Revista de la Asociación Geológica Argentina 64 (1): 43 - 54 (2009) 43

INTRODUCTION

Charles Darwin, a naturalist who thought

of himself as a geologist, crossed thePiuquenes and Portillo passes in 1835,during his voyage on board HMS Beagle.

The trip, which lasted from March 19th toMarch 24th, was a milestone in his kno-wledge of Andean geology. It was in this

THE HIGH ANDEAN CORDILLERA OF CENTRALARGENTINA AND CHILE ALONG THE PIUQUENESPASS-CORDON DEL PORTILLO TRANSECT: DARWIN'SPIONEERING OBSERVATIONS COMPARED WITHMODERN GEOLOGY

Laura GIAMBIAGI1, Maisa TUNIK2, Victor A. RAMOS3, and Estanislao GODOY4

1 IANIGLA - CONICET, Mendoza. Email: lgiambia@lab.cricyt.edu.ar 2 CONICET-CIMAR - Universidad Nacional del Comahue.3 Laboratorio de Tectónica Andina (FCEN) Universidad de Buenos Aires - CONICET.4 Geólogo Consultor, Santiago de Chile.

ABSTRACTThe geological observations made by Darwin in 1835 during his crossing of the Andes from Santiago to Mendoza via thePiuquenes Pass and Cordón del Portillo are compared with the present geological knowledge of the Cordillera Principal andCordillera Frontal at 33°-34°S. The analysis of the complex stratigraphy of the Cordillera Principal, the imbricated structureof the Aconcagua fold and thrust belt, as well as the stratigraphy and structure of the inter mountain foreland Tunuyán Basin,allows to assess the pioneer observations of Darwin. He recognized the old metamorphic basement and the granitoids andvolcanic sequences of late Paleozoic to Triassic age of the Cordillera Frontal, established the Cretaceous age of the marinesuccessions cropping out along the eastern Cordillera Principal and studied the conglomeratic deposits associated with theuplift of the Cordillera in the Alto Tunuyán Basin. Based on the study of clast provenance of the synorogenic deposits of theAlto Tunuyán Basin, Darwin recognized that the Cordillera Frontal was uplifted later than the Cordillera Principal. The pre-sent knowledge of this sector of the Andean Cordillera confirms his pioneer observations and show that Darwin was one ofthe first scientists ever in realizing that in an orogenic system the sequence of uplift and deformation proceeds from hinter-land towards foreland, according to a process that is exceptionally well-illustrated along the Piuquenes-Cordón del Portillotransect.

Keywords: Uplift, Provenance, Structure, Metamorphic rocks, Synorogenic deposits.

RESUMEN: La Alta Cordillera de los Andes del centro de Argentina y Chile a lo largo de la transecta del Paso Piuquenes-Cordón del Portillo: Las observaciones pioneras de Darwin comparadas con la geología moderna. Las observaciones geológicas efectuadas por Darwin en 1935durante su cruce de la Cordillera de Los Andes entre Santiago y Mendoza realizado en 1835 a través de los pasos del Portilloy Piuquenes son examinadas y comparadas con el conocimiento actual existente de las Cordilleras Principal y Frontal entre los33°-34°S. El análisis de la compleja estratigrafía de la Cordillera Principal, la estructura de las diferentes láminas imbricadas dela faja plegada y corrida del Aconcagua y la cuenca de antepaís intermonatana del Tunuyán, permiten ponderar las pionerasobservaciones de Darwin, quien reconoció el basamento metamórfico y los granitoides y secuencias volcánicas neopaleozoi-cas a triásicas de la Cordillera Frontal, estableció la edad cretácica de las sucesiones marinas del sector oriental de la CordilleraPrincipal y describió e interpretó los depósitos conglomerádicos asociados al levantamiento de los Andes en el Alto Tunuyán.Sobre la base del estudio de la proveniencia de los clastos de los depósitos sinorogénicos de la cuenca del Alto Tunuyán pudoreconocer que la Cordillera Principal, fue alzada con anterioridad a la Cordillera Frontal, la que a través de varios pulsos adqui-rió su altura actual en épocas más recientes. Darwin fue uno de los primeros en reconocer que en un sistema orogénico, lasecuencia de deformación asociada al levantamiento orogénico comienza en su región interna desde donde avanza progresi-vamente hacia el antepaís, como lo demuestran sus brillantes observaciones efectuadas ya hace más de 150 años.

