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131Revista de la Asociación Geológica Argentina 63 (1): 131 - 149 (2008)

VERY LONG PAHOEHOE INFLATED BASALTIC LAVAFLOWS IN THE PAYENIA VOLCANIC PROVINCE (MENDOZA AND LA PAMPA, ARGENTINA)

Giorgio PASQUARÈ1, Andrea BISTACCHI2, Lorella FRANCALANCI3, Gustavo Walter BERTOTTO4, Elena BOARI3, MatteoMASSIRONI5 and Andrea ROSSOTTI6

1 Dipartimento di Scienze della Terra, Università degli Studi di Milano, Via Mangiagalli 34, 20133, Milano, Italy.E-mail: [email protected] Università degli Studi di Milano-Bicocca, Dipartimento di Scienze Geologiche e Geotecnologie, Piazza della Scienza 4, 20126 Milano,Italy.3 Dipartimento di Scienze della Terra, Università degli Studi di Firenze, via La Pira, 4, 50121, Firenze, Italy.4 Facultad de Ciencias Exactas y Naturales, Universidad Nacional de La Pampa, Uruguay 151, 6300, Santa Rosa, La Pampa, Argentina.5 Dipartimento di Geoscienze, Università di Padova, Via Giotto 1, 35137, Italy.6 Dipartimento di Scienze Naturali Universitá degli Studi dell´Insubria, Via Valeggio 11, 20100, Como, Italy.

ABSTRACT Extremely long basaltic lava flows are here presented and described. The flows originated from the great, polygenetic, fissu-ral Payen Volcanic Complex, in the Andean back-arc volcanic province of Payenia in Argentina. The lava flows outpouredduring the Late Quaternary from the summit rift of a shield volcano representing the first volcanic centre of this complex.One of these flows presents an individual tongue-like shape with a length of 181 km and therefore is the longest known indi-vidual Quaternary lava flow on Earth. Leaving the flanks of the volcano this flow reached the Salado river valley at La Pampaand, in its distal portion, maintained its narrow and straight shape without any topographic control over a flat alluvial plain.It has a hawaiite composition with low phenocryst content of prevailing olivine and minor plagioclase. Rare Earth elementpatterns are typical of Na-alkaline basalts, but incompatible trace element patterns and Sr -Nd isotope ratios, suggest a geody-namic setting transitional to the orogenic one.The flow advanced following the thermally efficient "inflation" mechanism, as demonstrated by a peculiar association of welldeveloped morphological, structural and textural features. The temperature of 1130-1160°C and the viscosity of 3-73 Pa*s,calculated by petrochemical data, may be considered, together with a very low cooling rate and a sustained and long lastingeffusion rate, the main causes of the extremely long transport system of this flow. Both the extreme length of the flow andthe partial lack of topographic control may provide new constraints on the physics of large inflated flows, which constitutethe largest volcanic provinces on Earth and probably also on the terrestrial planets.

Keywords: Pahoehoe lava flows, Inflated flows, Payenia.

RESUMEN: Flujos de lava basáltica pahoehoe muy extendidos en la provincia volcánica Payenia (Mendoza y La Pampa, Argentina). En este trabajo se presentan y describen flujos de lava extremadamente largos. Estos flujos se originaron en el complejo vol-cánico fisural Payen, dentro de la provincia volcánica Payenia en el retroarco andino. Los flujos de lava fueron emitidos duran-te el Cuaternario tardío desde el sector superior de un volcán en escudo, el que constituyó el primer centro volcánico de estecomplejo. Uno de estos flujos presenta forma de lengua y tiene una longitud de 181 km, constituyendo el flujo de lava cua-ternario conocido, más largo sobre la Tierra. Este flujo alcanzó el valle del río Salado en La Pampa y mantuvo una formaangosta y recta sin ningún control topográfico, hasta su porción más distal sobre una planicie aluvial. La roca que forma estacolada es una hawaita con un bajo contenido de fenocristales de olivino y plagioclasa. Los patrones de elementos traza sontípicos de basaltos alcalino-sódicos de intraplaca, sin embargo el patrón de elementos traza incompatibles y las relaciones deisótopos de Sr y Nd, sugieren un marco transicional al orogénico. El flujo estudiado fluyó siguiendo un mecanismo térmica-mente eficiente denominado inflation, como demuestran las asociaciones de sus rasgos morfológicos, estructurales y textura-les. Las causas principales de este sistema de transporte extremadamente largo pueden ser una temperatura de 1.130º-1160ºCy viscosidad de 3-73 Pa*s (calculadas a partir de los datos petroquímicos), junto con una muy baja velocidad de enfriamien-to y una tasa de efusión grande y sostenida en el tiempo. La gran longitud y la falta parcial de control topográfico del flujopueden aportar nuevos conocimientos acerca de la física de los grandes flujos inflados, los que constituyen las más extensasprovincias volcánicas de la Tierra y probablemente también la de otros planetas terrestres.

Palabras clave: Coladas de lava pahoehoe, Flujos basálticos inflados, Payenia.

INTRODUCTION

Long individual basaltic lava flows (seve-ral ten of km in length), are rare on theEarth's surface but widespread on otherterrestrial planets. The longest flows,exceeding 100 km in length, are the Qua-ternary Undara and Toomba flows inAustralia (Southerland 1998, Stephensonet al. 1998, 2000) and the Thjorsa flow inIceland (Hjartarson 1994). Studyingshorter but younger and best exposedhawaiian basaltic lava flows some au-thors have suggested that a great trans-port efficiency can be explained througha process of endogenous growth calledinflation. This theory challenged the tra-ditional views that large and long basalticlava flows were formed by the rapideruption of relatively low viscosity lavasand proposed a relatively slow emplace-ment. This process was firstly envisagedby Anderson (1910) and later illustratedmore thoroughly by Walker (1991) andHon et al. (1994). An originally few deci-metres thick basalt flow is later inflatedby the underlying fluid core, which re-mains hot and fluid thanks to the ther-mal-shield effect of its visco-elasticcrust. Increasing and evenly distributedpressure in the liquid core leads to uni-form uplift of the crust and to thicke-ning and widening of the entire lava flow.Spreading of the flow is due to the con-tinuous creation of new inflated lobes,which develop at the front of the flow,where the crust is thinner and the lavapressure is higher. The inflation processis most pronounced in flat areas or onvery gentle slopes (< 1-2°), and a steadyflow rate, sustained for a long time, isprobably a necessary condition. Thethermal efficiency of this process is im-proved by the endogenous travel of lavainside tube systems (Greeley 1987, Pin-kerton and Wilson 1994, Keszthelyi andSelf 1998, Cashman et al. 1998). Certainmorphological, structural and texturalfeatures are considered to be "finger-prints" of inflation such as: tumuli, lavarises, columnar jointing patterns, dilata-tional surface fractures and inner layering

