Geomorphic expression of the southern Central Andes forebulge (37°S, Argentina)

Geomorphic expression of the southern Central Andes forebulge(37°S, Argentina)

Bertrand Nivi�ere,1 Gr�egoire Messager,1,2 S�ebastien Carretier3 and Pierre Lacan1,41LFCR - UMR 5150/CNRS/Total/University of Pau, IPRA, BP 1155, F-64013, Pau Cedex: France; 2Present address; Group of Dynamics

of the Lithosphere Group, Institute of Earth Sciences “Jaume Almera”, CSIC, Lluis Sole i Sabaris s/n 08028 Barcelona, Spain; 3GET - UMR5563/CNRS/UPS/IRD/CNES, University of Toulouse, 14 avenue Edouard Belin, F-31400, Toulouse: France; 4Present address: Centro de Geo-ciencias, Universidad Nacional Aut�onoma de M�exico, Blvd. Juriquilla, 3001, 76230, Juriquilla Quer�etaro, Mexico

ABSTRACT

We present a geomorphologic analysis of an east-west tran-

sect located east of the southern Andes of Argentina (~37°S).We observe a succession of zones that underwent erosion

and deposition during the Pleistocene. If the proximal Andean

foothills are incised, a proximal depozone receives sediments

feeding the megafan of the Rio Colorado on the Chadileuv�u

plain. More distally, the abandoned palaeo-valleys and bend-

ing of the valley floors reflect a localized uplift. Further to

the east, another depozone corresponds to the Pampa Depri-

mida lowland. This pattern is consistent with the presence of

a classical flexural geometry of the lithosphere. The distal

uplift of the foreland corresponds in terms of location, length

(150 km) and amplitude (240 m) to the Andean forebulge

modelled by a geophysical approach. In this study, we iden-

tify the morphological imprint of this bulge and show its

effect on the fluvial activity.

Terra Nova, 25, 361–367, 2013

Introduction

Shortening and thickening in orogensmay produce a flexural response inthe foreland lithosphere far awayfrom the mountain belt. Thisresponse depends on the thicknessand rheology of the lithosphere, andresults in a foreland basin systemconsisting of a forebulge depozonecorresponding to a broad area offlexural uplift between the foredeepand the back-bulge depozones (e.g.DeCelles and Giles, 1996).Thermo-mechanical modelling of

the lithosphere yields the range ofdimensions of each zone (e.g. Ven-ing-Meinesz, 1941). Thus, dependingon the flexural parameters of the An-dean lithosphere, the locus of theforebulge may be located at a dis-tance of 300–750 km from the moun-tain front (Chase et al., 2009; D�avilaet al., 2010; Prezzi et al., 2009;Tassara, 2005). The amplitude of theforebulge elevation ranges from 25 to500 m (Chase et al., 2009; D�avilaet al., 2010).

This type of flexural morphologyhas a clear geological imprint on thesedimentary record. In the foothillsof the Agrio fold belt (Fig. 1), Cob-bold and Rossello (2003) interpretedthe thinning of Aptian to Cenoma-nian strata as due to the influence ofa foreland bulge during the Late Cre-taceous. Farther north between 22and 26°S, the deposit thickness andother sedimentary features have alsobeen used to identify the Miocene–Present forebulge (Decelles et al.,2011). However, the topographicexpression of forebulges in the fore-land basins is weak, whereas directobservations are sparse and the geo-logical data restricted to seismic linesor boreholes. Therefore, the informa-tion is often concentrated along tran-sects and the lateral extent of theforebulge remains poorly con-strained.We analyse here a west-east geo-

logical transect located in the easternforeland of the southern CentralAndes of Argentina between latitudes35 and 39°S. The transect extendsover a distance of 700 km, whichspans all four flexural depozones. Wefocus on the morphological signatureof these depozones, based on dataprovided by the digital elevationmodel (DEM), satellite imagery andfield observations, and investigate

their development using past andpresent drainage patterns.