Palabras clave: Levantamiento, Procedencia, Estructura, Rocas metamórficas, Depósitos sinorogénicos.

sector of the Andes that Darwin suspec-ted that the giant mountains were uplif-ted as late as in the Cenozoic. The ideawas a radical one, as well as the interpre-tation that the different ranges wereuplifted one after the other, being theeasternmost one the youngest. Later ge-nerations of geologists demonstratedthat he was essentially right in both con-cepts.The Piuquenes and Portillo passes aresituated in the southern Central Andes, at33°40´S (Fig. 1). At these latitudes theAndes are formed by the CordilleraPrincipal and the Cordillera Frontal. ThePiuquenes Pass is situated in the Cor-dillera Principal, at the border betweenArgentina and Chile, and the PortilloPass is located in the high summits of theCordillera Frontal to the east (Fig. 2). Atthe time Darwin crossed the Andesthrough the Piuquenes Pass, this sectorof the Cordillera Principal was known asthe Peuquenes range or western line, andthe Cordillera Frontal, as the Portillorange. These two ranges are separated, bya wide valley filled by Neogene sediments(Depresión Intermontana del Alto Tunuyán,Polanski 1957). The Cordillera Principalcan be subdivided into a western sector,formed by a thick sequence of Oligoceneand Miocene volcanic rocks and intrusi-ves, and an eastern zone correspondingto the strata of Mesozoic sedimentaryrocks deformed into the Aconcagua foldand thrust belt (Ramos 1988, Cegarraand Ramos 1996, Ramos et al. 1996a). Itwas observing these rocks that Darwinbecame astonished to see "shells which wereonce crawling on the bottom of the sea, now stan-ding nearly 14,000 feet above its level"(Darwin 1845, p. 320).

TECTONIC SETTING

The Andes of central Argentina andChile are a linear orogenic belt formed atthe convergent plate margin between theNazca and South American plates. At33°-34° S the Nazca plate subducts bene-ath the South American plate at a relativevelocity of nearly 10 cm/yr (Pardo-Casas

and Molnar 1987). The distribution ofseismicity suggests that the Nazca Plateis characterized by alternating flat andnormal segments of subduction (Baran-zangi and Isacks 1976, Cahill and Isacks1992, Pardo et al. 2002). Several featuresof the Andes orogenic system appear tocorrelate with these changing subductiongeometries, such as volcanism and distri-bution of morphostructural belts andtectonic style (Isacks et al. 1982, Jordan etal. 1983). North of 33°S, the flat subduc-tion segment lacks active arc magmatismand underlies the Cordillera Principal,Cordillera Frontal, Precordillera andSierras Pampeanas structural provinceswhile south of 34°S the "normal" sub-duction segment of south-central Argen-tina and Chile shows the active volcanismof the Southern Volcanic zone (see Hil-dreth and Moorbath 1988, Stern et al.2007) and the orogenic system is much

narrower and formed only by the Cor-dillera Principal and Cordillera Frontal.The study area (33°-34°S) is locatedalong the transition zone between the flatand normal subduction segments (Fig. 3).

STRATIGRAPHY

The stratigraphy, examined in many pre-vious studies such as those of by Po-lanski (1964), Thiele (1980), Charrier etal. (1997), Ramos et al. (2000), Jordan etal. (2001), Giambiagi et al. (2001, 2002),Tunik (2003), Tunik et al. (2004), Kay etal. (2005), Fock ( 2005) and Farías et al.(2008) includes seven main major tecto-no-stratigraphic units (Fig. 4): (1) Prote-rozoic to Lower Triassic metamorphic,volcanic and intrusive rocks; (2) Triassicand Jurassic rift sequences; (3) Titho-Neocomian marine strata; (4) Cretaceousto Paleogene marine and non marine

L . GIAMBIAGI , M. TUNIK, V. A . RAMOS AND E GODOY44

Figure 1: a) Location map of the High Andes with main geological provinces and lithological sequencesbetween Santiago (Chile) and Mendoza (Argentina); b) Block diagram showing the main structures of theCordillera Principal and Cordillera Frontal with location of the Aconcagua fold and thrust belt.

