The Mercedario rift system in the principal Cordillera ofArgentina and Chile (328 SL)

P. Pamela Alvarez*, Victor A. Ramos

Laboratorio de TectoÂnica Andina, Departamento de Ciencias GeoloÂgicas, PabelloÂn II, Ciudad Universitaria, Facultad de Ciencias Exactas y

Naturales de la Universidad de Buenos Aires, Buenos Aires, Argentina

Abstract

Recent studies carried out in the High Andes of central-western Argentina in the provinces of San Juan and Mendoza haveestablished its stratigraphic and structural evolution. This paper presents new data on the Triassic±Early Jurassic rift system, the

depositional sequences, and a synthesis of the tectonic evolution of the region, along with a correlation with the Chileancontinental margin.The paleogeographic evolution of the Cordillera Principal at these latitudes is controlled by the development of the

Mercedario rift system. This rift began with the sedimentation of synrift deposits of the Rancho de Lata Formation, during the

Rhetian (about 190 Ma). Subsidence was driven by normal faults, locally preserved in spite of the severe tectonic inversion ofthe Andes during the Cenozoic. Di�erent authors have emphasized that an important extension dominated the transitionbetween the Triassic and Jurassic periods along the magmatic arc in the Coastal Cordillera of Chile on the western side of the

Andes. Extension was related to the bimodal magmatism that characterized the evolution of this segment (308±338 SL). Thegranitic plutonism and the associated ma®c volcanism indicate that they were controlled by extension during 220±200 Ma. The®rst subduction related granitoids at these latitudes are 170 Ma old (Bathonian).

The geometry of the Mercedario rift system may be reconstructed by the pattern of the normal faults. Rifting was followed bya thermal subsidence that expanded the original area of sedimentation and controlled the paleogeography of the Los PatillosFormation during Pliensbachian to early Callovian times. This period of cooling and thermal subsidence is correlated withmagmatic quiescence in the continental margin. The evolution of the basin closely matches the magmatic history of the Chilean

continental margin. Subduction at the continental margin began in the Bathonian, together with deposition of the upper sectionof Los Patillos Formation.Arc magmatism shifted to the Cordillera Principal during the Kimmeridgian, where it is represented by the volcanic and

volcaniclastic deposits of Tordillo Formation.Early Mesozoic evolution of the Andean system at these latitudes is, thus, reconstructed by a comparative analysis of these

two adjacent regions, driven by a common tectonic regime, but through di�erent geological processes. # 1999 Elsevier Science

Ltd. All rights reserved.

Resumen

Estudios recientes llevados a cabo en los Andes Principales del centro-oeste de Argentina, en las provincias de San Juan yMendoza han establecido la evolucio n estratigra ®ca y tecto nica de esta a rea. En este trabajo se presentan nuevos datos sobre el

sistema de rift del Tria sico±Jura sico Temprano, las secuencias deposicionales y una sõ ntesis de la evolucio n tecto nica de laregio n.La evolucio n paleogeogra ®ca de la Cordillera Principal a estas latitudes estuvo controlada por el desarrollo del sistema de rift

Mercedario. Este rift comenzo con la sedimentacio n de depo sitos de sinrift de la Formacio n Rancho de Lata, durante el Re tico

(aproximadamente 190 Ma). La subsidencia fue conducida por fallas normales, las cuales se encuentran localmente preservadas

Journal of South American Earth Sciences 12 (1999) 17±31

0895-9811/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.

PII: S0895-9811(99 )00004 -8

* Corresponding author.

