compare short-term versus long-term trends in erosional processes andto reconstruct spatio-temporal changes in sediment fluxes as controlled by partitioning of crustal shortening and rock uplift in theorogenic belt. Areas of active recent uplift are assessed by quantitative geomorphological techniques [digital elevation model (DEM)analysis of stream profiles and their deviation from equilibrium] and compared with regions of rapid exhumation over longer time inter-vals as previously determined by fission-track and cosmogenic-nuclide analyses. Complementary information from petrographic andheavy-mineral analyses of modern sands and ancient sandstones is used to evaluate erosion integrated throughout the history of theorogen. River catchments displaying the highest relief, as shown by channel-steepness indices, correspond with the areas of most rapidexhumation as outlined by thermochronological data. The region of high stream gradients is spatially associated with the highest topog-raphy around Mount Elbrus, where sedimentary cover strata have long been completely eroded and river sediments display the highestmetamorphic indices and generally high heavy-mineral concentrations. This study reinforces the suggestion that the bedrock–channelnetwork can reveal much of the evolution of tectonically active landscapes, and implies that the controls on channel gradient ultimatelydictate the topography and the relief along the Greater Caucasus. Our integrated datasets, obtained during a decade of continuingresearch, display a general agreement and regularity of erosion patterns through time, and consistently indicate westward decreasingrates of erosional unroofing from the central part of the range to the Black Sea. Copyright © 2014 John Wiley & Sons, Ltd.

KEYWORDS: tectonic geomorphology; concavity and channel-steepness indices; sedimentary petrology; heavy minerals; Kuban River; Laba River;Belaya River; Inguri River; Rioni River

Introduction

Drainage patterns in orogenic belts are primarily controlled bytectonic strain, and stream profiles may reorganize dynamicallyin response to tectonic perturbations. Landscape topographycan thus be used to reconstruct the distribution of tectonic upliftwithin zones of crustal deformation (Castelltort et al., 2012).Rivers, however, may also be passive features (Castelltort andSimpson, 2006), and record long-term continental conver-gence and collision (Hallet and Molnar, 2001) or past reorgani-zation events driven by surface uplift (Clark et al., 2004).Estimation of longer-timescale deformation requires trackinggeological features back in time, and can be investigated onlythrough a set of integrated techniques, including a variety of‘thermochronological’ methods (Lisker et al., 2009). The petro-graphic and mineralogical composition of sediments derivedfrom an evolving orogen is primarily dependent on plate-tectonic processes (Dickinson, 1985; Garzanti et al., 2007a),and thus represents a potentially effective additional tool totrace erosion processes both in space and time (Garzantiet al., 2007b; Najman et al., 2009).

In this article we illustrate the results of original geomorphologi-cal and quantitative petrographic-mineralogical analyses andcompare them with complementary data sets on the exhumationand erosional evolution of the western Greater Caucasus obtainedduring a decade of continuing research (Figure 1; Vincent et al.,2007, 2011). Like other high-relief collision orogens, the Caucasusrange is a spectacular product of plate tectonics and amajor sourceof terrigenous detritus. However, because it lies in a politicallytroubled region, its geological knowledge is largely incomplete.The aim of our analysis is to help fill this gap. By integrating a vari-ety of techniques, including landscape morphometry and prove-nance tracing plus short-term to long-term erosion chronometry,we present a picture of spatially variable but temporally steady ero-sion, mainly driven by tectonics and modulated by climate.

The Greater Caucasus

The Greater Caucasus is Europe’s highest mountain belt,reaching 5642 m above sea level (a.s.l.) at the summit of MountElbrus. Maximum topographic gradients coincide with the

EARTH SURFACE PROCESSES AND LANDFORMSEarth Surf. Process. Landforms (2014)Copyright © 2014 John Wiley & Sons, Ltd.Published online in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/esp.3567

central part of the belt, at an average elevation of ~2500 m. Thedrainage divide of the Greater Caucasus forms a major climaticboundary. The Pre-Caucasus foreland in the north belongs to adrier temperate zone, whereas the Transcaucasus in the south be-longs to awetter subtropical zone. Annual precipitation increasesfrom600 to 1800mm in the east to 1000 to 4000mm in the west,with maxima of 3000 to 4000 mm in the southwest. In the west-ern Greater Caucasus, precipitation is 1000–2000mm at 3000ma.s.l., and decreases with decreasing altitude (Figure 1; Krenke,1982; Dolgushin and Osipova, 1989).

Geological framework

The western Greater Caucasus is situated at the southern de-formed edge of the Scythian Platform and consists of accreted

arcs, continental fragments and sedimentary basins of theTethyside orogenic collage (Şengör and Natal’in, 1996). It com-prises a central zone of Gondwana-derived, predominantlylower to middle Paleozoic crystalline protolith and middlePaleozoic island arcs and ophiolites that were metamorphosedand intruded during their Variscan accretion to the southernmargin of Laurasia (Zonenshain et al., 1990; Somin et al., 2006;Zakariadze et al., 2007). Uppermost Paleozoic to Mesozoicsediments onlap this crystalline basement (Figure 2), and recorda series of tectonic events that resulted from Tethyan active-margin processes to the south (Gaetani et al., 2005). Middle toLate Triassic compression/transpression (the early Cimmerianorogeny, sensu Nikishin et al., 2001) was followed by EarlyJurassic extension/transtension. Extensive basaltic volcanismwas associated with formation of a deep sedimentary troughalong the southern flank of the range (the Greater Caucasus

Figure 1. Hillshaded DEM of the western Greater Caucasus showing the studied rivers and the position of modern-sand samples. Annual rainfallacross the Caucasus region is shown in inset. This figure is available in colour online at wileyonlinelibrary.com/journal/espl

Figure 2. Geological map and steepness indices for the studied river channels. Cross-sections through the western Greater Caucasus (×2 verticalexaggeration). This figure is available in colour online at wileyonlinelibrary.com/journal/espl

G. VEZZOLI ET AL.

Copyright © 2014 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms, (2014)

Basin; Nikishin et al., 2001; Saintot et al., 2006). Middle Jurassic(middle Cimmerian) deformation led to the partial inversion ofthis basin before further extension/transtension occurred inEarly Cretaceous time.Uplift of the western Greater Caucasus began around the