Geological setting

The southern Andes are a geologicalprovince analogous to the Laramideorogeny in the western USA, wherecrystalline basement is involved indeformation (e.g. Dickinson andSnyder, 1978; Erslev, 1993) above ashallow angle slab. This province isformed mainly by Proterozoic toPalaeozoic basement ranges (Chernic-off and Zappettini, 2004; Ramos,1988, 1996). The Andean front in thestudy area is made of the Agriofoldbelt of Eocene-to-Recent age(Cobbold and Rossello, 2003;Messager et al., 2010), which is bor-dered to the east by the PampeanPlain (Fig. 1). As the slab is steepen-ing, the deformation style may betransitionary between a thick-skinned‘pure shear’ type of deformation anda thinner skinned ‘simple shear’ style.In its foreland, the Cenozoic

deposits overlies Proterozoic toMesozoic units (Chebli et al., 1999;P�angaro and Ramos, 2012; Ramos,2008), and are composed ofa � 500-m-thick succession rangingin age from the Oligocene to Present(Marengo, 2006; Fig.1C). These for-mations are essentially alluvial

Correspondence: BertrandNivi�ere, LFCR -

UMR 5150/CNRS/Total/University of

Pau, IPRA, BP 1155, F-64013 Pau Cedex,

France. Tel.: +33 559 40 74 22; fax: +33

559 40 74 15; e-mail: bertrand.niviere@

univ-pau.fr

© 2013 John Wiley & Sons Ltd 361

doi: 10.1111/ter.12044

sequences inter-fingered by two shal-low marine horizons (Marengo,2006). From west to east, we can

distinguish four morphostructuraldomains (Fig. 1): (1) the Agrio base-ment thrust belt and its foothills, at

elevations of between 4,000 and1,000 m; (2) an extensive alluvial andlake depression drained by the

(A)

(B) (C)

Fig. 1 (A) Relief of the foreland of the southern Central Andes of Argentina (SRTM data, 90 m). We focus here on the topo-graphic ridge referred to as the La Pampa High (LPH). Light grey shading indicates the alluvial fan of the R�ıo Colorado. Dashedlines indicate topographic profiles along strike of the LPH and along the palaeo-valley floors of transverse valleys (Fig. 3). Rednumbers represent the thickness in metres of the Miocene deposits as observed in boreholes with references : (1) Tapia (1935), (2)Melchor and Casadio (1999), (3) Cordini (1967), (4) Terraza et al. (1981) and (5) Giai and Tullio (1998). Inset map shows loca-tion of the study area in the southern Andean setting. (B) Contour line in metres of the current Andean forebulge as modelledfrom geoid anomalies by Chase et al. (2009). Concordance of the LPH with the 250-m contour suggests that the LPH is the traceof the current Andean forebulge. (C) Late Proterozoic–early Palaeozoic suture zones (after P�angaro and Ramos, 2012; Ramos,1988) and Mesozoic Colorado Basin (light grey). Alternatively, the LPH could result from tectonic reactivation of these sutures.

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Chadileuv�u River, bounded to theeast by (3) a low-relief topographichigh (<300 m), referred to here asthe “La Pampa High” (LPH); and(4) a wetland area making up a largeelongate depression known as thePampa Deprimida plain. The dis-tance between the mountain chainand the LPH is approximately900 km (Fig. 1).