The High Andean Cordillera of central Argentina and Chile along ... 45

sedimentary rocks; (5) Eocene?-Oligo-cene to Lower Miocene volcanics; (6)Upper Cenozoic foreland basin deposits;and (7) Upper Cenozoic intrusives andvolcanic-arc rocks.The oldest rocks correspond to Protero-zoic mica-schists cropping out at Cordóndel Portillo, in the Cordillera Frontal.These rocks were examined by Darwin atthe "base of the Portillo range, ...at a placecalled Mal Paso. The mica-schist here consists ofthick layers of quartz, with intervening folia offinely-scaly mica, often passing into a substancelike black glossy clay-slate: in one spot, the layersof quartz having disappeared, the whole massbecame converted into glossy clay-slate. Wherethe folia were best defined, they were inclined ata high angle westward, that is, towards the

range... I think it more probable, considering itsmore perfect metamorphic character and its well-pronounced foliation, that it belongs to an ante-rior epoch" (Darwin 1846, p. 184). Theserocks are unconformably overlain by Up-per Paleozoic marine black shales andsandstones and are intruded by Carbo-niferous to Permian granitoids (Polanski1964). These granitoids were clearlyidentified by Darwin who stated that "thered granite, from being divided by parallel joints,has weathered into sharp pinnacles, on some ofwhich, even on some of the loftiest, little caps ofmica-schist could be clearly seen: here and thereisolated patches of this rock adhered to themountain-flanks, and these often correspondedin height and position on the opposite sides ofthe immense valleys. Lower down the schist pre-

vailed more and more, with only a few quitesmall points of granite projecting through.Looking at the entire eastern face of the Portillorange, the red color far exceeds in area the black;yet it was scarcely possible to doubt, that the gra-nite had once been almost wholly encased by themica-schist" (Dar-win 1846, p. 184).Permian-Triassic intermediate and acidvolcanic rocks unconformably overliethe previously deformed rocks. As Dar-win pointed out, these volcanics reach athickness of more than 1,700 m. He des-cribed these rocks and assigned them arelative age based on their metamor-phism and vein intrusion. Rifting waswidespread in central Argentina andChile during Triassic - Early Jurassic ti-mes. This was a consequence of exten-sional processes linked to the initial sta-ges of the fragmentation of Gondwanaand the opening of the South Atlantic(Uliana et al. 1989). A series of NNWtrending rift systems was formed at thistime along the western margin of Gond-wana (Charrier 1979, 1984, Uliana et al.1989). In the Cordillera Principal, theserift basins, which at this latitude onlycrop out in the Chilean side of the Cor-dillera Principal, were filled by Lower toMiddle Jurassic black shales (Godoy1993, Alvarez et al. 1997, 1999). In theArgentinean side, a thick layer of evapo-rites (Auquilco Formation) unconforma-bly rests upon Jurassic marine strata. Thiswas observed by Darwin who was stun-ned by its more than 600 m thickness.The evaporites grade into the red sands-tones and claystones with subordinatedconglomerates of the KimmeridgianTordillo Formation.The Jurassic units in the Cordillera Prin-cipal are capped by black shales, mudsto-nes, limestones and sandstones, deposi-ted on a stable marine platform duringTitho Neocomian thermal times (Biro1964, Polanski 1964, Aguirre-Ureta1996). These sedimentary rocks, nowgrouped into the Mendoza Group, weredescribed in detailed by Darwin whoassigned them a Neocomian age basedon his collection of invertebrate fossils(see Aguirre-Urreta and Vennari, 2009).