E-mail addresses: pamela@gl.fcen.uba.ar (P.P. Alvarez), andes@gl.fcen.uba.ar (V.A. Ramos)

a causa de la fuerte inversio n tecto nica ocurrida en los Andes durante el Cenozoico. Diferentes autores han enfatizado que una

extensio n importante domino la transicio n entre el Tria sico y el Jura sico a lo largo del arco magma tico en la Cordillera de laCosta de Chile, en el sector occidental de los Andes. El magmatismo bimodal es asociado a un perõ odo de extensio n quecaracteriza la evolucio n de este segmento (308±338 SL). El plutonismo granõ tico y el volcanismo ma ®co asociado indica que ellos

fueron controlados por extensio n durante los 220±200 Ma. Los primeros granitoides asociados a subduccio n a estas latitudesdatan de aproximadamente 170 Ma (Bathoniano).La geometrõ a del sistema de rift Mercedario pudo ser reconstruida por los patrones de fallas normales. El perõ odo de rifting

fue seguido por subsidencia te rmica que produjo la expansio n del a rea original de sedimentacio n y controlo la paleogeografõ a de

la Formacio n Los Patillos durante el intervalo Pliensbachiano±Caloviano. Este perõ odo de enfriamiento y subsidencia termal escorrelacionado con un episodio de quietud magma tica en el margen continental chileno. La subduccio n en el margen continentalcomenzo en el Bathoniano, junto con la sedimentacio n de la parte superior de la Formacio n Los Patillos.

El arco magma tico migro en los Andes Principales durante el Kimmeridgiano, donde esta representado por depo sitosvolca nicos y volcanicla sticos de la Formacio n Tordillo.La evolucio n del sistema andino durante el Mesozoico temprano a estas latitudes es asõ reconstruida mediante el ana lisis

comparativo de estas dos regiones adyacentes, pero con diferentes procesos geolo gicos, en un re gimen tecto nico comu n. # 1999Elsevier Science Ltd. All rights reserved.

1. Introduction

The Triassic continental basins of central Argentinapresent a northwest trend, oblique to the main struc-ture of the Andes. Most of the present knowledgefrom these basins was derived from the eastern foot-hills, where the Cuyo basin is one of the main oil pro-ducers of the Mendoza region. Due to the extremetectonic inversion of the basins developed along theaxis of the Andean fold and thrust belt, their charac-teristics and tectonic settings were poorly understood.

La Ramada basin is developed between 318 and 328South latitude, close to the international border ofArgentina and Chile. It is composed of a synriftsequence of conglomerates, sandstones and shales as-sociated with tu�s, pyroclastic breccias, and bimodalvolcanic rocks. Calcareous sandstones, mudstones,wackestones and black shales characterize the sagsequences. The basin ®ll is covered by a thick series ofgypsum, red beds and marine deposits of late Jurassic±early Cretaceous age and a Cenozoic volcanic cover.

The Mesozoic half graben system together withdi�erent mechanisms of tectonic inversion that tookplace during the Andean orogeny may be unraveledthrough a structural analysis. Its oblique orientation inrelation to the main Andean stresses produced a com-plex en echelon structure, carried by the di�erentthrust sheets.

The Mesozoic deposits in the High Andes of SanJuan have been widely known since the last centurydue to the exploration accomplished by Stelzner(1873), Schiller (1907, 1912) and Lambert (1943),among others. However, the tectonic setting and thepaleogeography of Early Mesozoic sequences wereunknown. Recent studies performed in the last yearsshed some light on the understanding of the structure,biostratigraphy, and tectonic evolution of the region(Alvarez, 1996a; Ramos et al., 1996a, b). This paper

reviews the present knowledge of the tectonic evolutionand paleogeographic development of the Triassic andJurassic deposits of the northernmost sector of theCordillera Principal at the latitude of San Juan (Fig.1). At the same time, we aim to link the sedimentaryhistory of the basin to the evolution of the continentalmargin through the tectonomagmatic events recordedin Chile and Argentina.