Morphological imprint

The main rivers of the easternAndean foothills (R�ıos Diamante,Atuel, Colorado and Neuqu�en) havelocally deeply incised into pre-Quaternary bedrock to form up tosix fill and strath terraces along theirpiedmont reaches (Baker et al., 2009;Messager et al., 2010). Mid and lateQuaternary uplift of the piedmontarea is a likely cause for the incisionor alternatively, it may represent adelayed response to pre-mid Quater-nary uplift (Baker et al., 2009).The Chadileuv�u river valley

extends from north to south over aplain that decreases in elevation from320 to 240 m and widens from 40 to90 km. Numerous swamps andmarshes occur along the main drain-age course. Towards the south, the

plain narrows to 7 km and the watercourse eventually disappears in a flatarea of lakes and marshes at an ele-vation of �200 m. About 80 km far-ther south, the R�ıo Curac�o takesover the drainage towards the R�ıoColorado. As the R�ıo Chadileuv�u isonly connected episodically with theR�ıo Curac�o (Bojanich Marcovich,1980; K€uhn, 1922), the R�ıo Chadi-leuv�u can be considered as endoreic(Fig. 1).In the northern part of the Chadi-

leuv�u depression, some boreholeshave encountered 108 m of fine sedi-ment (Tapia, 1935; Fig. 1A). In thesouthern part of this depression,Melchor and Casad�ıo (1999) havedescribed a 55-m-thick sequence oflacustrine deposits with some fluvialand aeolian intercalations. Fossils inthese deposits are not older thanMiocene. To the south-west, thedepression is delimited by the oldestR�ıo Colorado terrace, situated at430 m above sea level, which Mel-chor and Casad�ıo (1999) ascribed tothe Pliocene (dark grey in Fig. 1).The thickness of the Miocene depos-its, the growth of the Colorado Riveralluvial plain and the migration ofthe R�ıo Chadileuv�u over a wide allu-vial plain suggest that the Chadi-

leuv�u plain has functioned as adepozone ever since the Miocene.The LPH is a NW–SE topographic

ridge extending over a distance of300 km between 35°S and 38°30′S(Fig. 2B and C), which disappears tothe south of the R�ıo Colorado(Fig. 2D). Approximately 150 km inwidth, the elevation of this ridgegently decreases southward from400 m around 37°S to about 170 maround 38.30°S, where its top iseroded (Fig. 3). In an E–W transect,the ridge is asymmetrical with alonger slope on the eastern limb(Fig. 3A and B). This asymmetryprobably results from the superimpo-sition of the ridge onto the regionaleast-dipping depositional slope.Along strike, the rivers have formedsix main SE- to NE-trending windgaps (Fig. 1) called “valles transver-sales” by Calmels and Casad�ıo(2004). These gaps are 100 km ormore in length, 1–2 km wide and 80–100 m deep, being drained only afterheavy rains.The granitic basement crops out

locally on the western slope. It isconcealed elsewhere by: (1) the flu-vial and aeolian Upper Miocene“Cerro Azul Formation” (Giai andTullio, 1998); (2) a sedimentary body

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(B)

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Fig. 2 West-east topographic profiles across the eastern Andes and their foreland (SRTM data) showing location and ampli-tude of the LPH relief. Locations are given in Fig. 1.

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attributed to the uppermost Miocene(Llamb�ıas, 1975; Lorenz, 2001; Mehland Z�arate, 2007; Visconti, 2007);and (3) the early Holocene cover (Iri-ondo and Kr€ohling, 1995; Iriondoand Garcia, 1993). The thickness ofsediments ranges from 34 to 250 m(Fig. 1A).The Miocene sedimentary environ-

ment is characterized by low-energyflows that partly rework the sub-strate, with fine material filling awide and shallow palustrine basin.Higher energy flows supply sandtransported from far away. There-fore, the Plateau landforms appearto result from fluvial processes linkedto the Sub-Andean piedmont. How-ever, the transverse valleys suggestthat erosion later reshaped the area.

The Pampa Deprimida plain situ-ated to the east is less than 150 mhigh, and the groundwater tableintersects the surface at 120-m eleva-tion forming numerous salt ponds.There are numerous shallow lakesand swampy environments in thissector (Fig. 1); the lakes coalescetemporarily during flooding, butremain separated from the upstreamsource that shapes them. Below500 m of Pleistocene deposits, 1300and 600 m of Cenozoic and Meso-zoic successions (Bojanich Marco-vich, 1980; Salso, 1966), respectively,the Palaeozoic substratum is reachedat 2500-m depth (Salso, 1966).The sediment thickness gradient

shows that the LPH was alreadyundergoing a lower rate of subsi-

dence than the Deprimida plainduring the Miocene.