Figure 2: a) Water divide bet-ween Chile and Argentina withindication of the PiuquenesPass. The international land-mark (hito) is located onNeocomian limestones; b)Portillo Pass across theCordillera Frontal. Glacialdeposits are overlying latePaleozoic granites; c)Approaching the internationalboundary along Darwin's trail.

L. GIAMBIAGI , M. TUNIK, V. A . RAMOS AND E GODOY46

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The High Andean Cordillera of central Argentina and Chile along ... 47

The Upper Cretaceous strata, se-paratedfrom the Lower Cretaceous units by anerosional unconformity, consist of con-glomerates, sandstones and volcaniclasticrocks. The presence of a continuousinterval of Maastrichtian to Da-niansiltstones and limestones depositedduring an Atlantic ingression that rea-ched up to the eastern slope of the pre-sent-day Cordillera Principal (Tunik2003, Tunik et al. 2004) indicates a sub-dued relief close to sea level for this

region at that time.After at least 10 Ma of magmatic quies-cence, volcanic activity considerablyincreased in the western Cordillera Prin-cipal during the Eocene?-early Miocene(Godoy et al. 1996, Charrier et al. 1997,Jordan et al. 2001, Kay et al. 2005, Fock2005). This volcanic event correspondsto the Abanico and Coya Machalí For-mations of Late Eocene to Middle Mio-cene age (Wyss et al. 1990, Flynn et al.1995, Gana and Wall 1997, Charrier et al.

2002, Farías et al. 2008). that crop out intwo north-trending belts made up ofbasic lavas, tuffs and intermediate pyro-clastic rocks, interbedded with sedimen-tary rocks totalling at least 2,000 m thick.Eocene ages are reported only on thewestern belt while, in general, volcanicrocks have a primitive isotopic signatureindicating crustal thinning and high pale-othermal gradient (Nystrom et al. 1993,Vergara et al. 1988, 1999) during the ac-cumulation of these sequences, probably

Figure 4: Structural cross sections across the High Andes: a) Section of the Piuquenes pass and b) Portillo pass after Darwin (1846); c) Modern structural cross-sec-tion based on Giambiagi and Ramos (2002). Location of the section in figure 3.

in a volcano-tectonic extensional /trans-tensional basin (Charrier et al. 1997, 2002,Godoy et al. 1999, Jordan et al. 2001, Fock2005). The Abanico and Coya MachalíFormations are covered by 2,000 m ofMiocene calc-alkaline andesitic lava andacid pyroclastic flows of the FarellonesFormation from which radiometric agesspanning from 20 Ma to 7 Ma have beenreported (Rivano et al. 1990). This unit isintruded by several epizonal granitoidplutons and stocks, some of them asso-ciated with the giant copper-porphyrycopper deposits of Río Blanco and ElTeniente with show radiometric agesconstrained between 10 to 6 Ma (Cor-nejo and Mahood 1997, Kurtz et al. 1997,Maksaev et al. 2004, Deckart et al. 2005).During this journey across the CordilleraPrincipal Darwin observed massive rockswhich "rarely contain any quartz" intrudedamong the volcanic strata that probablycorrespond to sub volcanic dykes andsills emplaced in the volcanic sequences.Finally, the youngest geological units ofthe Cordillera Principal at this latitudecorrespond to large composite Quater-nary stratovolcanoes such as the Tupun-gato, San Juan, Marmolejo and San José,being these the northernmost volcaniccenters of the Southern Volcanic Zone