due to the distribution of vesicles andcrystallinity (Walker 1991, Glaze et al.2005, Cañón-Tapia and Coe 2002, Du-raiswami et al. 2001). Hon et al. (1994)introduced the term of "sheet flows" forlarge inflated pahoehoe flows of Kilauea,emplaced as a series of interconnectedlobes on sub horizontal slopes and dueto sustained input of lava during a long-lived eruption. Eventually, in the endoge-nous liquid core of the flow preferredpathways may develop, generating eithera system of anastomosed and transientsmall lava tubes or larger and more stableconduits. The transport thorough lavatubes over 100 km in length lava in infla-ted flows permits the molten lava core toreach the flow front with a cooling rateof less than 0.5°C/km (Keszthelyi 1992,1995, Keszthelyi and Self 1998, Saki-moto et al. 1997, Sakimoto and Zuber1998). This provides feasible explanationfor the development of extremely longlava flows.The above described genetic models we-re applied also to the terrestrial flood ba-salts, especially to those of the Colum-bia River Plateau (Self et al. 1996, Tho-rardson and Self 1998) but in this casediscrete single lava flows cannot be resol-ved because they are tightly intermingledinto very large lava fields to be recogni-zed. Their sources are very problematicbecause they actually correspond to com-plex and mostly hidden swarms of dikes.This paper describes a very large com-pound basaltic lava flow, previously citedby Pasquarè et al. (2005) with the name ofPampas Onduladas lava flow. It outpou-red in Late Quaternary during a singlelong-lasting eruption from the summitrift of a large basaltic shield volcano,which represents the first step of themagmatic and structural evolution of thePayen Volcanic Complex. This Complexis located in the Andean back-arc volca-nic province of Payenia in Argentina(Polanski 1954, Ramos 1999, Ramos andKay 2006). The total volume of PampasOnduladas compound lava flow can beestimated in 33 km3 and it covers an areaof 1,870 km2. The greatest part of the

lava flow descended along the northernflank of the shield. The flow is separated10 km away from the source into twobranches whose main one travelled to-wards ESE for a total distance of 181km, therefore being the longest Quater-nary individual lava flow on Earth. Thesecond branch flowed towards NW for60 km and the distance covered by lavabetween the two opposite fronts; a-mounts to 210 kilometres. Minor but stillconspicuous quantity of lava flowedalong the southern flank of the shield.From now onward the first branch willbe cited as Pampas Onduladas flow whilethe second one as Pampas Onduladasnorthern branch. Both branches weresampled for geochemical and petrologi-cal analyses while the morphologic andtextural observations herein presentedare devoted only to Pampas Onduladasflow description and interpretation.Thanks to the excellent outcropping con-ditions of Pampas Onduladas flow, andthe desertic unvegetated environment,the topographic, morphologic and struc-tural features of the flow can be obser-ved in great detail at different scales. Allof them are indicative of an inflated andpartly deflated pahoehoe basaltic lavaflow. This Argentinean lava flow alsoshows unusual inflation values, reaching25 m in the large sheet flow forming theupper-middle portion.An unusual emplacement behaviour, itsmedian and distal portions, forming cha-racterizes also a narrow tongue with avery low width-to-length ratio.This paper could be a basis for furtherinvestigations on the transport mecha-nisms of terrestrial long lava flows andon the conditions under which the lavaflows lengthens downstream rather thanwidens, as in the case of many long pla-netary homologues.

THE PAYEN VOLCANICCOMPLEX

The Payen is a fissural volcanic complex,developed during the Quaternary (Ger-ma et al. 2007) with a 70 km long align-

132 G. PASQUARÈ, A . BISTACCHI , L . FRANCALANCI , G. W. BERTOTTO, E . BOARI , M. MASSIRONI AND A. ROSSOTTI

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ment of hundreds of eruptive centresalong the W-E Carbonilla fault system(Llambías 1966, González Díaz 1972a).Its stratigraphic basement is representedby the northernmost one of the TertiaryPatagonian basaltic plateaus, defined asPalauco Formation by González Díaz(1979).The activity of the Payen Volcanic Com-plex probably started during the EarlyQuaternary, with a pahoehoe basaltic lavashield and related scoria and spattercones representing the basal portion ofthe Payen Volcanic Complex, here afterPayen Eastern Shield. Its product partlycorrespond to the Morado Alto For-mation of González Díaz (1972b) andpartly to the Basalts III and IV of Groe-ber (1946).They were emplaced over the eastern endof the Carbonilla Fault system (Fig. 1).These lavas are pahoehoe basalts rich inhorizontal vesicular zones showing largeinflated lobes with elliptical croos section(Fig. 2). After a long volcanic hiatus mar-ked by erosion, the Payen Eastern Shield

was involved in a new phase of strongeruptive activity and the Carbonilla faultacted again as its summit rift. A largevolume of olivine-basaltic lavas coveredthe shield with large fields of compoundand single pahoehoe inflated lava flows.The longest ones reached the Saladoriver valley at La Pampa province and theLlancanelo Lake to the North. These lavaflows were previously mapped by Gro-eber (1929, 1946), González Díaz (1972a,b, 1979) and Núñez (1976) as a uniquehuge lava field but were not distinguishedthe single flows and their exceptional di-mensions. Moreover González Díaz(1972a) attributed a sector of this lavaflow to the Holocene El Mollar For-mation. In addition to this single basalticlava flow González Díaz (1972a) propo-sed a vent area near Escorial de la Cruzand a length of about 80 km.Besides the already cited Pampas Ondu-ladas compound flow, Pasquarè et al.(2005) distinguished the Los Carrizaleslava field reaching about the same lengthbut showing a very much greater volume

(Fig.3). Radiometric dating of Los Carri-zales lava field, stratigraphically underl-ying the younger Pampas Onduladas La-va Flow, yields a K-Ar age of 0.40 ± 0.1Ma (Melchor and Casadío 1999).Another pahoehoe basaltic inflated lavafield with similar morphology, textureand age, was emplaced along the westernflank of the Payen Volcanic Complex.These lavas represent the base of a largeand still preserved shield volcano, herenamed as Payen Western Shield. It wasformed by repeated emissions of pahoe-hoe olivine basalts covered by aa lavaflows, showing spectacular channel-leve-es systems. Three cosmogenic 3He expo-sure ages on primary lava flow surfacesbelonging to El Puente Formation (at ~36.31°S; 69.66°W) near the Río Granderiver, indicate a Pleistocene age of 41±1ka (37 ± 3 to 44 ± 2 ka) (Marchetti et al.2006). By the way, the last aa lava flowswere very recent as proved by the localoral tradition since the colonial times(González Díaz 1972b). The stratigra-phic relations between lavas of the Payen

Very long pahoehoe inflated basaltic lava flows in the Payenia volcanic province ...