Post-Miocene uplift of the LaPampa High

At 37°20S, a late Miocene lagoonalfacies observed in the LPH at an ele-vation of 220 m has also been recov-ered in a borehole 30 km away inthe Pampa Deprimida, at only 60 m,i.e. 180 m lower in elevation (Fabre-gas, 1986; Vogt et al., 2010). Thisobservation is compatible with apost-late Miocene tilt of the PampaHigh. Vogt et al. (2010) report simi-lar tilts of lower intensity fathernorth.Because of their straight course, a

tectonic origin had been proposed to

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Fig. 3 Longitudinal profile showing folding of former valley floors of the Chadileuv�u (3A and B) and Colorado (3C) rivers(SRTM data). The crest line is a west-east projection of the swath profile of maximum relief. A and B in Fig. 3A–C refer tolocations in Fig. 1. Figure 3D is an along-strike topographic profile of the top of the LPH. The deeper entrenchment to thesouth of former valleys is interpreted as representing a progressive southward migration of rivers due to the uplift of the LPH.The longitudinal profile of the R�ıo Colorado (3E) shows a knick-point located above the LPH, which suggests that uplift isstill in progress.

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explain the valleys that cut throughthe LPH (Cordini, 1967; HerreroDucloux, 1978; Stappenbeck, 1926).However, no evidence has beenfound to indicate that the valley sidesare fault scarps.Numerous flint stones of Sub-An-

dean origin are widely scattered overthe valley floor of Valle del Tigre(Fig. 1A; Vogt et al., 2010). More-over, these valleys are too wide to beattributable to local incision, andshould therefore correspond to majorrivers of the study area with largecatchments. To the south, one ofthese broad open valleys can be fol-lowed to the Atlantic coast at Bah�ıaBlanca (Cordini, 1967; Herrero Duc-loux, 1978; Stappenbeck, 1926). It isthe continuation of a dry valley ofthe Colorado River, located to thenorth of its alluvial plain and west ofthe LPH (Fig. 1). Both valleys arevery similar in width. So, we mayinfer that this dry valley, now form-ing a wind gap over the LPH, repre-sents a former course of the R�ıoColorado that has been uplifted. Byanalogy, we interpret the northern-most air gaps of the LPH as formervalleys of the R�ıo Chadileuv�u, assuggested by Malagnino (1988) andClapperton (1993).Figure 3 shows the longitudinal

profiles of former valleys of the R�ıoChadileuv�u (Fig. 3A and B) and theR�ıo Colorado (Fig. 3C). The valleyfloor does not show a linear profilewith a regular downstream decreasein elevation, as would usually beexpected for river beds. In fact, thesevalleys are W-sloping on the westernlimb of the ridge and E-sloping onthe eastern limb. Thus, the river pro-files exhibit a slope inversion on thewestern limb, indicating that the riv-erbeds have been deformed. Thisdeformation is consistent with appar-ent folding of the crests, and offersan alternative to the previously pro-posed interpretation involving head-ward erosion from the subsidingborders of the LPH (Vogt et al.,2010). Cumulative folding began toleave its imprint on the inheritedmorphology after the shift in riverflow direction. The amplitude of thisfolding reaches 110 m in the north-ern part of the LPH.The timing of deformation is indi-

cated by the following constraints:(1) the reduced thickness of late Mio-

cene deposits above the LPH, syn-depositional or due to erosion; (2)the late Miocene deposits underlyingthe crest, which are tilted to the sameextent as the valley floor; (3) the latePliocene plain of the Colorado riveron the Chadileuv�u plain is in conti-nuity of a tilted valley floor; (4)deposits in the valley bottoms areattributed to the late Pleistocene–Holocene (Calmels et al., 1996; Ca-sad�ıo and Schulz, 1986; Tapia, 1935);(5) the Colorado River shows a 55-m-high knick-point on its longitudi-nal profile (Fig. 3E), which mayreflect ongoing folding activity. Thus,uplift postdates the Late Miocene.