(SVZ) of the Andes (Hildreth and Moor-bath 1988, Stern et al. 2007).Along the eastern slope of the CordilleraPrincipal, in Argentina, the oldest Ceno-zoic volcanic unit is the Contreras For-mation, exposed at the base of a thicksequence of Neogene synorogenic depo-sits, and consisting of basaltic lava flowsand breccias. A sample from the middlesector of this unit yielded a whole rockK-Ar age of 18.3 Ma (Giambiagi et al.2001). Its geochemistry suggests a retro-arc setting in a thickening crust during itsextrusion (Ramos et al. 1996b). Late Ce-nozoic foreland clastic deposits occur tothe east of the Cordillera Principal.Miocene sedimentary rocks filling theAlto Tunuyán Basin include three units.The oldest Tunuyán Conglomerate con-sists of up to 1,400 m of conglomeratesand sandstones deposited in an alluvial-fan setting (Fig. 5b).The vertical variation in clast composi-tion and paleocurrent data reflect theprogressive erosion and unroofing of theAconcagua fold and thrust belt (Giam-biagi 1999). The synchronism of deposi-tion of the upper 400 m of this unit withthe emplacement of thrusts within thebasin is documented by syntectonic un-conformities. Deposition of the Tunu-

yán Conglomerate started around 18 to17 Ma, and continued to ~10 Ma. Theintermediate Palomares Formation filleda gentle paleorelief in the Tunuyán Con-glomerate between 8.5 and 7 Ma andconsists of 200 m of volcaniclastic andclastic sediments deposited in an alluvial-fan setting (Fig. 5c). The Cordillera Fron-tal was the main source area of this unit,as indicated by clast composition, paleo-current measurements and syntectonicgeometries (Giambiagi et al. 2001). TheButaló Formation records the final infi-lling of the sedimentary trough, reachinga thickness of more than 300 m, and ismade up of fluvial and lacustrine depo-sits (Fig. 5d). It is unconformably over-lain by andesitic volcanic rocks dated at5.9 Ma (whole rock K/Ar age, Ramos etal. 2000). Darwin's description of theConglomerate of Tenuyan (sic) indicatedthat "the included pebbles are either perfectly oronly partially rounded: they consist of purplishsandstones, of various porphyries, of brownishlimestone, of black calcareous, compact shaleprecisely like that in situ in the Peuquenes range,and containing some of the same fossil shells;also very many pebbles of quartz, some of mica-ceous schist, and numerous, broken, roundedcrystals of a reddish orthitic or potash feldspar(as determined by Professor Miller), and these

L. GIAMBIAGI , M. TUNIK, V.A . RAMOS AND E GODOY48

Figure 5: a) Tunuyán andPalomares rivers, lookingsouthward. Cretaceous mari-ne strata unconformably restover the Paleozoic basementrocks of the Cordón delPortillo; b) TunuyánConglomerate in thePalomares river; c) Tuffs andconglomerates of thePalomares Formation uncon-formably resting over theTunuyán Conglomerate; d)Butaló Formation.

The High Andean Cordillera of central Argentina and Chile along ... 49

Figure 6: a) Geological map of the Aconcagua fold-and-thrust belt in the Yeso and Palomares river (modified from Giambiagi et al. 2003) See locationin figure 3; b) Structural cross-section A-B (Giambiagi and Ramos 2002); c) The Chacayal thrust in the Chilean slope of the Cordillera Principal, uplif-ting Late Jurassic red beds (Tordillo Formation) over Early Cretaceous continental and marine sequences (Colimapu Formation and Mendoza Group);d) Southern view of the Cerro Palomares. The Palomares fault system uplifts Cretaceous limestones over the Neogene synorogenic strata of theTunuyán Conglomerate (a) and the Palomares Formation (b).

L. GIAMBIAGI , M. TUNIK, V. A . RAMOS AND E GODOY50

from their size must have been derived from acoarse-grained rock, probably granite. From thisfeldspar being orthitic, and even from its exter-nal appearance, I venture positively to affirmthat it has not been derived from the rocks of thewestern ranges; but on the other hand it may wellhave come, together with the quartz and meta-morphic schists, from the eastern or Portillo line,for this line mainly consists of coarse orthiticgranite. The pebbles of the fossiliferous slate andof the purple sandstone, certainly have been deri-ved from the Peuquenes or western ranges."(Darwin 1845, p. 182).