Figure 1: Geological sketch-map of the central sector of the Payen Volcanic Complex. Black represents the final basaltic lavas; light grey shows thePayen western shield volcano; dark grey indicate the Pampas Onduladas compound lava flow; horizontal lines correspond to the Los Carrizales lavafield; diagonal striped pattern is the Payen eastern shield volcano; dotted area is covered by tephra from Payún Matru volcano and its caldera (PM),vvv denotes the trachytic, and trachyandesitic lavas of Payún Matru and Payen Liso (PL); thick red line refers to the Carbonilla Fault system.


Western Shield and the remaining por-tion of the Payen Volcanic Complex,suggest that the Payen Western Shielddeveloped its activity during the entirelife of the Complex itself.After the main growth of the Payen Eas-tern Shield a new widespread volcanicactivity, along the central sector of theCarbonilla Fault system was responsibleof the formation of two great trachyticand trachyandesitic stratovolcanoes, ca-lled respectively Payen Liso and PayúnMatru volcanoes. While Payen Liso was

not involved anymore in other relevantvolcanic events, the original cone ofPayún Matru was partially destroyed by a8 km wide caldera collapse (dated ataround 168 ka; Germa et al. 2007). Thepresence of abundant loose or weldedtephra deposits were formerly recogni-zed by Groeber (1933, 1937) and laterclassified as ignimbrites by GonzálezDíaz (1972b).After the collapse, highly viscous trachy-tic and trachyandesitic lavas were eruptedfrom intra and extracalderic vents for-

ming thick and levee-bordered flows,well described by Llambías (1966).The Carbonilla fault provided once morea very efficient feeding system for a newphase of basaltic volcanism developedon the fault terminations. The main ventsof this great effusion phase are associa-ted with dense clusters of cinder conesclearly controlled by the main fault andby its several conjugate fractures. Thisyoungest phase of olivine-basaltic lavaproduction was very noticeable in thewestern side of the Payen Volcanic Com-plex. Also along the eastern sector of theCarbonilla Fault system both aa andpahoehoe, olivine-basaltic flows outpou-red on the south-eastern flank of thePayen Volcanic Complex, and partlycovered Pampas Onduladas and Los Ca-rrizales lava fields. The very recent age ofthe volcanic activity along the easternsector of the Payen Volcanic Complex isproved by the Holocene age of somecinder cones called "El Rengo Group"(Inbar and Risso 2001). However agedetermination and statigraphic relation-ships among the last volcanic activities ofthe complex needs to be still clarified.

THE EASTERN ARM OFPAMPAS ONDULADASCOMPOUND LAVA FLOW

General descriptionThe Pampas Onduladas flow is an indivi-dual basaltic pahoehoe lava flow very cle-arly recognizable in the satellite imagesand aerial photos and remarkably evidentin the field (Figs. 3, 4, 5).The uppermost crust of Pampas Ondu-ladas flow, commonly with a maximumthickness of 40-50 cm, appears to be aspongy pahoehoe (Walker, 1989) witharound 40 % vol. bubbles covered by avery thin chilled top and whose sphericalvesicles have a nearly constant averagesize of 3-5 mm in diameter (Fig. 6a). Im-mediately under this skin, microlites be-gin to appear in the same vesciculatedlava. This coupled surface textures andfeatures of Pampas Onduladas flow areremarkably diffused over the entire lava

Figure 3: Areal distribution of the inflated pahoehoe lava flows representing the late activity ofthe Payen Eastern Shield volcano. Black represents the Pampas Onduladas compound flow withits eastern branch (POF) and its northern branch (PNB). Light grey: Los Carrizales lava field,dark grey: central sector of the Payen Volcanic Complex, horizontal lines: pre-Quaternary base-ment, white: Quaternary Andean piedmont deposits and basaltic lavas from Chachahuen volcaniccentres along the southern margin of the figure. The arrows indicate the longitudinal axes of themain branches of the Pampas Onduladas compound flow.po: Pampas Onduladas, lc: Los Carrizales, ec: Escorial de la Cruz, ls: La Salinilla, co: El Cortaderal,col: Las Coloradas, ro: Rincón del Olmo, cg: Cerro Guadaloso, pm: Payún Matru, pl: Payen Liso.

Figure 2: Section of an inflated pahoehoe lobe with its dense elliptical core from the early acti-vity of the Payen eastern shield volcano.

135Very long pahoehoe inflated basaltic lava flows in the Payenia volcanic province ...

flow. Under this crust the crystallinity in-creases while the relative abundance ofthe vesicles sharply decreases. Their sizepresents a marked variability reaching si-zes of many centimetres and their shapesare often irregular (Fig. 6b). This denselava layer is dissected by regular joint setswhich form short and rough polygonalcolumns and merge at shallow depth intodilated, widely spaced, and curvilineardiscontinuities.Under this layer, and often separatedfrom it by a subhorizontal and billowyphysical discontinuity, the dense basaltloses most of its joints and cracks andshows horizontal alignments of great o-blate or prolate vesicles, commonly offew decimetres (Fig. 6c). This layer, whenentirely visible, gradually passes down-ward to a more massive rock hostinghorizontal short discontinuities formedby coalesced large oblate bubbles (Fig.6d), or to a dense and massive layer cros-sed by subvertical joints and microvesi-culated pipes (Fig. 6e). This ideal section,collected from few isolated verticalscarps, do not exceed a total thickness offour metres.The scarcity of erosional scarps or artifi-cial excavations all over the lava flow ma-kes difficult to estimate the total thick-ness of Pampas Onduladas flow but in ahand-made water-well crosscutting athick central part of the flow, it has beenobserved a 15 m thick layered crust for-med by several repetitions of the sequen-ce above described. The dense massive

layers can reach up to two metres ofthickness while the vesiculated layersmaintain a decimetric average thickness.Between 15 and 22 metres the water-wellreached a massive basalt with high crysta-llinity (up to 90 % vol.) and larger grainsize. Vesicle rich layers are not frequent.The base of the flow is composed by athin glassy vesicular crust (15-50 cmthick), including fragments of the subs-tratum, which is composed by older vol-canic rocks (close to the volcano), or byMiocene-Pliocene deposits (Cerro AzulFormation) near the Salado river valley atLa Pampa Province. Its superficial textu-re is mostly that of a shelly or slabbypahoehoe flow forming a basal, manymetres wide, thin platform flanking themain flow.The Pampas Onduladas flow emissionarea is located on the eastern sector ofthe master E-W feeding fracture systemof the Payen Volcanic Complex. Thevents are constituted by long fissuresmarked by alignments of scoria conesand spatter ramparts with associated lavaponds, showing evidences of quick tem-poral and spatial changes of the eruptionsources.In the near-vent area, the Pampas On-duladas flow appears to be constituted bya great number of pahoehoe differentuninflated cooling units formed by ove-rriding ropy and slabby pahoehoe flowssometimes channellized into levees. Theleeves are formed by broken and tiltedlava slabs, and therefore suggest a transi-