Eocene-Oligocene boundary (Lozar and Polino, 1997; Nikishinet al., 2001; Saintot et al., 2006; Vincent et al., 2007). Thisresulted in the partial inversion of the Greater Caucasus Basinand the exhumation of its northern rift margin to expose thecrystalline core of the range. The amount and style of deforma-tion are disputed, with both thick- and thin-skinned tectonicmodels having been applied (Saintot et al., 2006). South-vergent thin-skinned deformation is apparent along its southernflank. However, steeply dipping, elongate sinuous fault traceswithin the core and internal parts of its southern flank suggestthat thick-skinned, strike-slip deformation dominates, most likelyreflecting oblique convergence and presence of inherited struc-tures (Somin, 2000; Vincent et al., 2011). The range is highlyasymmetricwith its northern flank having undergone only limitedamounts of fault-related deformation (Figure 2).Foreland-basin development took place during two syn-

collisional stages, each displaying markedly different subsidencepatterns and sedimentary facies. The 34–16 Ma ‘Maikopian’ pre-foreland stage displays a long-wavelength subsidence of a broadarea, whereas the 16–0 Ma foreland stage displays asymmetricalsubsidence (Ershov et al., 1999). The stratigraphic change fromwidespread deposition of the Maikop Group mudrocks duringthe Oligocene-Burdigalian to ‘molassic’ sedimentation in theLanghian-Serravallian testifies to much coarser sediment supplysince ~16 Ma (Ershov et al., 1999). This turning point in the ero-sional evolution of the mountain belt marks the change from the‘underfilled’ to the ‘overfilled’ stage of foreland-basin sedimenta-tion. Rapid exhumation is documented by thermochronologicaldata to have taken place between 18 and 13 Ma all along theBitlis Orogen in the south (Okay et al., 2010). This coincidencein time suggests a link between the onset of the morphogenicphase of mountain building in the Caucasus (Gansser, 1982)and the transition from ‘soft’ to ‘hard’ collision between Arabiaand Asia.

Methods

To understand the landscape evolution of the western GreaterCaucasus and to assess spatial and temporal patterns of erosion,we have integrated existing information on river sediment fluxes(Jaoshvili, 1986, 2002), sandstone petrography (Vincent et al.,2013, 2014), zircon fission-track, apatite fission-track, apatite(U–Th)/He, and beryllium-10 (10Be) cosmogenic-nuclide data(Avdeev and Niemi, 2011; Vincent et al., 2011) with originalgeomorphological analysis and bulk-petrography and heavy-mineral data on modern sands. Specifically, we have deter-mined quantitatively the concavity and steepness indices ofselected river catchments in the western, north-western andsouth-western parts of the Greater Caucasus, and estimated therelative sediment contributions from different catchment areas.

River profile analysis

For each of 18 rivers draining the western Greater Caucasus(between 42°N 39°E and 45°N 44°E; Figure 1), we performedstream-profile analysis by evaluating stream gradients and theirdeviation from ideal equilibrium. The fluvial network was de-lineated in TecDEM (software shell implemented in MATLAB;Shahzad and Gloaguen, 2011) from a 30 m resolution digitalelevation model (DEM) provided by ASTER GDEM (http://

www.gdem.aster.ersdac.or.jp). Where streams are graded, thenthe steepness index ks and the concavity index θ values areconstant along the entire stream. These values change wheretransient perturbations are observed (e.g. changes in precipitation,tectonic uplift, and lithology). At topographic steady state, con-cave longitudinal profiles of fluvial channels can be describedusing a power-law relationship between local channel slope Sand contributing drainage area A (Hack, 1957; Flint, 1974):

S ¼ ksA"θ (1)

Whipple (2004) identified four degrees of concavity (θ). Lowconcavities (< 0.4) are associated either with short steep drain-age influenced by debris flows, or with downstream increase inincision rate or rock strength, commonly related to knickpoints.Moderate concavities (0.4–0.7) are associated with activelyuplifting bedrock channels in homogeneous substratesexperiencing uniform rock uplift. High concavities (0.7–1.0)are associated with the downstream decrease in rock-uplift rateor rock strength. A downstream transition to fully alluvial condi-tions and disequilibrium conditions results from a temporaldecline in rock-uplift rate. Extreme concavities (negative or> 1)are associated with abrupt knickpoints, related to either pro-nounced along-stream changes in substrate properties or spatialor temporal differences in rock-uplift rate, including transitionsfrom incisional to depositional conditions. Following Snyderet al. (2000); Whipple (2004); Wobus et al. (2003, 2006);Schlunegger et al. (2011); Norton and Schlunegger (2011), afixed reference concavity (θ ref = 0.45) was used to calculate anormalized steepness index (ksn). Using a reference concavityovercomes the inherent correlation of regression slope and in-tercept in Equation 1, and facilitates comparison of gradientsin channels with widely varying drainage areas. However,where profile concavity differs substantially from the referencevalue, the ksn inferred from the regression along the entirechannel contains little information about the variation ofchannel gradient, because it reflects only the mean slope (Kirbyet al., 2003). Lithologic contrasts may produce differences inchannel-steepness indices similar to those associated withgradients in uplift rates (Whipple, 2004).

Sediment fluxes

Modern erosion rates in a mountain catchment can be esti-mated by measuring fluxes of river sediments (Ahnert, 1970).The sediment load of a river is the mass of detritus transportedpast a point in unit time, usually expressed in tons per year (a).The sediment yield (SY), usually expressed as tons/km2 a, isobtained by dividing the sediment load by the area (A) of thedrainage basin. The erosion rate, usually expressed in mm/a, iscalculated from the sediment yield and the density (ρ) of thematerial removed. Besides the large inaccuracies associatedwith the uncertain assessment of sediment fluxes, the calcula-tion of erosion rates is affected by an imperfect knowledge ofrock densities (Einsele and Hinderer, 1997; Hinderer, 2013).

Between summer 1998 and summer 2004, 27 samples ofloose bedload sand were collected from active fluvial barsand deltaic spits in Russia and Georgia. Sand samples wereimpregnated with Araldite, cut into standard thin sections,stained with alizarine red to distinguish dolomite and calcite,and analysed by counting 400 points under the microscope(Gazzi-Dickinson method; Ingersoll et al., 1984). Sands wereclassified according to their main components (Q=quartz;F = feldspars; L = lithic fragments), considered only whereexceeding 10%QFL and listed in order of abundance (e.g. in