2. Tectonic setting

The Triassic paleogeography of Argentina and Chileat these latitudes presents some important links onboth sides of the Andes, as shown by the early recon-struction of Charrier (1979) and the most recent ofUliana and Biddle (1988), Ramos and Kay (1991) andRamos (1992). It is evident that Pangea was a�ectedby an important extensional regime related to thealmost complete cessation of subduction along the per-ipheral continental margin of the supercontinent (Kay,1993). The high temperature thermal anomalies pro-duced by blanketing of the mantle by supercontinentsas proposed by Anderson (1982), later modelled byGurnis (1988), controlled the Pangea break-up, andthe consequent dispersal of the di�erent blocks. Thisprocess led to the inception of important rift systemsalong weakness zones of the supercontinent, such ashanging-wall sutures of several collisional orogensaround the world. The peripheral development of riftsystems in southern South America was controlled byancient major crustal boundaries among previouslyamalgamated terranes (Ramos and Kay, 1991; Ramoset al., 1996b) (Fig. 2). Soon after a major period ofshortening during the middle Permian, known as theSanrafaelic orogenic phase, an important interval ofquiescence and mild extension originated the Choiyoiacidic magmatism. This extensive rhyolitic plateau,

P.P. Alvarez, V.A. Ramos / Journal of South American Earth Sciences 12 (1999) 17±3118

Fig. 1. Location map of the study area with the main tectonic units (based on Ramos and Alvarez, 1996) and location of late Triassic±Jurassic

granitoids in the Coastal Range in Chile (based on Gana, 1991 and Parada et al., 1991).

P.P. Alvarez, V.A. Ramos / Journal of South American Earth Sciences 12 (1999) 17±31 19

developed along hundreds of kilometers from northernChile to southern Argentina (Kay et al., 1989), withvolcanic piles from 2±4 km thick, is only locally con-nected with rifting and active extension.

The Mercedario rift system was severely invertedduring the Andean compression in Cenozoic times,however, a detailed study of Andean structures per-mitted a palinspastic restoration of the previous exten-sional units (see Cristallini, 1996, for details).

The major rift systems were developed during lateTriassic times between 235 and 230 Ma. This riftingepisode is recorded in several basins, such as the Cuyo,Marayes, Ischigualasto and Puesto Viejo basins by theage of the synrift deposits and associated ma®c alka-line magmatism.

The available paleogeographic reconstructions ofthese rift systems, as described by Uliana and Biddle(1988), Ramos and Kay (1991) and Ramos et al.(1993) have not identi®ed any rifting episode along theHigh Andes of San Juan and Mendoza. In theMendoza region (Fig. 2) Yrigoyen (1979) and Ramos(1985 a, b), as well as Cegarra et al. (1993), point outthe absence of an Early Jurassic rift. The middle toupper Jurassic marine La Manga Formation was heredeposited on an unbroken peneplain developed on the

Choiyoi Group volcanics. The studies of Alvarez andPe rez (1993), Alvarez et al. (1995), Cristallini et al.(1995) and Cristallini (1997) demonstrated a di�erenttectonic setting in La Ramada basin developed alongthe High Andes of San Juan.

3. La Ramada basin

La Ramada basin is a composite basin that began inthe late Triassic to early Jurassic as a rift depocenterisolated from the main Andes basins of Argentina andChile and continued during early Cretaceous time as aretroarc basin. An important topographic high, whichwas present from the Triassic to the mid Jurassic, separ-ated the La Ramada basin from the Neuque n basin tothe south. The high, known since the early studies ofGroeber (1918), was located between the Las Vacas andDiamante rivers, in the northern part of the Mendozaprovince (Fig. 3) and separated two di�erent depocen-ters with similar geologic history (Alvarez, 1996a).

These basins began as rift systems in a period ofnull or very slow convergence in the Paci®c margin.The beginning of the subduction in the earlyBathonian (approx. 170 Ma), as inferred from themagmatic arc activity at these latitudes, marked theinception of thermal subsidence in a retroarc setting.The retroarc basin recorded several depositionalsequences, mainly related to eustatic sea-level changes,locally modi®ed by tectonism. Fig. 4 illustrates thestratigraphy of the region.

The early Jurassic outcrops are distributed in threebelts: the eastern, central, and western belts, as a resultof the Andean tectonics. Each belt presents distinctivestratigraphic and magmatic histories.