tion to aa lavas. When the slope decrea-ses from 2° to around 1°(*), the proximalcooling units are to be transformed intoseveral progressively inflated lobes andtoes cooled one against the other. Thefronts of the single lower lobes appear tobe arranged in a unique very large front.In this sector the Pampas Onduladasflow drowned and levelled the underlyingnearly horizontal topography can be con-sidered as an uniform very wide flowed,a sheet flow (Fig. 7a, b). The contact bet-ween some lobes and the sheet flow ismarked by open arcuate tension gashesshowing downstream oriented convexity.Several kilometres long fractures, occuralso inside the initial portion of the sheetflow. All of them are probably due to theinner traction of the liquid core of theflow on its solidified crust. Some gashesare partly filled by great squeeze-upshaving the shape of long vesicle-like swe-llings, sharply cut by marginal and axialclefts.Reaching the E-W faulted margin of Ce-rro Guadaloso Tertiary volcano, thesheet flow divides into two opposite di-rections: one moving eastward belongsto the Pampas Onduladas flow mainbranch, the other flowing westwardconstitutes the Pampas Onduladas flownorthern branch (Fig. 3). The total areacovered by this huge sheet flow amounts

(*) Slopes were calculated by SRTM (ShuttleRadar Topogra-phy Mission) altimetry data,using ArcInfo.

Figure 4: POF lon-gitudinal elevation(top) and slope(botton profile).Vertical scale ishighly exaggerated.The rectanglesindicate the relati-vely steep descentof the lava flowEast of LaSalinilla- El Cor-taderal road.

to 978 km2. The sheet flow portion ofthe main branch of Pampas Onduladasflow, entirely subdues all the underlyingtopography except some older isolatedvolcanic cones (Fig. 5a). Its morphologyis characterized by a considerable densityof tumuli, flat lava rises, and other infla-tion-related features, which will be des-cribed in the next paragraph. From thenorthern margin of this large lava fieldsome apophyses, up to 10 km in length,spreads in fillings of valleys previouslygoing down towards the area now cove-red by the main sheet flow. It indicates amarkedly depressed palaeotopographyhere covered by the sheet flow. This isalso confirmed by the E-W normal faultscarp representing the margin betweenPampas Onduladas flow and the Tertiaryvolcanic centres of Cerro Guadaloso.Because of the thick aeolian deposits co-vering the basal contact of the lava flows,it is difficult to find a complete section ofthe flow itself and to evaluate its strati-graphy and thickness. Some topographicmeasurements were performed on theflow apophyses above cited, and an esti-mate of 20 m as the minimum sheet flowthickness was derived in this area.East of the La Salinilla-El Cortaderalroad, occurs a remarkable altimetric andmorphological change, with a relativelysteep 40 m descent of the lava flow overa distance of one kilometre (Fig. 4) and asharp reduction of the average widthfrom 10 to 3 km up to its end. Here thePampas Onduladas flow is confinedalong its southern side by a Tertiary vol-canic tabular relief and on the northernside by a river channel coming from theSan Rafael massif. This place marks alsothe sudden disappearance of the lavarises, above described as the more wides-pread superficial features of PampasOnduladas sheet flow.In this central sector of the Pampas On-duladas flow, the distribution of tumulibecomes very discontinuous and theytend to be associated in isolated clusters.Their size generally is uneven and in so-me cases they reach a height of 10-12metres, never observed in the sheet flow

portion of Pampas Onduladas. Tumuliare mostly concentrated in the more

depressed portions of the flow, but someof them were placed along the axis of


Figure 5: LANDSAT false colour images along POF. LANDSAT 7 ETM + false color compositeimages (RGB= 742) of different sectors of POF. For their locations see Fig. 3.


the long marginal ridges characterizingpart of the central portion of PampasOnduladas flow, at an intervening distan-ce of hundreds of metres (Fig. 5b).The topographic confinement of Pam-pas Onduladas flow disappears nearPuesto Las Coloradas where the flowsharply deviates towards SSE and obli-quely crosses various small creeks belon-ging to the pampean alluvial piedmontplain 22 km far from Las Coloradas. Af-ter another sharp deviation, Pampas On-duladas flow recovers its ESE directionunder the obstacle effect of a thick lava

fan previously formed by the lava flowitself. From here until the lava flow front,Pampas Onduladas flow does not showany external confinement but neverthe-less maintains its tongue-like shape in awell directioned ESE pathway (Fig. 5c).In the median and terminal sector,Pampas Onduladas flow loses its long-distance morphological and structuraluniformity and acquires significant va-riety along its still long remaining seg-ment. In the initial part of this segmentthe flow is self-confined by lateral ridgeswhile its flat internal portions present a

depressed, slightly hummocky surface.More downstream, near Rincón del Ol-mo, the flow reduces, for a short stretch,its width to 500 m after which presents acentral longitudinal hundreds of metreswide tabula ridge, showing a deep axialcleft partly filled with squeeze-ups of va-rious sizes and shapes, as well as by hori-zontal clefts indicating a consistent com-ponent of lateral inflation. Lateral brea-kouts from the central ridge producedmany short lobes and toes. After this 13km long segment of Pampas Onduladasflow another bottle neck-like narrowing

Figure 6: Cross-section of the upperpart of the POF inflated sheet flowidealized from different outcrops repre-sented in the joined images (a, b, c, d,e) (see the text for their descriptions).

of the lava flow marks the end of thepreviously described tubular ridge andthe onset of a new morphological andtextural feature of Pampas Onduladasflow, characterizing its final stretch for 40km onwards. From here the flow is for-med by a great number of interconnec-ted, very wide, long and billowy braidedlobes, separated by flat elongated depres-sions covered by aeolian sand fromwhich several large smooth lava hum-mocks appear. After reaching the alluvialterrace of the Salado river, the lava flowjumps down a 25 metres scarp, resumesits course for 4 km more and ends inchaotic block piles.