EROSION PATTERNS IN THE WESTERN GREATER CAUCASUS

Copyright © 2014 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms, (2014)

a litho-feldspatho-quartzose sand Q> F> L> 10%QFL); anadjective reflecting the most common rock-fragment type maybe added (e.g. carbonaticlastic, volcaniclastic). Full quantitativeinformation was collected on coarse-grained rock fragments,and metamorphic types were classified according to protolithcomposition and metamorphic rank. Average rank of rock frag-ments in each sample was expressed by the MetamorphicIndices MI and MI*. MI varies from zero (in detritus shed byexclusively sedimentary and volcanic cover rocks) to 500(in very-high-rank detritus shed by exclusively high-grade base-ment rocks), whereas MI* considers only metamorphic rockfragments and thus varies from 100 (in very-low-rank detritusshed by exclusively very low-grade metamorphic rocks) to500 (Garzanti and Vezzoli, 2003). Very low- to low-rank meta-morphic lithics, for which protolith can still be inferred, aresubdivided into metasedimentary (Lms) and metavolcanic(Lmv) categories. Medium- to high-rank metamorphic lithicsare subdivided into felsic (metapelite, metapsammite andmetafelsite; Lmf) and mafic (metabasite; Lmb) categories.Median grain size was determined in thin section by rankingand visual comparison with sieved standards.From a split aliquot of the 63–250 μm class, dense minerals

were separated by centrifuging in sodium (Na) metatungstate(density 2.90 g/cm3) and recovered by partial freezing with liq-uid nitrogen. From each sample, 200 to 250 transparent detritalminerals were counted in grain mounts by the area method(Mange and Maurer, 1992). Heavy-mineral concentrations(HMCs) were calculated as the volume percentage of totalHMC and transparent heavy-mineral concentration (tHMC)(Garzanti and Andò, 2007). Heavy-mineral suites are describedas ‘extremely poor’ (HMC< 0.1), ‘very poor’ (0.1≤HMC<0.5),‘poor’ (0.5≤HMC< 1), ‘moderately poor’ (1≤HMC<2),‘moderately rich’ (2≤HMC< 5), ‘rich’ (5≤HMC< 10), and‘very-rich’ (10≤HMC<20). The ‘Source Rock Density’ (SRD)index is defined as the weighted average density of extrabasinalterrigenous grains, and used as an estimator of the averagedensity of source rocks in the absence of hydraulic effects. TheZTR index is the sum of zircon, tourmaline and rutile over totaltransparent heavy minerals (Hubert, 1962). The ‘HornblendeColour Index’ HCI and ‘Metasedimentary Minerals Index’MMI (Andò et al., 2013) were used to estimate the averagemetamorphic grade of metaigneous and metasedimentarysource rocks, respectively. They vary from zero in detritus fromgreenschist-facies to lowermost amphibolite-facies rocks yield-ing exclusively blue/green amphibole and chloritoid, to 100 indetritus from granulite-facies rocks yielding exclusively brownhornblende and sillimanite. Detrital components are listed in or-der of abundance throughout the article. The main petrographicand heavy-mineral parameters are provided in Table I.

River Morphometry and Sediment Load

The concavity and channel-steepness indices were derivedfrom regression analysis of channel-gradient and drainage-areadata relative to the length of the channel encompassed by theregression interval (Table II). The geographic distribution ofvalues allowed us to group river catchments in eastern (Malka,Baksan), north-eastern (Podkumok, Kuma), northern (Kuban, Urup),north-western (Laba, Fars, Belaya), western (Ashe, Shakhe, Mzimta)and southern regions (Inguri, Tskhenistskali, Rioni; Figure 2). Toquantify modern erosion rates, specific attention was dedicatedto the Kuban, Mzimta, Inguri and Rioni Rivers, for whichsediment fluxes are available (Table III).River channels in the western Greater Caucasus display a wide

range of concavity indices, ranging from quasi linear (θ =0.15) tohighly concave profiles (θ = 0.99). Normalized channel-steepness

(ksn) also vary widely, from 57 to as high as 222, indicating strongvariability in mean channel gradients. These changes in channelsteepness vary markedly among river catchments in the northernand southern parts of the western Greater Caucasus.

Eastern and north-eastern catchments

The Malka River and its Baksan right-bank tributary are sourcedfrom Mount Elbrus glaciers and eventually drain eastwardstowards the Caspian Sea. They have high concavities in theirupper reaches (θ = 0.89 and 0.99, respectively) and moderateconcavities in their lower reaches (θ = 0.52 and 0.69, respec-tively). The steepness indices are high and increase furtherdownstream (from 128 to 149 for the Baksan, and from 117to 181 for the Malka (Figure 3).

Markedly different are the Kuma River and its Podkumokright-bank tributary, which rise in the low-altitude foothillregion and flow across a Jurassic to Cenozoic sedimentarysuccession (Figure 2). They show generally smooth profile withnearly uniform concavities (θ = 0.52 and 0.57, respectively) andlow channel steepness indices (ksn = 72 and 85, respectively;Figure 3), consistent with the expectation for concave channelprofiles where rock properties, patterns of rock uplift andprecipitation rates are nearly constant (Kirby et al., 2003;Whipple, 2004).

Northern catchments

The Kuban River is sourced from the western slopes of MountElbrus at ~1340 m a.s.l., and flows northwestward throughthe Indolo-Kuban plain before reaching the Sea of Azov. Thetotal length of the river is 870 km and its drainage basin is 57900 km2. The alluvial plain has elevations≤ 200 m, and repre-sents 52% of the catchment area. Foothills and highlands(elevations up to 5500 m a.s.l.) represent the remaining 48%(Lourie et al., 2005). The Krasnodar reservoir, the largest inthe northern Caucasus, is located in the Kuban basin 242 kmupstream of the mouth. After construction of the KrasnodarDam in 1975, the original sediment load of the river(8.4× 106 ton/a) decreased by ~90% (Table III; Algan et al., 1999).Our geomorphological analysis focused on the highland andfoothill regions. In the upper reaches close to Mount Elbrus,the Kuban River has a high-gradient channel (ksn = 169) andmoderate concavity (θ = 0.66). In the lower reaches, whereJurassic, Cretaceous and Cenozoic strata are exposed, theconcavity value decreases (θ = 0.45), whereas the channel-steepness index remains constant.

The Urup River, a left-bank tributary of the Kuban, rises in thePaleozoic crystalline core of the Greater Caucasus (Figure 2),and crosses into the sedimentary foothill region ~10 kmdownstream. Such fault-bounded lithological change is reflectedby markedly decreasing concavity and steepness indices fromθ = 0.78 and ksn = 219 upstream, to θ = 0.32 and ksn = 76downstream (Figure 4).

North-western catchments

The headwaters of the Bolshaya (Greater) Laba River drain diversetectonic units, including Paleozoic metamorphic rocks, a tectonicsliver of Jurassic strata, and next an Upper Carboniferous-Permiansuccession. About 70 km downstream, it begins to cross aMesozoic to Cenozoic sedimentary succession. The BolshayaLaba has uniform concavity (θ = 0.43) and moderate steepness

G. VEZZOLI ET AL.

Copyright © 2014 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms, (2014)

TableI.

Petrog

raph

ican

dhe

avy-mineral

mod

esformod

ernsand

sfrom

thewestern

Greater

Cau

casus

NQ

FLv

LcLsm

LmLu

MI

MI*

HMC

tHMC

ZTR

Brt

Ttn+Ap

EpGrt

MM

Amp

Px+Ol

&HCI

MMI

Northeastrivers

Kum

a1

5318

221

60

010

0.0

28n.d.