4. The Rancho de Lata synrift sequence

The detailed surveys performed along the easternslope of the Cordo n del Espinacito by Alvarez (1991)and Benoit (1992), showed that a sequence of conti-nental deposits developed between the rhyolites and as-sociated pyroclastics of the Choiyoi Group (Permian±upper Triassic), and the marine Los PatillosFormation (early and middle Jurassic) (Alvarez,1996c). These continental deposits consist of conglom-erates, red beds, and tu�s associated with bimodalmagmatism that are included in the Rancho de LataFormation (uppermost Triassic±early Jurassic) (Fig. 4)by Alvarez (1991) and Alvarez et al. (1995). This unitis locally separated by normal faults from the volcanicrocks of the Choiyoi Group.

Chaotic conglomerates at the base; ®ning-upwardsandstones and conglomerates with cross-bedding and®ne epiclastic beds bearing abundant micro¯ora and

Fig. 2. Inception of the La Ramada basin in the Triassic Rift System

of Argentina (modi®ed from Ramos, 1994).

P.P. Alvarez, V.A. Ramos / Journal of South American Earth Sciences 12 (1999) 17±3120

fossil plants, which corroborate its continental origin,characterize the Rancho de Lata Formation. Thefacies corresponds to ¯uvial deposits at the base whichgrade to lacustrine facies towards the top. Locally,some calcareous black shales and thin stromatolitescharacterize lacustrine facies. The sedimentarysequence is interbedded with pyroclastic products re-lated to explosive volcanism.

The pyroclastic facies consist of ignimbritic breccias,¯ow tu�s, and Plinian fall tu�s. Surge deposits havebeen identi®ed at the base. Rhyolitic domes and ®s-sural volcanism are controlled by north-south frac-tures. Olivine basalts in dikes and sills are widespreadwithin this unit.

The thickness of the Rancho de Lata Formationmeasured in the eastern belt of outcrops ranges from100 meters at RõÂ o Colorado (Fig. 6), to 150 meters atEspinacito and more than 500 meters in Arroyo

Rancho de Lata. In the Central belt, thicknesses ofthis unit are 150 meters in Arroyo de las Garzas, 140meters in Cie naga del Gaucho, and more than 300meters with unexposed base in Arroyo Las Flores. TheRancho de Lata Formation is 15 meters thick at ElPacho n, in the western belt (Fig. 5). These thicknessesdenote a pattern of thickening towards the south andthe east.

4.1. Lithofacies and depositional system

This sequence corresponds to an early ®ll of thebasin that is associated with acidic volcanism, devel-oped in an extensional regime and controlled by nor-mal faulting. Based on sedimentological characteristics,these synrift deposits are interpreted as continentaldeposits, as con®rmed by terrestrial ¯ora and pollenstudies (Ottone et al., 1992).

Fig. 3. Schematic section showing the relationship between La Ramada basin and Neuque n±Mendoza basin.

P.P. Alvarez, V.A. Ramos / Journal of South American Earth Sciences 12 (1999) 17±31 21

The epiclastic facies were deposited in proximal allu-vial fans, ¯uvial, and lacustrine environments, whilethe pyroclastic rocks are associated with an acidicexplosive volcanic event. These pyroclastic facies rep-resent ignimbritic breccias, ¯ow-tu�s, and ash-falltu�s. They are lithic in composition with the dominantrhyolitic clasts being derived from the Triassic ChoiyoiGroup. These sequences are locally chloritized, mainlydue to the thermal overprint of basaltic dikes. Thecoarse facies at the base may represent surge deposits,where pyroclastics with contorted lamination are as-sociated with a distinctive preserved breccia.

The upper half of the section belongs to anotherignimbritic episode, where several facies have beenrecognized. It starts with welded tu�s characterized byabundant vitric ®amme. A large volume of crystalsand lithics denote the explosive nature of the deposit.Toward the top, ¯attening of the ®amme decreasesand the rock is poorly sorted. The upper part of thedeposit includes vitric shards and vesicules, ®lled withzeolites, clays and glass, indicative of fumarolic ac-tivity. At this level lithics and crystals are scarce.

Within the epiclastic facies, chaotic, ®nning upwardconglomerates grade to cross laminated sandy tu�s.Clast composition is mainly volcanic, with a domi-nance of rhyolites and tu�s. This facies is interpretedas having formed in a braided ¯uvial environment.