Relevant morphological and structu-ral features of Pampas Onduladassheet flow

The surface morphology of the PampasOnduladas sheet flow is represented by agreat variety of small scale inflation fea-tures, strictly depending from the physi-cal processes of lava injected and trans-ported beneath the surface crust. Theinflated sheet flow is commonly flankedby a very thin and flat uninflated lavasheet, mostly formed by slabby pahoe-hoe basalts (Fig.10).The most common features of this kindare lava rises in the sense proposed by

Walker (1991) but showing some mor-phological and structural charactersrather different from their original defini-tion. Other common surface features aretumuli, indirectly connected to the lavarises thorough their apophyses. Finallyother positive features are represented bynarrow and elongated ridges close to pre-existing obstacles. The lava rises are scat-tered all over the sheet flow and assumevery different sizes and forms, from fewthousands of square metres up to squarekilometres. Their elevation does not ex-ceed 10 m. The largest lava rises are mar-kedly flat-topped and commonly boun-ded by monoclinal flexures showing dip-ping values between 20° and 40°. Thehinges are marked by long tensional frac-tures, sometimes hosting vertical squee-ze-ups of a dense basalt, enriched inscattered large plagioclase phenocrysts(Fig. 9).The upper crust of the lava rises every-where polygonally jointed, but the size ofthe polygons varies in width, from fewdecimetres up to one or two metres. A-long the external side of the lava rises,groups of polygons of similar size areoften separated from other groups withdifferent size by cooling joints generallyat high angles relatively to the margin ofthe lava rise.The edge of the flat-topped lava rises isextremely irregular for the presence ofelongated, sinuous and convolute apo-physes giving to these structures a polyp-form aspect (Fig. 11). The apophyses areusually lower than the related flat-toppedlava rises, except when they contain atumulus that commonly reaches thetopographic level of the lava rise itself.Their longitudinal development registersmany axial elevations and depressions,the latter often filled by sandy aeoliandeposits. In the axial depressions of theapophyses may appear sub circular, densebasaltic squeeze-ups (Fig. 12). Common-ly the apophyses act as connections bet-ween adjacent flat-topped lava rises ma-king the sheet flow surface as an uninte-rrupted network of lava rises and relatedapophyses.


Figure 7: a) Aerial photo of the transitional area between POF near-vent area and POF sheetflow. b) Proposed interpretation of the photo image. 1) overriding and coalescing inflated lobesof the POF near-vent area; 2) POF sheet flow; 3) older volcanic centres; 4) liquid core squeeze-ups; 5) tensional fissures.

a

b

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Tumuli are widespread, randomly distri-buted features, always arising from theabove described apophyses and neverfrom the lava rises flat tops. They are ei-ther circular or elliptical in plan view (Fig.

13a, b, c) and dissected by summit exten-sional fissures. They usually measure lessthan 7 m in height, around 20 m in widthand not over 50 m in length. The centralfissure clefts often end into triple junc-

tions or more complex branched systemsof dilatation fissures. The short colum-nar joints, always perpendicular to thecooling surfaces, regularly follow thebending of the tumuli surfaces.Some peculiar surface features wereobserved along the margins of older iso-lated cinder or lava cones surrounded bythe Pampas Onduladas sheet flow. Theyare bundles of parallel ridges and chan-nels following the pre-existing obstaclesborder; the ridges are flat-topped and thechannels show a ditch-shaped morpho-logy, ranging from less than 1 m to up toabout 10 m in depth and from few me-tres to over 100 m in width. Their spatialarrangement suggests that they whereoriginated because of a preferential coo-ling of the sheet flow around the pre-existing obstacles and by the develop-ment of thermally preferred pathwaysinside the liquid core of the inflatingflow. The evolution of this frozen area,after the evacuation of the remnantselongated magma batches, was conse-quently dominated by the partial roofcollapse of these previously active path-ways. Similar channels were also noticedon some inflated lava flows moving late-rally from the main sheet flow body,along its northern margin. Each channelexhibits the side facing towards the flowrim (external side) always steeper and lesselevated respect with the side facingtowards the inner part of the lava flow(internal side) which, on the other hand,shows gentle inclination and higher ele-vation (Fig. 8).Typically the external flanks have an ele-vation slightly smaller than the main flowbody and are characterised by internalslopes steeper than the main body bor-ders. In addition, evidences of brittle tosemi-brittle collapse associated to align-ments of narrow arched trenches havebeen found locally at the internal slope ofthe levee. Such film slides have not beenfound at the more gentle slope betweenchannels and the main body. Both cha-racteristics suggest they were originatedby mechanisms of preferential coolingalong the margin of Pampas Onduladas


Figure 8: Aerial photo of the northern margin of the POF sheet flow with a lava tongue movingeastward counter current into a pre-existing valley. LR lava rises, CH collapse channels.

Figure 9: A large, flat lava rise in the central part of the POF sheet flow. Its large marginal cleftis filled by a denser, less vesiculated lava, squeezed-up from the liquid core.

flow and consequent rapid withdrawal ofmagma from no more alimented trans-port pathways.The main general attributes of the Pam-pas Onduladas sheet flow are a relevantthickness, a great volume of lava, a long-lasting extrusive activity and a vigorousendogenous growth and inflation. Theypermit to suppose that older flow unitsof this compound flow are overtoppedby the youngest one, whose surface

shows a continuous network of flat-top-ped lava rises and sinuous ridges contai-ning tumuli, without any evidence ofimportant breakouts and subsequentlobe by lobe activity. We consider that thesurface morphology of this huge sheetflow is the result of its latest evolutionarystep leading to the concentration of thelava transport into narrow pathways,mixing flat sheets with features typical ofhummocky flows, partially driven by

interconnected capillary tubes but notrelated to the great distributaries tubes asin the examples proposed by Self et al.(1998).

Petrochemical and mineralogical stu-dies of Pampas Onduladas rocks

Several samples have been collected fromPampas Onduladas and adjoining lavas inorder to investigate their petrology andgeochemistry. The magmatism of the Pa-yen Volcanic Complex mainly belongs tothe Na-alkaline series and is prevailinglyconstituted by basaltic rocks rangingfrom hawaiites to slightly sub-alkaline ba-salts (Fig. 14). More evolved magmas, upto trachytic compositions, are howeverrelatively abundant, especially in the vol-canism occurred after Pampas Ondula-das compound lava flow (González Díaz1972b). Furthermore, a sub-alkaline se-ries, ranging between basalt to andesiteand displaying higher MgO and lowerincompatible element contents, seems al-so to be present among the magmatismolder than Pampas Onduladas flow (Figs.14, 15).


Figure 10:Uninflated,thin slabbypahoehoesheet (inforeground)along thesouthernmargin ofPOF sheetflow, hereinrepresentedby a largeflat-toppedlava rise withseveralsinuousapophyses.

Figure 11: Satellite image of aPOF lava rise and its apophy-ses, partly buried by a thindeposit of aeolian sand. Asterfalse color composite image(RGB=321).