0.3

0.1

305

92

404

53

210

0.0

n.d.

56Po

dkum

ok1

4014

1623

61

010

0.0

2318

92

0.4

2316

84

160

528

010

0.0

n.d.

n.d.

Northernrivers

Kub

an1

4128

170

48

110

0.0

140

277

76

00

0.5

163

0.5

782

010

0.0

20n.d.

Urup

156

266

25

50

100.0

177

298

21

210

435

1410

130

210

0.0

1155

North-w

estern

rivers

BolshayaLaba

257

257

05

61

100.0

219

289

64

30.2

143

71

441

0.3

100.0

683

53

20

10

110

201

11

0.3

23

01

30

0.4

1MaliL

aba

343

214

114

135

100.0

217

257

85

20.2

643

50.3

431

010

0.0

2n.d.

15

11

41

05

92

22

0.3

12

10.3

51

02

Fars

171

82

135

10

100.0

177

300

11

120

648

155

140

010

0.0

545

Upp

erBelaya

227

134

247

70

100.0

138

175

43

0.2

01

365

0.2

562

010

0.0

3n.d.

81

10

43

00

120

00

01

33

0.3

01

02

Lower

Belaya

344

173

427

40

100.0

124

179

21

30.2

2.9

325

252

30

100.0

350

64

13

55

017

151

13

0.3

0.8

50

12

10

2Western

rivers

Ashe/Sh

akhe

217

96

2441

30

100.0

6314

12

13

04

396

019

290

100.0

20n.d.

17

212

14

056

301

13

03

1315

04

40

0Mzimta

213

723

550

20

100.0

9112

85

30

01.2

51

015

790

100.0

7n.d.

62

123

242

010

103

20

00.2

40

02

20

South-western

rivers

Rioni

519

1113

1830

90

100.0

123

173

52

04

15

10

1079

0.2

100.0

36n.d.

32

36

63

015

123

10

41

31

03

90 .3

8Tskren

i1

183

66

5810

010

0.0

165

175

10.1

50

54

00

878

010

0.0

n.d.

n.d.

Ingu

ri2

3937

11

158

010

0.0

235

284

21

32

1118

118

425

0.5

100.0

1688

215

01

75

064

930.4

15

110

90

26

00.7

1110

Note:

meanvalues

inbo

ldtype

face,stand

ardde

viationin

italic

s.N=nu

mbe

rof

samples;Q

=qu

artz;F

=feldspars;L=ap

hanite

lithicgrains

(Lvm

=vo

lcan

ican

dlow-ran

kmetavolcanic;

Lc=carbon

ate;

Lsm=othe

rsed-

imen

tary

andlow-ran

kmetased

imen

tary;Lm

=high

-ran

kmetam

orph

icinclud

ingmetab

asite

;Lu

=serpen

tinite

).MI*

andMI=

Metam

orph

icIndicesrefle

ctingaveragemetam

orph

icgrad

eof

source

rock.HMC

and

tHMC=vo

lumepe

rcen

tage

oftotala

ndtran

sparen

theavy

mineral

conc

entrations

onbu

lksample.

ZTR

=zircon

,tou

rmaline,

rutilean

dothe

rTi

oxides;B

rt=ba

rite;T

tn=titan

ite;A

p=ap

atite

;Ep=ep

idote;

Grt=garnet;

MM

=ch

loritoid,staurolite

,and

alusite

,kyanite,sillim

anite

;Amp=am

phiboles;P

x=py

roxene

;Ol=

olivine;

&=othe

rs(Cr-spinel,m

onazite

,xen

otim

e).H

CI=

Hornb

lend

eColou

rInd

ex;M

MI=

Metased

imen

tary

Minerals

Inde

x(afte

rAnd

òet

al.,20

13);n.d.

=no

tdetermined

.

EROSION PATTERNS IN THE WESTERN GREATER CAUCASUS

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index (ksn = 96), consistent with the expectation for steady-statechannel profiles experiencing uniform uplift.The Malaya (Lesser) Laba River is sourced in Paleozoic

metabasite and granitoid basement units of the Greater Caucasus(Figure 2). In the upper reaches, it flows through a series oftightly folded and tectonically-bounded Upper Carboniferousto Anisian strata including serpentine-schist conglomerates(Gaetani et al., 2005), and displays moderate concavity(θ = 0.37) and steepness index (ksn =118). In the lower reaches,where Mesozoic to Cenozoic sedimentary strata are exposed,concavity increases (θ = 0.46) whereas the steepness indexdecreases (ksn =101)most likely due to a decrease in rock strength.The Fars River rises in the foothills and drains a Mesozoic-

Cenozoic sedimentary succession. It displays relatively lowconcavity (θ = 0.33) and low steepness index (ksn = 73). The Be-laya River flows through Triassic-Jurassic and fault-boundedUpper Carboniferous-Permian strata, before cutting across thefault-bounded Paleozoic granitoids of the Dakhovsky Massifand finally through Jurassic to Miocene sedimentary covers.The Belaya River shows moderate concavity (θ = 0.45) andsteepness index (ksn = 117; Figure 5).

Western catchments

The Ashe and Shakhe Rivers rise in the foothills on the southernside of the western Caucasus and flow through Jurassic toCenozoic strata (Figure 2). They exhibit high concavities(θ = 0.70 and 0.75, respectively) and low steepness indices(ksn = 57 and 78, respectively), associated with the downstream

decrease in rock strength of younger sedimentary rocks(Whipple, 2004).

The largest river flowing into the Black Sea along the Russiancoast is the Mzimta River, with a drainage area of 885 km2, anaverage annual runoff of 1.6 km3, and a suspended sedimentload of 0.3 106 ton/a (Table III; Jaoshvili, 2002; Mikhailova,2009). The Mzimta is sourced in Jurassic sedimentary and vol-canic rocks, and flows through Cretaceous and Cenozoic strataof the foothills. Lithologic transitions are mostly fault-bounded,as reflected in a composite river profile characterized by sev-eral knickpoints, high concavity (θ = 0.69), and high steepnessindex (ksn = 122; Figure 6).