STRUCTURE

Aconcagua Fold and Thrust Belt

During his ride through the PiuquenesPass Darwin felt admiration for the incli-ned to overturned Mesozoic layers whichhad been formed undersea and lateruplifted and deformed in an "extraordi-nary manner", reaching an altitude ofmore than 4,000 m (Figs. 4 a and b).Presently, the Argentine eastern sector ofthe Cordi-llera Principal is interpreted toform part of the Aconcagua fold-and-thrust belt. This belt can be divided intotwo do-mains (Fig. 6b, c).The structure of the western domain isdominated by open NNW trending foldsand N-S thrust faults. The majority ofthese thrusts cut the previously develo-ped fold structures and show an eastwardvergence, with the exception of a belt ofwest verging back-thrusts located at theborder between Argentina and Chile (Fig.4c). Although the basement does notcrop out, its involvement is deducedfrom the geometry of the structure andsignificant thickness variations of thedeformed Mesozoic successions (Giam-biagi et al. 2003). The eastern domain isbroad and narrows towards the southuntil it disappears south of Volcán Mar-molejo. This domain is characterized by adense array of imbricate low-anglethrusts which exhibits flat and ramp geo-metries, with flats corresponding to decó-llement levels located on the Late Jurassicevaporites and Early Cretaceous shales.

In this sector of the belt it is common toobserve thrusts cutting previously deve-loped structures, some of them showingyounger-over-older relationships. This isthe reason why it is interpreted as evol-ved by a forward propagating thrust se-quence with periods of out-of-sequencethrusting

The Alto Tunuyán Foreland Basin

The Alto Tunuyán Basin is a Neogeneforeland basin located between the Cor-dillera Principal and the Cordillera Fron-tal, from 33°30´ to 34°S. The basin wasgenerated by lithospheric flexure as res-ponse to thrust belt load of the Acon-cagua fold and thrust belt and was filledwith more than 1,800 m of continentalsynorogenic deposits of the TunuyánConglomerates and the Palomares andButaló Formations. These strata weredeformed as the Aconcagua fold-and-thrust belt deformation migrated eas-tward and cannibalized the foreland ba-sin. One of Darwin's first geological ob-servation was that the constitutions ofthe two ranges are totally different andthat "one part of the double line of mountains(Cordilleras Principal and Frontal) is ofan age long posterior to the other", which isnowadays known as the normal sequenceof deformation and uplift of an orogenicsystem, from the hinterland to the fore-land. It is interesting to remark how Dar-win, based on a provenance analysis ofconglomeradic clasts, was able to inferthe order and the relative uplift age ofeach mountain chain across this sectionof the Andes. He noted that between thetwo main ranges "there rest beds of a conglo-merate several thousand feet in thickness, whichhave been upheaved ... and dip at an angle of45° towards the Peuquenes line. I was astonis-hed to find that this conglomerate was partlycomposed of pebbles, derived from the rocks,with their fossil shells, of the Peuquenes range;and partly of red potash granite, like that of thePortillo. Hence we must conclude, that both thePeuquenes and Portillo ranges were partiallyupheaved and exposed to wear and tear, whenthe conglomerate was forming; but as the beds of

the conglomerate have been thrown off at anangle of 45° by the red Portillo granite ..., wemay feel sure, that the greater part of the ...upheaval of the already partially formed Portilloline, took place after the accumulation of theconglomerate, and long after the elevation of thePeuquenes ridge. So that the Portillo, the loftiestline in this part of the Cordillera, is not so oldas the less lofty line of the Peuquenes. Evidencederived from an inclined stream of lava at theeastern base of the Portillo, might be adduced toshow, that it owes part of its great height to ele-vations of a still later date. ... In most parts,perhaps in all parts, of the Cordillera, it may beconcluded that each line has been formed by repe-ated upheavals" (Darwin 1845, p. 320).These conglomerates are now part of theAlto Tunuyán Basin (Polanski 1957), anintermontane basin studied by Giambiagi(1999), who confirmed the uplift sequen-ce by means of precise provenance stu-dies. The Alto Tunuyán Basin structurecomprises thin-skinned fold-thrust she-ets above a decóllement developed in LateCretaceous siltstones. Within this zone,faults related to syntectonic unconformi-ties recorded in the upper part of theTunuyán Conglomerate indicate sedi-mentation above an active thrust (Giam-biagi et al. 2001). Other faults indicateactivity after deposition of the Neogeneunits. The structure in this domain intri-gued Darwin, who observed that thenewer coarse conglomerates dipped di-rectly under the much older beds of gyp-seous and limestone units. He stated thatthe Mesozoic strata has been broken upby "several, distinct, parallel, unilinal lines ofelevation" and estimated a vertical throwof over 600 m. These lines of elevationrepresent the complex thrust system ofthe Cerro Palomares (Fig. 6d).