The Pampas Onduladas, the northernbranch rocks and the Los Carrizales lava-field are slightly silica-undersaturated tosilica-saturated hawaiites with a Na-alka-line character (mainly between 3.8% ofnormative ne and 2.0% of normative hy)(Table 1). The two samples of the olderLos Carrizales lava-field are slightly lessalkaline than Pampas Onduladas rocks,whereas the basic lava flows youngerthan Pampas Onduladas tend to be morealkaline than them (Figs. 14, 15). Thus, ageneral increase of the alkaline characterwith time seems to be occurred.The Pampas Onduladas, the northernbranch rocks and the Los Carrizales lava-field have similar petrographic characte-ristics. They are holocrystalline rockswith low phenocryst contents of olivine(usually ca. 5 vol.%) ± plagioclase(usually


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70,

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50,

30,

30,

40,

30,

70,

70,

80,

20,

20,

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315

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326

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293

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238

298

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332

266

240

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35,9

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5,45

5,82

5,35

3,91

4,45

4,64

4,46

4,44

3,85

4,40

5,12

5,44

4,03

4,06

3,75

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5,4

5,8

5,28

4,28

4,98

5,00

4,93

5,13

4,47

4,78

5,13

5,36

4,57

4,81

4,61

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773,

57-

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831,

971,

841,

571,

821,

841,

841,

881,

631,

731,

811,

861,

641,

801,

64-

1,69

1,42

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4,94

5,48

4,96

4,27

4,86

4,94

4,81

5,00

4,39

4,52

4,8

5,14

4,48

4,97

4,43

-4,

663,

87-

Tb0,

80,

880,

80,

690,

790,

800,

780,

820,

720,

760,

780,

810,

730,

790,

71-

0,74

0,66

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4,49

4,94

4,53

3,86

4,41

4,37

4,37

4,45

4,03

4,24

4,52

4,53

4,26

4,52

3,96

-4,

253,

75-

Ho0,

840,

920,

810,

710,

850,

830,

820,

810,

740,

790,

820,

830,

780,

850,

74-

0,79

0,70

-Er

2,27

2,48

2,25

1,88

2,26

2,21

2,17

2,20

2,03

2,14

2,24

2,26

2,12

2,32

2,05

-2,

221,

93-

Tm0,

322

0,34

60,

314

0,25

70,

303

0,30

40,

290

0,29

90,

272

0,29

50,

312

0,31

70,

294

0,31

80,

285

-0,

319

0,26

1-

Yb1,

962,

161,

931,

531,

801,

801,

731,

781,

621,

831,

961,

961,

771,

931,

71-

1,95

1,56

-Lu

0,28

20,

320,

277

0,22

20,

271

0,25

40,

257

0,26

60,

224

0,26

90,

288

0,29

10,

252

0,27

30,

239

-0,

271

0,23

1-

Hf3,

74,

13,

82,

53,

33,

43,

23,

33,

03,

13,

63,

63,

03,

42,

9-

3,2

2,8

-Ta

1,25

1,33

1,21

0,73

0,68

1,00

0,95

0,93

0,71

0,55

1,27

1,12

0,50

0,78

0,56

-0,

550,

44-

Pb-

--

3-

22

1-

6-

-2

21

-2

23

Th2,

843,

213,

091,

461,

641,

681,

611,

621,

372,

272,

992,

731,

791,

171,

49-

2,35

1,25

-U

0,78

0,8

0,71

-0,

490,

490,

460,

460,

350,

650,

720,

70,

280,

360,

33-

0,65

0,36

-87Sr

/86Sr

0,70

3762

0,70

3759

0,70

3811

-0,

7039

000,

7038

33-

-0,

7038

970,

7041

510,

7037

710,

7038

48-

--

-0,

7041

88-

-

2 σ

0,00

0007

0,00

0007

0,00

0007

-0,

0000

070,

0000

08-

-0,

0000

070,

0000

060,

0000

070,

0000

07-

--

-0,

0000

06-

-14

3 Nd/

144 N

d0,

5128

200,

5128

230,

5128

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0,51

2794

0,51

2807

--

0,51

2810

0,51

2752

0,51

2822

0,51

2808

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--

0,51

2749

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0,00

0005

0,00

0005

0,00

0005

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0000

050,

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0000

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--

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05-

-

east

ing

4446

0344

0785

4580

8353

2164

5212

9555

4782

5726

2157

3798

6342

9163

4291

5261

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4854

4048

2408

4916

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1253

4959

3563

1469

5122

55

north

ing

5982

382

5969

489

5955

635

5972

301

5983

640

5964

8835

9633

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5426

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0737

359

8535

459

8336

760

3317

160

1368

960

0524

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3427

6005

137

5897

830

5955

121

PY2a

rPY

3ar

PY6a

rPY

16PY

20PY

23PY

24PY

25PY

34a

PY34

bSA

L1SA

L4PY

39PY

40PY

42PY

43PY

44PY

36PY

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alys

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y X

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Flu

ores

cenc

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ata

in it

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s) a

t th

e D

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oren

ce,I

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,and

by

ICP-

MS

at A

ctla

bs,C

anad

a.Sr

and

Nd

isot

ope

ratio

s w

ere

anal

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at

the

Dep

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ent

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arth

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ence

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ence

usi

ng t

he a

naly

tical

pro

cedu

res

repo

rted

in A

vanz

inel

li et

al.(

2005

).PO

F =

Pam

pas

Ond

ulad

as la

va f

low

;PO

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= N

orth

ern

Bra

nch

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mpa

s O

ndul

adas

lava

flo

w;L

osC

ar =

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riza

les

lava

-fie

ld.

These results have shown that PampasOnduladas composition is slightly varia-ble and form variation trends togetherwith the Pampas Onduladas NorthernBranch rocks (Fig. 16). In particular, onesample collected at the Pampas Ondu-ladas lava front (PY34b) has compositio-nal characteristics very similar to thePampas Onduladas Northern Branchrocks (more close to the PY44 composi-tion). These evidences strongly suggestthat the Pampas Onduladas northernbranch flow was outpoured togetherwith the main Pampas Onduladas lavaflow, at least during the eruption ofPY34b lavas and similar.The compositional variations of PampasOnduladas and the northern branchrocks can be explained by a little fractio-nal crystallization of olivine + Cr-spinel,which leads to increase TiO2 togetherwith K2O and incompatible trace ele-

ments and to decrease MgO, Cr andother compatible trace element contents.Sr and Nd isotope ratios are negativelycorrelated with the degree of magmaevolution. Indeed, the highest 87Sr/86Srand lowest 143Nd/144Nd values have beenregistered in the rocks with the highestMgO and Cr contents. These correla-tions suggest the occurring of a crustalassimilation process similar to that pro-posed by Huppert and Sparks (1985) andDevey and Cox (1987) and found to haveacted in several magma systems of arcvolcanoes (e.g., Francalanci et al. 2007and reference therein). According to thisprocess, hotter mafic magmas are able toassimilate higher amounts of crustalmaterial during their ascent to the surfa-ce than cooler, more evolved liquids, thuschanging at higher extent their isotopicsignatures. The fact that higher Sr isoto-pe ratios are found in the samples PY44