Southern catchments

The Inguri River rises from a number of glaciers and springs in thecore of the Greater Caucasus (Mount Shkhara, 5068 m a.s.l.) andflows into the Black Sea 213 km downstream. Basin area is4060 km2, average altitude 1840 m a.s.l., and average annualrunoff 1.1 km3. Upstream of the Inguri Dam, one of the highestarch dams in the world (height 272 m), the river flows acrossMesozoic and next Upper Carboniferous-Permian sedimentarystrata. After dam construction in 1978, total sediment flux hasbeen reduced by 86%, from 2.9 to 0.5 106 ton/a (Khalatyan,1977; Jaoshvili, 1986). The Inguri River displays relatively lowconcavity (θ = 0.32) and high steepness index (ksn = 198). Itstwo right-bank tributaries, sourced in the southwestern slopesof Mount Elbrus within Paleozoic metamorphic rocks, similarly

Table II. Geomorphological parameters of studied river catchments in the western Greater Caucasus

Rivers

Area Perimeter River length Mean elevation Mean slope Concavity Steepness

(km2) (km) (km) (m) (%) (θ) (ksn)

Kuma 3196 475 125 631 7 0.52 72Podkumok 1328 260 76 1355 17 0.57 85Malka 2794 545 182 1372 18 0.99–0.69 117–181Baksan 2554 514 168 2045 36 0.89–0.52 128–149Kuban 57900 1027 304 1325 23 0.66–0.45 169Urup 3229 597 227 865 13 0.78–0.32 219–76Bolshaya Laba 1661 403 131 1652 35 0.43 96Malaya Laba 4842 640 176 1331 28 0.37–0.46 118–101Fars 642 165 48 515 9 0.33 73Belaya 2343 409 131 1156 31 0.45 117Ashe 279 114 32 588 32 0.75 57Shakhe 554 185 59 878 42 0.70 78Mzimta 885 286 90 1301 42 0.69 122Inguri 4060 626 200 2032 47 0.32 198Inguri tributaries 625 190 45 2195 49 0.28–0.15 178–222Rioni 13400 871 274 1048 35 0.47 128Tskhenistskali 2138 527 168 1679 43 0.23–0.45 146–77

Table III. Sediment-load data (sources cited in text) and calculated average sediment yield and erosion rates for the studied river catchments in thewestern Greater Caucasus

Rivers

Area Precipitation Suspended load Total load Sediment yield Erosion rates

(km2) (mm) (ton/a) (ton/a) (ton/km2 a) (mm/a)

Kuban 57900 400–1600 n.d. 8400000 145±45 0.05±0.02Mzimta 885 >2000 260000 371429 420±214 0.16±0.08Inguri 4060 1600–2000 2700000 2910000 717±222 0.27±0.08Rioni 13400 1600–2000 n.d. 9550000 713±264 0.26±0.10

Note: n.d. = not determined.

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show low concavities (θ = 0.15 and 0.28, respectively) and highsteepness indices (ksn = 222 and 178, respectively; Figure 2).The Rioni River, originating from the Pasi mountain glacier at

3785 m a.s.l. on the southern slope of the main Caucasus ridge,flows into the Black Sea. River length is 327 km, basin area13 400 km2, average inclination 7.2‰, and average elevation1084 m. In the upper reaches, the Rioni cuts across Jurassic,Cretaceous and Cenozoic sedimentary and volcanic strata alonga deep narrow gorge. Because of steep mountain topography,with slope inclination commonly exceeding 12°, mudflowsand landslides occur frequently after periods of intense rainfall.The upper Rioni River displays moderate concavity (θ = 0.47)and high steepness index (ksn = 128; Figure 7). The Rioni Riveris regulated by the Gumati Dam and by three other major damsplus three hydroelectric reservoirs on its tributaries. Because ofhigh sediment load, the storage capacity of the Gumati Reservoirhas been reduced from the initial 18×106 m3 in 1958 to only

1×106 m3; sediment flux downstream of the dam is estimated tohave varied from the original 7.6×106 ton/a to 3.7×106 ton/a(www.adaptation-fund.org). Other studies variously estimatedthe pre-dam total sediment load at 6.7×106 ton/a (Khalatyan,1977), 8.5×106 ton/a (Cagatay, 1997) or as high as 13.5×106

ton/a (Khmaladze, 1978). The suspended sediment load was esti-mated at 6×106 ton/a (Table III; Jaoshvili, 2002; Mikhailova,2009).

The Tskhenistskali River (a right-bank tributary of the Rioni) is176 km long and drains an area of 2120 km2. Rising in theglaciers of the Vodorazdel’nyi Range, it cuts across tectonicslivers of Upper Paleozoic, Mesozoic and Cenozoic strata

Figure 4. Area, longitudinal river profile and slope–area data for thenorthern catchments of the western Greater Caucasus. This figure isavailable in colour online at wileyonlinelibrary.com/journal/espl

Figure 3. Area, longitudinal river profile and slope–area data for theeastern and north-eastern catchments of thewesternGreater Caucasus. Thisfigure is available in colour online at wileyonlinelibrary.com/journal/espl

Figure 5. Area, longitudinal river profile and slope–area data for thenorth-western catchments of the western Greater Caucasus.

Figure 6. Area, longitudinal river profile and slope–area data for thewestern catchments of the western Greater Caucasus.

EROSION PATTERNS IN THE WESTERN GREATER CAUCASUS

Copyright © 2014 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms, (2014)

exposed along the walls of a deep canyon with rapids andwaterfalls. In these upper reaches the channel has low concavity(θ = 0.23) and high steepness index (ksn =146), whereasmoderateconcavity (θ = 0.45) and low steepness index (ksn = 77) character-ize the lower course (Table II).

Sediment Provenance

Rivers draining the southern side of the Caucasus and flowing tothe Black Sea carry quartzo-lithic (Ashe, Mzimta, Tskhenistskali;Figure 8E) to feldspato-quartzo-lithic (Rioni, Shakhe) sedimentaclasticto low-rank metasedimentaclastic sands with common carbon-ate and volcanic rock fragments (Table I). Moderately poor tomoderately rich heavy-mineral suites are either pyroxene-dominated with subordinate amphibole and epidote (Rioni,Tskhenistskali, Mzimta), or richer in epidote (Shakhe). Onlythe Inguri River carries sand notably rich in feldspars, granitoidrock fragments and higher-rank metamorphic lithics, withmoderately rich amphibole> epidote> garnet heavy-mineralsuites (Figure 8A).Along the northern side of the Caucasus, sands carried by rivers

chiefly draining the foothills range from feldspato-quartzo-lithicto litho-quartzose carbonaticlastic with variable amounts ofvolcanic rock fragments (Podkumok, Kuma, Fars; Figure 8F).Poor heavy-mineral assemblages include garnet, ZTR (zircon,tourmaline, rutile) and barite (Kuma, Podkumok), or are epi-dote-rich with garnet, amphibole and ZTR (Fars). Commonclinopyroxene and hypersthene derived from local volcanicdomes characterize Podkumok sand. Rivers sourced in the

crystalline core of the orogen carry feldspatho-litho-quartzoseto litho-feldspatho-quartzose sands, with feldspars, rank ofmetamorphic rock fragments and HMCs increasing from west(Belaya; Figure 8D) to east (Laba, Kuban; Figures 8B and 8C).Downstream, detritus increases in quartz due to recycling ofCenozoic siliciclastic units exposed in the foothills (e.g. lowerBelaya). Malaya Laba sand contains serpentine-schist rock frag-ments derived from Variscan basement rocks or recycled fromLower Triassic conglomerates. Volcanic rock fragments derived fromMount Elbrus are conspicuous in Kuban sand. Rich, amphibole-epidote (Laba and Malaya Laba) or amphibole-dominated (Kuban)heavy-mineral assemblages with HCI indices up to 20 point toprovenance from lower and middle amphibolite-facies base-ments, respectively. The ZTR index is higher and HMC lowerin Urup sand, indicating extensive recycling of molassic clasticwedges exposed along the front of the range.