Discussion and conclusion

The flexural response of the litho-sphere to surface loads couples thepulses of tectonic activity in thethrust belt with the sedimentaryrecord in the foreland basin: the loadexerted by the migrating forelandfoldbelt leads to the development ofa foredeep, which accumulates sedi-ments eroded from the orogen. Thesuccession observed in the Pampeanforeland at these latitudes is compati-ble with a flexural morphology. TheChadileuv�u plain has functioned as adepozone since the Miocene, andwould appear to represent the cur-rent Andean foredeep at the toe ofthe Agrio foldbelt. East of the fore-deep, the post-Miocene upliftingLPH would represent the forebulgeand the Pampa Deprimida the backbulge. The location and timing ofthis uplift is compatible with the lateMiocene–Present forward migrationof the Andean deformation front atthis latitude (Messager et al., 2010).Moreover, the regional morphologiesof the present-day Argentine Pam-pean Plain match remarkably wellwith the distribution and intensity ofuplift predicted for the Andean fore-bulge by Chase et al. (2009). Geoidanomalies determined from satelliteobservations and flexural analysispredict a foredeep width of ca.250 km and a peripheral bulgeamplitude of >250 m at this latitude,dimensions that are in agreementwith the results of our analysis(Fig. 1B).Our information could also suggest

that the region has changed fromoverfilled (with a buried forebulge) to

underfilled while the forebulge is nowbeing eroded. This occurred at theend of Miocene and could beexplained by the rising of the SanRafael block to the west (Ramos andKay, 2006) that trapped Andean ero-sional products in the MalargueBasin.Alternatively, this succession could

record an evolution from an unde-formed foreland to a tectonic domainthat is compartmentalized by theuplift of distinct blocks. The uplift ofthe LPH, with coeval lateral aggra-dation, follows a pattern observedfarther north in the Sierras Pampe-anas (Ramos et al., 2002) and thePuna Plateau (Coutand et al., 2006),as well as in the Laramide orogenyof the USA (Erslev and Koenig,2009) and the Canadian Cordillera(Mazzotti and Hyndman, 2002).Geophysical investigations (Chernic-off and Zappettini, 2003; HerreroDucloux, 1978, 1983) have indeedrevealed lineaments with a dominantN0°E to N60°E trend in conformitywith the main relief orientation.Moreover, the LPH uplift took placeabove the pre-existing ColoradoBasin, which began to developduring the late Jurassic–early Creta-ceous, as well as above the late Pro-terozoic to early Palaeozoic crustaldiscontinuities (P�angaro and Ramos,2012; Ramos, 1988; Fig. 1C). Vogtet al. (2010) stated that scarps alongthe edges of the LPH are fault traces.Reactivations are minor and weinterpret them as cracking the fore-bulge as it happens in many places(e.g. the forebulge south of theTimor through foredeep on the northAustralian shelf).Consequently, we would interpret

the LPH as a flexural forebulge thatdeveloped in response to topographicloading of the South American plateby the Andes. Our data provideunique evidence of the morphologicalimprint of an active forebulge in asetting of very low topographicgradient. The results presented heremay also help our understanding ofspatiotemporal patterns of forelandevolution in other mountain belts.

Acknowledgements

This study represents part of an ongoingcollaboration with Total and TotalAustral scientists. M. Carpenter helped

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with translation. Careful reviews by P. G.DeCelles, S. E. Baker, an anonymousreviewer and Associate Editor helped us tosubstantially improve the manuscript.

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Received 22 October 2012; revised version

accepted 19 March 2013

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