and PY34b (Fig. 18), which also showpetrographic textures indicating fastercooling probably in the external part ofthe lava flows, might indicate some sortof lava-bed-rock digestion by the mostmafic lava flows during their emplace-ment.According to light and heavy REE frac-tionation (Fig. 17a), the parental magmasof Pampas Onduladas rocks are genera-ted in a garnet lherzolite by small degre-es of mantle partial melting, leaving gar-net in the residue of melting. The negati-ve anomalies of Nb and Ta in patterns offigure 17b suggest that, in spite of theirgeodynamic setting, mantle source ofPampas Onduladas parental magmas wasprobably affected by slab-derived meta-somatism.Temperature and viscosity of Pampas Ondu-ladas lavas. Due to the high mobility ofthis flow, it is of fundamental importan-


Figure 13: Examples of tumuli from POF sheet flow. a) largetumulus with the Payen Volcanic Complex in the background;b) elongated and axially collapsed tumuli; c) polygonal colum-nar jointing of the flank of a tumulus.


ce to estimate the temperature and visco-sity of these magmas. Thus, we usedmineral and whole rock chemistry to cal-culate these rheological parameters.Temperature calculations of Pampas On-duladas lavas, based on mineral/liquidgeothermometers (Roeder and Emslie1970, Roeder 1974, Nielsen and Drake1979), give values of 1130-1160 °C. Vis-cosity of Pampas Onduladas flow hasbeen calculated by chemical analyses, u-

sing algorithms of Shaw (1972) and Bo-ttinga and Weil (1972), at 1100 and 1200°C and with 0.5-2% volatile elements(assuming a phenocryst-free and voids-free magma). The calculated viscosity va-lues are low, ranging between 3-73 Pa*s,in agreement with the high mobility ofPampas Onduladas lava flows.

CONCLUSIONS

Pampas Onduladas compound lava flowbelongs to the fissural Payen VolcanicComplex and originated during a singlelong-lasting eruption, when a volume of33 km3 was emitted from the eastern tipof the Carbonilla Fault system, the hugeE-W structure which controlled thegrowth of the Complex during its entirelife. Pampas Onduladas flow outpouredfrom the summit rift of a large elongatedshield volcano, now partially coveredwith younger volcanic products. Theactive rift fissures measured about 16 kmin length and the lava moved mos-tlynorthward with minor outflows in theopposite direction. After reaching a verylarge saddle of the Tertiary basement,the lava flow diverged into two oppositepaths, one of them flowing in ESE direc-tion (Pampas Onduladas flow), for a totaldistance of 181 km from its source, andthe other following a NW direction for60 km away from the divergence point(Pampas Onduladas northern branch).Pampas Onduladas flow is the longest

known Quaternary individual lava flowon Earth, approaching in dimensions andshape some long lava flows of the terres-trial planets.From a compositional point of view,Pampas Onduladas samples are hawaiiteswith a low phenocryst content of olivineand plagioclase. Trace element patternssuggest that mantle source of PampasOnduladas parental magmas was proba-bly affected by metasomatism from thesubducted slab below the Andean conti-nental margin. The small compositionalvariations observed among the differentPampas Onduladas samples indicate oli-vine + Cr-spinel fractional crystallizationassociated with crustal assimilation. Thelatter process, in particular, has beenmore efficient in the most mafic magmasof Pampas Onduladas rocks, probablydue to its higher temperature. Petroche-mical data also confirm that PampasOnduladas northern branch flow be-longs to the same eruptive event of Pam-pas Onduladas flow. On the bases ofmineral and whole rock chemistry a tem-perature of 1130-1160 °C and a viscositybetween 3-73 Pa*s (at 1100 and 1200 °Cand with 0.5-2% volatile elements) havebeen calculated. Those physical valuescan be considered, together with a verylow cooling rate and a sustained and longlasting effusion rate, the main causes ofthe extremely long transport system ofPampas Onduladas flow.The textural analysis of Pampas Ondu-ladas flow shows that the upper crust ofthe lava flow is commonly layered andcharacterized by an upper layer of spon-gy pahoehoe (Walker 1989), rich in sphe-rical vesicles rapidly passing below to amassive basalt containing a few largerirregular scattered vesicles. The upper la-yer, with an average thickness of 40-50cm, shows a rough columnar jointingwhose roots are commonly interruptedby column-normal cracks at the contactwith the underlying layer. The latter isconstituted by a massive basalt, up to 2-3m thick, showing large, horizontally o-riented oblate and prolate vesicles, oftencoalescing into discrete planar disconti-

Figure 14: Total Alkali-Silica classification dia-gram (Le Bas et al. 1992)for rocks belonging to thePayen Volcanic Complex.The grey fields report thecompositions of theanalysed samples fromolder rocks (light greyfields) and younger rocks(dark grey field) than thePampas Onduladas com-pound lava flow and theLos Carrizales lava-field.I.B. = Irvine and Baragar(1971) line. Rocks arereported on water-freebasis.

Figure 15: Diagrams of silica versus MgO andCe for POF and PNB lava flows and Los Ca-rrizales lava-field. The compositional fields ofolder rocks (light grey fields) and youngerrocks (dark grey field) than the Pampas On-duladas compound lava flow and the Los Ca-rrizales lava-field are also reported.


nuities as well as vertical vesicle pipes.The upper layer must have been formedinitially under low strain rates, when thevesicles and the dissolved gas were ca-rried away from the vent by a fluid lavahaving probably a Newtonian rheology.The bubbles failed to escape because ofthe sharp increase of the viscosity due tothe great number of such undeformedrigid spheres (Pinkerton and Stevenson1992) enclosed in a very rapidly coolinglava. Under this thin primitive crust theliquid core moved and inflated, progres-sively losing its gas without significantcooling. The upper part of this densecore appears to be disrupted by systemsof oblique and curved joints, probablybecause of changes in directions of localmain stresses and thermal perturbationsduring a long-time extended cooling pro-cess.The morphological and textural analysisof Pampas Onduladas flow permits todistinguish four main sectors in its longi-tudinal development (Fig. 19) which canhelp the understanding of the long andcomplex emplacement history of theflow itself.The first sector is the product of sustai-ned extrusion and rapid emplacement ofbasaltic lava during a single, long-lastingeruptive event and is characterized by ve-ry large flow lobes, coalescing and overri-ding each other on a slope of about 2°-3°, often originated at outbreaks fromthe inflated front of a previous lobe.The second sector is represented by apahoehoe sheet flow covering a large tec-tonic depression extended beyond theEastern foothill of the Payen VolcanicComplex, showing a uniformly levelledsurface with a slope of 1°-0.80° and mis-sing significant outbreaks. The overallmorphology, shape and dimensions ofthis sector should suggest an origin regu-lated by a powerful, continuous and long-lasting magma emplacement, where auniform liquid interior cooled as a conti-nuous sheet-like body, as in the continen-tal flood basalts (Hon et al. 1994, Self etal. 1998). The initial pahoehoe flow sub-merged and levelled the underlying topo-

Figure 16: Diagrams of TiO2 plotted against FeO, K2O, P2O5, Cr, Nb and Sr for POF and PNBlava flows and the Los Carrizales lava-field. The names (e.g., PY34a) of some samples are alsoreported on the diagrams.