Modern Erosion Rates and Controls

On a short-term perspective (101–104 a) erosion rates mainlydepend on the interplay of area, relief, climate, lithology andhuman activity (Meybeck, 1976; Brozović et al., 1997; Hay,1998; Molina et al., 2008; Korup and Schlunegger, 2009;Hoffmann et al., 2010), notwithstanding the role of tectonics insetting the pace of longer-term (105–106 a) orogenic exhumation(Raymo and Ruddiman, 1992; Koppes and Montgomery, 2009).In this section we discuss present-day erosion patterns as de-duced from sediment fluxes and morphometry of river slopes,and investigate the effects of climate and lithology as potentialcontrols of channel gradient in mountain catchments. Later inthe paper, the larger-scale spatio-temporal picture of erosion willbe investigated using thermochronogical and compositional data.

Climate

Direct relationships between precipitation and erosion rateshave been inferred since the classical study of Langbein andSchumm (1958), and numerous subsequent works have focusedon the strong link between climate and erosion (e.g. Wobuset al., 2003; Burbank, 2005; Whipple, 2009). Higher erosionrates should be associated with a reduction of channel gradients(e.g. Whipple and Tucker, 1999; Roe et al., 2002), because riverincision more effectively balances the effect of rock uplift.

We calculated sediment yields and modern erosion rates forthe Kuban, Mzimta, Inguri and Rioni Rivers, for which esti-mates of suspended or total sediment load are available(Table III). Uncertainties could be assessed only for the RioniRiver, for which more than a single estimate is available. A sim-ilar error was assumed for the other three rivers. Where onlysuspended load was available (e.g. Mzimta River), we have as-sumed that bedload represents 32% of the total flux followingthe regional study by Khmaladze (1978). Considering the largeuncertainties involved, and that bedload may reach up to 50%of total load in mountain catchments (Turowski et al., 2010),we assumed an error of ± 20%. Integrated petrographic andmineralogical data indicate that the grain density of fluvialsands, considered as a proxy for average density of sourcerocks, has a limited range of variability in different catchments,from 2.67 to 2.74 g/cm3. A value of 2.70±0.03 g/cm3 was thusassumed in our calculations.

The very large Kuban catchment in the northwest has moderaterainfall andmoderate average sediment yield (145±45 ton/km2 a)and erosion rates (0.05 ± 0.02 mm/a). The much smallerMzimta catchment in the west has higher rainfall, averagesediment yield (420 ±214 ton/km2 a) and erosion rates

Figure 7. Area, longitudinal river profile and slope–area data for thesouthern catchments of the western Greater Caucasus.

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(0.16 ±0.08 mm/a). The Inguri catchment in the southwesthas the highest rainfall (> 2000 mm/a; Figure 1), sediment yield(717±222 ton/km2 a) and erosion rate (0.27±0.08 mm/a). TheRioni catchment in the south has similar climatic conditions,sediment yield (713±264 ton/km2 a) and erosion rate(0.26±0.10 mm/a).Our study on the western Greater Caucasus thus lends

full support to the close relationship between orographicprecipitation and sediment yield (Whipple, 2009). However,we did not observe the predicted decrease in channel gradientsassociated with higher precipitation and erosion rates(Kirby et al., 2003). Notwithstanding the widely establishedfeedback between climate and short-term denudation incollisional orogens (Masek et al., 1994; Burbank, 2005;Gabet et al., 2008; Schlunegger et al., 2011), the high gradientsin the southern slopes of the Greater Caucasus appear tobe maintained in spite of high rainfall, indicating activetectonic uplift.

Lithology

The differential resistance to erosion offered by contrasting lithol-ogies has long been recognized to control channel gradient andlandscapemorphology (e.g. Hack, 1957, 1973). To assess the po-tential role of lithology, we have compared the spatial pattern ofchannel-steepness indices with mapped geological units. In gen-eral, we did not observe a strong influence of lithology on chan-nel profiles, and changes in slope–area arrays seldom correspondwith marked lithological changes (Figure 2). For instance, theKuban River cuts across resistant Paleozoic granitoid and meta-morphic rocks or weaker Mesozoic and Cenozoic strata down-stream, and yet it does not show any significant gradientchange along its course. Changes in channel steepness coincidewith lithologic boundaries only in a few cases, where river pro-files are characterized by marked convexities and correspondingsteps in slope–area arrays. Most evident is the Urup River case,which displays a prominent decrease in gradient, and changes

C

B

D

E F

MALAYA LABA (MI* 264 - MI 216)

MZIMTA RIVER (MI* 121 - MI 98) KUMA RIVER (MI* n.d. - MI 28)

BELAYA (MI* 184 - MI 139)

LABA (MI* 303 - MI 226)

Ls

A INGURI RIVER (MI* 350 - MI 280)

K

Lu

a

ms

Ls

Lc

bs

PK

Q

a

Q

P

m

Q

P

eLms

Lms

Lc

Lc

Q

Ls

Lms

Figure 8. Contrasting petrography of modern sands in the western Greater Caucasus. Metamorphic indices MI* and MI (Garzanti and Vezzoli, 2003)faithfully reflect the different unroofing stages reached in different catchment areas. Crystalline basement is exhumed in the upper catchments of theInguri and Laba Rivers, whereas cover strata are undissected in the Belaya, Mzimta and Kuma basins. Q=quartz; K=K-feldspar; P = plagioclase;L= lithic fragment (Lc=carbonate; Ls= sedimentary; Lms=very-low to low-rank metasedimentary; Lu= serpentine-schist). Micas: b=biotite; m=musco-vite. Heavy minerals: a =amphibole; e= epidote; s = sillimanite. Photographs with crossed polars; blue bar =250 μm. This figure is available in colouronline at wileyonlinelibrary.com/journal/espl

EROSION PATTERNS IN THE WESTERN GREATER CAUCASUS

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from high concavity to low concavity after entering sedimentarystrata of the foothills (Figure 4). Other examples are the upperBaksan and Malka catchments in the Elbrus region of high-gradient channels. About 10 km downstream of the source, theMalka River passes from erodible Quaternary volcanic depositsto resistant Paleozoic metamorphic basement, and the riverprofile displays a prominent convex section coinciding with aband of crystalline rocks (Figure 3), where the steepness indexincreases abruptly (Table II). Besides these examples, however,lithological effects account poorly for the systematic regionalpatterns in channel-steepness indices.