Cordillera Frontal

The Cordillera Frontal consists of seve-ral basement blocks uplifted east of theAconcagua fold-and-thrust belt. Duringthe Late Cenozoic compression - at thelatitude of the Portillo Pass- it behavedas a rigid block disrupted by medium tohigh angle faults showing two regional

The High Andean Cordillera of central Argentina and Chile along ... 51

trends of structures, i.e., NNE-trendingfaults in the northern region and N toNNW-trending faults in the southernhalf, related to Late Paleozoic structures.A deep crustal detachment level is requi-red in order to propagate shorteningwithin the Cordillera Frontal. This de-tachment is inferred to be located betwe-en 15 and 20 km deep, probably within abrittle-ductile transition.

GEOMORPHOLOGY

Darwin stated "in the course of ages in this(referring to the Cachapoal) and othervalleys, events may have occurred like between,but even on a grander scale than, that describedby Molina, when a slip during the earthquake of1762, banked up for ten days the great riverLontué, which then bursting its barrier inunda-ted the whole country and doubtless transportedmany great fragments of rocks" (Darwin1846, p. 66-67). However, "not withstan-ding this one case of difficulty" he then clai-med that "one of the most important conclu-sions to which my observations on the geology ofSouth America have led me", is his lengthilyexposed conviction that the "terrace-likefringes, which are continuously united with thebasin-shaped plains at the foot of the Cordillera,have been formed by the arrestment of river-borne detritus at successive levels, in the samemanner as we see now taking place at the headsof all those many, deep, winding fiords intersec-ting the southern coasts." Due to this viewDarwin failed to recognize the MesonAlto landslide and the plain in the Yesovalley (Embalse El Yeso in Fig. 6a) andrefers to it as "the one exception in which arange was dislocated by a great and abruptfault" In page 178 this plain is describedas a "drained lake", yet once more no rela-tion to a landslide is mentioned. The damof this paleolake was later described as amoraine and only recently has it beenrecognized as part of a big landslide deri-ved from the granitic southern slopes ofthe Yeso valley (Abele 1981).

STRUCTURAL EVOLUTIONOF THE ANDES AT 33°40´S

Here we integrate recent structural andsedimentological studies carried out at33°-34° S (Godoy 1993, Godoy et al.1996, 1999, Ramos et al. 1996b, 1997,Giambiagi et al. 2001, Giambiagi andRamos 2002, Giambiagi et al. 2003) as away to understand what was envisaged byDarwin's during his early observations.During the early Miocene - between 20?and 15 Ma - shortening associated withinversion of the Mesozoic and Early Ce-nozoic extensional systems started diach-ronically in the Cordillera Principal. Ataround 18 Ma deformation and upliftgenerated a western sediment source forthe lower part of the foreland TunuyánConglomerate, which at 15 Ma shows anabrupt change in clast composition indi-cated by the first appearance of Me-sozoic sedimentary debris (Giambiagi1999, Irigoyen et al. 2000). This is consi-dered as evidence for a second phase ofdeformation, linked to E-W shorteningalong the eastern Cordillera Principal.Continued migration of deformation to-ward the foreland is documented by theoccurrence of growth structures in theupper section of the Tunuyán Conglo-merate. At that time basement thruststransported eastwards earlier CordilleraPrincipal thrusts and folds, forming thusa hybrid thin and thick-skinned belt ofdeformation.Overprinting relationships indicate thatthe fold and thrust belt evolved by meansof a forward propagating thrust sequen-ce including periods of out-of-sequencethrusting. The denudation of the foldand thrust belt provided the material forthe uppermost levels of the TunuyánConglomerate. The unconformity thatseparates the Palomares Formation fromthe Tunuyán Conglomerate, the changesin paleocurrents measured in the formerunit, and the occurrence of locally deri-ved conglomerates, provide evidence forthe beginning of a third phase of defor-mation, associated this time with the on-set of uplifting in the Cordillera Frontal.This is recorded by the regional influx ofthick coarse conglomerates, the genera-tion of a broken foreland basin system,