Figure 17: Patterns of Rare EarthElements (REE) and incompatibletrace elements normalised to thea) Chondrite and b) Primor-dialMantle compositions (Sun andMcDonough 1989), respectively,for POF and PNB lava flows andLos Carrizales lava-field.

147

graphic irregularities and created a smo-oth, subhorizontal lava field and createdthe essential external conditions for thedevelopment of the Pampas Onduladassheet flow. These conditions changedduring the late stage of its life, probablybecause of thermal perturbations in thepreviously uniform process of conducti-ve cooling inside the sheet flow itself,according to the model proposed by Honet al. (1994). This change is attested bythe development of a network of si-nuous, tumulus-bearing ridges, intercon-necting the flat-topped lava rises, pro-bably related to a system of capillary lavatubes of late generation. This evolution,deserving in this case further investiga-tions, determined the coexistence ofsheet flow and hummocky flow featuresin the same lava flow.This sheet flow could have been the mainmagma storage area, capable of provi-ding lava supply to the very long and na-rrow following sectors, despite possibleirregularities in lava input from vents.The third sector of Pampas Onduladasflow begins where the sheet flow entersinto a short, narrower and lower tectonicdepression and flows across a relativelyinclined step, like a rapid-forming stream.Downflow of this critical point the thirdsector of Pampas Onduladas flow movesSE with a slope of 0.25° over the alluvialdeposits of the proximal Andean pied-mont as a gently sinuous tongue partlydeviated by small Permian-Triassic out-crops or self-deviated by its own tempo-rary lateral lava fans. In this sector theliquid-cored flow tends to be concentra-ted in preferential pathways constitutedby very long longitudinal inflation ridgespreferentially located near the margins ofPampas Onduladas flow and delimitatinga large, flat central depression. The di-mensions of these ridges, up to 1 km wi-de and 20 m high, indicate a very effi-cient and probably rapid uplift, preven-ting in time the opposite effect of thecrustal thickening of the lava flow. Theoccasional presence of tumuli alignmentsalong the hinge of the ridges shows theevolution of the remaining liquid-core

pathways into lava-tube systems. Thecentral depression seems mostly to bethe negative counterpart of the lateral in-flation ridges, even though some collap-sed escarpments limiting the depressionscould be partly attributed to late defla-tion sinks.The fourth sector of Pampas Onduladasflow follows a rectilinear path over theflat distal Andean piedmont plain, in ab-sence of any topographic confinement.The initial part of this sector presents avery long and straight inflation ridge,flanked by several flow lobes repeatedlyemitted from lateral breakouts of thisrelevant structure. The ridge axis pre-sents several intermitted breakouts ofthe underlying liquid core, forming na-rrow strips of very dense and glassy lava.The internal pressure of the lava storedinside the ridge was not enough to gene-rate tumuli, which on the contrary appe-ar on top of the inflation ridges of theprevious sector.Moving downflow, where the central rid-ge disappears, this sector is formed by atight boundle of long and wide flow lo-bes, generated by the progressive brea-kouts from the fronts of the preceding

lobes.The main conceptual problem of thissector, as well as the previous one, is theorigin of the exceptional length of Pam-pas Onduladas Flow with respect to itswidth. This particular behaviour requiresa self-confinement mechanism, still to beidentified, which avoided the radial spre-ad of the flow over the plain near Saladoriver valley at La Pampa province.Further investigations, including detailedtopographic and morphometric surveysof the flow surface, precise assessmentsof the flow thickness, evaluation of rhe-ological and thermal parameters (inclu-ding the effect of vesiculation), and esti-mates of the duration of the eruptionand flow rates, are needed to explain thisbehaviour.Understanding the processes controllingPampas Onduladas Flow emplacement isof paramount importance for the com-prehension of lava emplacement, also inthe other terrestrial bodies of the solarsystem, where similar flows have beenfound, like in the case of the Tharsisrelated flows on Mars, the long flows ofthe Imbrium basin on the Moon, and theflows of Strenia Fluctus on Venus (e.g.


Figure 18: 87Sr/86Sr versus143Nd/144Nd diagram for POFand PNB lava flows. 2σerrors for Sr and Nd isotoperatios are within the size ofsymbol. PY34a, PY34b andPY44 are sample names.

Figure 19: The four main sectors in which the lava pathway of POF can be divided according toits morphology and structure as well as to its small-scale surface features and its overall topo-graphy.

Schaber 1973, Zimbelman 1998, Zimbel-man et al. 2003, Giacomini et al. 2007).For example on Mars lava rises, tumuli,lava ridges and huge squeeze-ups similarto the one showed by Pampas Onduladasflow have been detected at Daedalia pla-num (Giacomini et al. 2007), whereaschannels morphologically similar to thechannel-like depression of Pampas On-duladas flow were already observed inthe low-viscosity lava flows encounteredin the Tharsis region (Zimbelman 1998).These channels were recently ascribed toa thermal erosion process (Zimbelman etal. 2003), but our analysis of PampasOnduladas flow suggests that also syn-emplacement mechanisms cannot beruled out.

ACKNOWLEDGEMENTS

The present research was performed wi-thin a scientific agreement between Di-rección de Recursos Naturales Renova-bles of the Province of Mendoza, Ar-gentina and the Associazione ArditoDesio, Roma, Italia. Victor A. Ramos en-couraged the preparation and revised thepresentation of this paper. AlexanderMcBirney provided a first evaluation ofthe viscosity of the described lava flows.Francesco Mazzarini is thanked for hisfield visit and very profitable discussions.Anibal Soto and his family helped thefield campaigns with their logistic assis-tance and warm hospitality. Andrea Or-lando is thanked for petrochemical calcu-lations, whereas Maurizio Ulivi and Ric-cardo Avanzinelli for technical assistanceduring isotope analyses.

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Recibido: 17 de septiembre, 2007 Aceptado: 22 de noviembre, 2007


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