Erosion Rates in Space and Time

In this section we characterize average erosion levels reached indiverse catchments through time. Spatial and temporal patternsof uplift and exhumation are reconstructed by integrating mor-phometric features of river profiles and mineralogical data onmodern sands, with the available information from fission-trackand cosmogenic-nuclide analysis (Avdeev and Niemi, 2011;Vincent et al., 2011) and petrographic analysis of Oligo-Miocenesandstones (Vincent et al., 2013, 2014).

Erosion chronometers

Zircon and apatite fission-track data as well as (U–Th)/He dataand detrital 10Be cosmogenic nuclide concentration allowedVincent et al. (2011) to outline two major phases of coolingand exhumation. The first one followed the major Cimmerianorogenic event in the Upper Triassic/Middle Jurassic, whereasthe second one took place during the Oligo-Miocene moun-tain-building phase. Thermochronological data and field rela-tionships within the core of the range west of Mount Elbrusrequire ~5 km of Permo-Triassic exhumation, and restrict theoverall amount of Cenozoic exhumation to ~2.5 km. To the

east of Mount Elbrus, overall exhumation is calculated to be nomore than ~6 km, with up to ~4.5 km of this occurring since theOligocene (Avdeev and Niemi, 2011). Exhumation was 3–5 kmin the Inguri catchment (Vincent et al., 2011). Current cooling ratesare typically low, except to the south and east of Mount Elbrus,where mineral cooling ages suggest Pliocene exhumation ratesof ~1 mm/a (Vincent et al., 2011) and ~0.5 mm/a, respectively(Avdeev and Niemi, 2011). Exhumation rates are based on an es-timated geothermal gradient of 40°C/km.Despite a general lack ofsignificant seismicity within the study region, this exhumationpeak characterizing the southern flank of the range in northwestGeorgia corresponds with one of the most seismically active areasof the Greater Caucasus (e.g. Racha earthquake of April 29, 1991,Ms=7.1).

River profiles and exhumation

The observed pattern of channel profiles in the western GreaterCaucasus cannot be explained by climatic and lithological ef-fects alone. The role of tectonic activity must be examined aswell. Thermochronological data suggest a direct relationshipbetween active rock uplift and channel steepness indices. Inthe western Greater Caucasus, the region of high-gradientchannels (ksn≥ 180; Figure 2) is located close to Mount Elbrus(Baksan and Kuban Rivers), with maximum values in the south(Inguri basin). Apart from minor lithological effects, the upperKuban and Baksan Rivers display moderate concavity (meanθ = 0.48 and 0.49, respectively) and high steepness indices(mean ksn 169 and 135, respectively), suggesting uniform rockuplift (Whipple, 2004; Kirby et al., 2003). The Inguri Riverand its tributaries display relatively low concavity of channelprofiles and high steepness indices unrelated to precipitationand lithology but consistent with high, tectonically-drivenexhumation rates. In contrast, the generally smooth profiles withuniform concavities of the Kuma, Podkumok, Bolshaya Laba,

Figure 9. Framework petrography and heavy minerals in modern sands of the western Greater Caucasus. Q=quartz; F = feldspar; L = aphanitic lithicfragments (0–1=unmetamorphosed to very low rank; 2–3= low to medium rank; 4–5=high to very high rank; Garzanti and Vezzoli, 2003).Unroofing trends for Collision Orogen Provenance after Garzanti et al. (2004). MM=metasedimentary minerals (chloritoid, staurolite, andalusite,kyanite, sillimanite). &HM=other transparent heavy minerals. Samples with<20% and>20% pyroxene/total transparent heavy minerals are plottedin the upper and lower parts of the diamond-shaped diagram. In order to obtain a better visualization, the compositional data in the upper part havebeen centred (von Eynatten et al., 2002). This figure is available in colour online at wileyonlinelibrary.com/journal/espl

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Figure 10. Petrographic, mineralogical and thermochronological evidence concur to indicate that exhumation rates are highest in the upper Inguricatchment at the crystalline core of the orogen, where rainfall and relief are maximum, and lowest towards the Black Sea coast in the west. (A) Erosionrates (after Vincent et al., 2011). (B) Areal distribution of Metamorphic Indices MI* and MI in modern sands. (C) Areal distribution of heavy-mineralconcentration indices HMC and tHMC in modern sands. These indices may be markedly influenced locally by hydraulic-sorting processes or litho-logical factors (e.g. high for mafic rocks, low for granites). This figure is available in colour online at wileyonlinelibrary.com/journal/espl

EROSION PATTERNS IN THE WESTERN GREATER CAUCASUS

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Malaya Laba, Fars and Belaya Rivers, associated with low tomod-erate channel steepness, indicate moderate to low exhumation.

Sandstone petrography through time

The western Greater Caucasus has been a subaerial source ofdetritus since the earliest Oligocene (Lozar and Polino, 1997;Vincent et al., 2007). Oligocene to Lower Miocene sandstones(34–16 Ma) range from litho-quartzose sedimentaclastic tofeldspatho-quartzose in southern and northern parts of thewestern Greater Caucasus in Russia, respectively. Feldspato-quartzo-lithic sedimentaclastic/volcaniclastic sandstones withabundant mudrock and carbonate rock fragments characterizethe western Greater Caucasus in Georgia (Vincent et al., 2013).Sandstones shed northwards contain more plutonic and meta-morphic detritus derived from the crystalline basement of therange (Vincent et al., 2013). Differences in the composition ofsandstones shed southwards are largely dependent on differencesin the along-strike lithology and age of strata recycled during theinversion of the Greater Caucasus Basin (Vincent et al., 2014).Middle to Upper Miocene sandstones in Russia have less

quartz than their older ‘Maikopian’ counterparts, probably be-cause of less prolonged weathering than during the Oligocene-Burdigalian stage of slower exhumation (Vincent et al., 2013).Middle to Upper Miocene sandstones in Georgia do not showmajor compositional changes, and are still feldspato-quartzo-lithic to quartzo-feldspato-lithic sedimentaclastic/volcaniclastic.River sands derived from the western Greater Caucasus today

display two similar end-member compositions (Figure 8). Lithicto litho-feldspatho-quartzose sedimentaclastic/volcaniclasticsands are recycled from sedimentary (e.g. Kuma River;Figure 8F), very-low-rank metasedimentary, and volcanic coverstrata (e.g. Mzimta River; Figure 8E), whereas feldspatho-quartzose sands are eroded from axial basement rocks (e.g.Inguri and Laba Rivers; Figures 8A and 8B). Such differentcompositions testify, respectively, to only incipient and welladvanced unroofing of the crystalline backbone of the range(Figure 9). Sediments carried by most major rivers display inter-mediate composition, documenting mixed provenance includ-ing neovolcanic (e.g. Kuban River draining Mount Elbrusvolcano) or paleovolcanic sources (e.g. Rioni and TskhenistskaliRivers draining largely Jurassic volcanic rocks).