and the accumulation of the wedge ofproximal synorogenic deposits of thePalomares and Butaló Formations.The uplift of the Cordillera Frontal gene-rated a sticking point, preventing the pro-pagation of the thrust belt towards theforeland. As a result, a series of out-of-sequence thrusts developed in the eas-tern Cordillera Principal and the Alto Tu-nuyán foreland basin was partially canni-balized by the advancing thrust sheets.After the emplacement of granitoidswith isotopic signatures indicating mag-mas sources in a thickened crust (Kurtz etal. 1997, Kay et al. 2005) in the westernCordillera Principal, magmatism shiftedeastward to its present position along theactive volcanic front of the Tupungato-San José line. Thrusting along the Cordi-llera Principal ended by ~4 Ma and sub-sequently the deformation front migra-ted to the Precordillera - north of 33°S-and to the Cuyo Basin and the easternCordillera Frontal foothills south of33°S. Folding in the Cuyo Basin startedafter the beginning of deposition of theMogotes Formation at ~ 3.0 Ma (Yri-goyen 1993, Chiaramonte et al. 2000).The last phase of deformation accom-modating ESE-WNW and ENE-WSWshortening concentrated in the CuyoBasin and in the La Carrera thrust systemin the Cordillera Frontal. Earthquake fo-cal mechanisms and neotectonic activityindicate that the eastern border of thePrecordillera and the Cuyo Basin are stillactive (Cortés et al. 1999, Chiaramonte etal. 2000).

CONCLUDING REMARKS

Darwin was the first geologist to descri-be the stratigraphy of the CordilleraPrincipal and the Cordillera Frontal ofcentral Chile and Argentina recognizingthe presence of an old metamorphic ba-sement - much older than its "red grani-tes" - presently considered to be late Pa-leozoic intrusives in the Cordillera Fron-tal. He correctly assigned a Cretaceousage to the marine sediments croppingout along the eastern Cordillera Principal

L. GIAMBIAGI , M. TUNIK, V.A . RAMOS AND E GODOY52

and also showed that the uplift of theAndean range was much younger thanthe marine sedimentation of the Creta-ceous deposits. By studying the composi-tion of clasts included in Tertiary sedi-ments of the Alto Tunuyán Basin he wasable to establish the relative upliftingtimes for the Cordillera Principal andCordillera Frontal conluding that thePiuquenes range (Cordillera Principal)was uplifted before the Portillo range(Cordillera Frontal). To explain the drai-nage pattern of the Tunuyán-Cordón delPortillo region, he proposed a gradualelevation of the Cordón del Portillo afterthe uplift of the Piuquenes range, whichallowed the waters of the Tunuyán De-pression to cut a deep canyon throughthe latter as the range was been uplifted.As a tribute to his exceptional observa-tional skills we should emphasize that theprogressive advance of an orogenic de-formation front from hinterland towardsforeland, now considered a commonprocess in most mountain chains, wasearly envisaged by Darwin more than 150years ago during is historic 1835 journeyacross the Andes.

ACKNOWLEDGEMENTS

This work was found by the University ofBuenos Aires (UBACYT x182) andCONICET (PIP 4162, 5965). The au-thors acknwoledge with special thanksthe reviews of Reynaldo Charrier, MiguelGriffin, and Constantino Mpodozis.

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Recibido: 3 de octubre de 2008 Aceptado: 21 de noviembre de 2008