Unroofing of the Greater Caucasus

The mineralogy of modern sands helps us to quantify the de-gree of unroofing reached in different areas of the orogenicbelt. As a proxy for erosion level in each river catchment, wecan use a series of petrographic and mineralogical parametersthat allow us to quantify the average metamorphic grade of ex-posed parent rocks (Garzanti et al., 2010). In particular, MI andMI* can be employed to trace integrated erosion since the on-set of surface uplift, provided that the original isopach patternand basin architecture are taken into due account.The highest indices characterize upper Inguri sand, fed by

granitoid and upper-amphibolite-facies metasediments of thecrystalline core and subordinately from Paleozoic to Mesozoicstrata (MI* 350, MI 280; Figure 10B). Indices decrease down-stream, reflecting increasing supply from Mesozoic strata ofthe Greater Caucasus Basin (MI* 219, MI 190). Metamorphicindices are also relatively high for the Laba (MI* 289±20,MI 219±10) and its Malaya Laba tributary (MI* 257±9,MI 217±5), and decrease markedly westward in the Belaya(MI* 177±12, MI 130±15) and in the Mzimta and other catch-ments of westernmost Caucasus (MI* 134± 20, MI 77±37).

Indices similar as those of Belaya sands characterize thesoutherly-draining Rioni and its tributary, the Tskhenistskali(MI* 173±11, MI 130±22). The lowest indices characterizethe Kuma and Podkumok Rivers draining only cover strata(MI 25±4).

Because the continental crust is stratified in terms of chemi-cal composition and density (Rudnick and Gao, 2003), and be-cause denser rocks contain and thus can shed a larger quantityof dense minerals, also HMC and tHMC indices can be used astracers of the average erosion level reached in each catchment.HMC, however, may be influenced strongly by lithological fac-tors or hydraulic-sorting processes, which must be carefullyconsidered and corrected for (Garzanti et al., 2009). For in-stance, mafic volcanic or metabasite sources (e.g. Malaya Laba)supply large amounts of heavy minerals, whereas very few areshed by sedimentary or granitic rocks (Garzanti and Andò,2007). HMCs display a westward-decreasing trend along thenorthern edge of the western Greater Caucasus (Figure 10C),being higher in Laba and Malaya Laba sands (HMC 7.4 ±1.7,tHMC 6.3±1.6) than in Belaya sands (HMC 2.7±1.4, tHMC2.2 ±1.3). Although pyroxene is shed locally in abundance byvolcanic rocks, lower values characterize river catchments inthe southern (e.g. HMC 2.8 ±0.7 and tHMC 2.0 ±0.4; Rionisand), western (e.g. HMC 2.5 ±1.2 and tHMC 1.4 ±1.2;Tskhenistskali, Mzimta, Shakhe, Ashe), and northern foothillregions (e.g. HMC 1.3 and tHMC 0.4; Podkumok).

Complementary information is provided by the relativeabundance of key heavy minerals. Sands derived from amphib-olite-facies metaigneous and metasedimentary rocks exposedin the crystalline core of the Greater Caucasus are eitherhornblende-dominated (HCI 20; Kuban sand) or include horn-blende, epidote, garnet and both fibrolitic and prismatic silli-manite (HCI 24, MMI 94; Inguri sand). Hornblende> epidotesuites characterize Laba (HCI 6 ± 1, MMI 83), Malaya Laba(HCI 2 ±2, MMI 50) and Belaya sands (HCI 3 ±1, MMI 50),whereas Rioni, Tskhenistskali and Mzimta sands are pyrox-ene-dominated (Figure 9).

To calculate integrated erosion, we need to establish theinitial thickness of sedimentary or volcanic covers overlyingthe Variscan basement in various parts of the western GreaterCaucasus prior to initial Oligocene uplift. The available dataindicate that cover strata originally increased southward inthickness from 2 to 3 km in the north-western Caucasus (Ershovet al., 2003; Gaetani et al., 2005) to 8 km or more in the south-western Caucasus (Saintot et al., 2006), and were much re-duced on basement highs (e.g. Dakhov Salient in the BelayaValley; Somin et al., 2007). These assumptions allow us to cal-culate an integrated erosion rate of at least 0.07 mm/a for thenow fully exhumed crystalline core of the orogen. This is simi-lar to the modern day erosion rate calculated from the sedimentflux in the Kuban River catchment. Integrated-erosion ratesdecrease progressively westwards in catchments where onlyvery-low-grade to unmetamorphosed cover rocks are exposed(e.g. Belaya), and reach minimum values in westernmost riverbasins draining into the Black Sea, where successions of sedi-mentary and locally volcanic rocks are largely undissected.

Conclusion

Complementary datasets obtained by quantitative geomorpho-logical, petrographic, mineralogical and thermochronologicaltechniques in a decade of continuing research on the westernGreater Caucasus display a general agreement and a regularityof erosion patterns throughout the history of the range. Differentmethods at different timescales consistently indicate westwarddecreasing rates of erosional unroofing, from the Bolshaya

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and Malaya Laba, to the Belaya, to the Mzimta and other riversdraining into the Black Sea. Steepness indices as well as in-ferred exhumation rates reach maximum values in the upperInguri catchment (Figure 10), incising into the crystalline coreof the orogen along its southern flank where rainfall is highest(Figure 1). River catchments displaying the highest channel-steepness indices are spatially associated with the highest to-pography around Mount Elbrus, where sedimentary strata havelong been eroded completely, and correspond closely to theareas ofmost rapid exhumation outlined by thermochronologicaldata (Avdeev and Niemi, 2011; Vincent et al., 2011). This rein-forces the suggestion that the bedrock-channel network revealsmuch of the evolution of tectonically active landscapes (Whippleand Tucker, 1999), and implies that the controls on channel gra-dient ultimately dictate the topography and the relief along theGreater Caucasus.

Acknowledgments—The manuscript benefited from very helpful con-structive comments by Fritz Schlunegger and two anonymous reviewers.Riccardo Scotti kindly provided the upper Inguri sample; MatteoMagnaghi and Ivan Mariano helped in heavy-mineral separation.

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