Siltstones and light coloured clays are interbeddedwith dark levels of calcareous shales, some organicrich, bearing abundant micro¯ora. Within thissequence, stromatolitic levels are locally observed.They have a ®ne fenestral fabric, characteristic of an

unstable intertidal substratum that is cemented by cal-careous and fragmentary crusts. This facies indicatesrapid sedimentation in an environment characterizedby blue and green algae build-ups in warm water,located at the shoreline of a lake in maximum ¯ood-ing.

A ®ne-grained conglomerate made up of stromato-lite clasts covers the previously described sequence.This may be interpreted as a contraction of the lakeshore and subsequent erosion of the inter and suprati-dal deposits. Fining-upwards cycles of braided ¯uvialfacies are unconformably developed on top of thelacustrine deposits.

A new level of dark gray organic rich shales, withabundant carbonaceous plant remains, was depositedin a lacustrine anoxic environment. This sequencebears abundant palynomorphs and fossil plants, suchas Dicroidium and Cordaicarpus (Alvarez et al., 1995).

An ash-fall tu� in the upper section was depositedin a subaqueous environment. This is inferred by thepresence of fresh glass shards, ®nely oriented parallelto the bed lamination. Ostracods were found in theselevels, associated with plant debris and micro¯orasimilar to the previous level. The fact that the asheshave not been transported and the occurrence of ripplemarks in the sequence shows that deposition tookplace near a shore lake.

Siltstones with wavy bases are frequent towards thetop of the section and are inferred to represent erosionintervals associated with deposition of distal turbiditesin a lacustrine environment. These turbidites may berelated to water level falls, as deduced from synexten-sional minor normal faults.

5. The Los Patillos sag phase sequence

The continental synrift deposits are covered by themarine Los Patillos Formation (Fig. 4). This unit con-sists of calcareous sandstones, packstones, nodularmudstones and tu�s. The sequence starts with a re-sidual conglomerate immediately overlain by fossilifer-ous limestones. These bioclastic limestones inter®ngerwith characteristic near-shore facies of cross-beddedcoarse sandstones. The sequence grades upward to ®nesandstones with parallel strati®cation and massivesandy bioclastic limestones, the latter with denselypacked fossil invertebrates. The presence of large logsis indicative of a near-shore, high-energy environment.High energy levels related to storm events are presentalong this marine sequence up to the Late Aalenian.The deepening of the basin during Bajocian times per-sists up to the Early Callovian with a minor regressionduring the Early Bathonian.

The thickness of the Los Patillos Formation in theeastern belt varies from 250 meters in Arroyo Las

Fig. 4. Stratigraphic chart of the Triassic and Jurassic deposits of La

Ramada basin.

P.P. Alvarez, V.A. Ramos / Journal of South American Earth Sciences 12 (1999) 17±3122

Fig. 5. The Mercedario rift system in the High Andes of San Juan (based on Cristallini et al., 1996).

P.P. Alvarez, V.A. Ramos / Journal of South American Earth Sciences 12 (1999) 17±31 23

Vegas, to 120 meters in Paso del Espinacito to 200meters in La Ramada (Fig. 7). In the central belt ofoutcrops, thicknesses are 220 meters at Cie naga delGaucho, 115 meters in Arroyo de las Flores (top unex-posed), and 150 meters in Arroyo de las Garzas. Inthe western outcrops, the thickness is 100 meters atPacho n. These thicknesses are indicative of a more

homogenous ®ll than that of the Rancho de LataFormation and of northwest depocenter trend. Thelower Jurassic deposits are only developed in thesouthern end of the central and eastern belts, whereasthe Middle Jurassic is extensive throughout most ofthe area (see Alvarez, 1996b).

5.1. Lithofacies and depositional system

The lithology and sedimentary structures, as well asthe fossil content of the Los Patillos Formation, indi-cate a typical marine sedimentation. The great abun-dance of fossils in most of the section suggests thatliving conditions were extremely favorable. The marinesequence has a sharp contact with the residual con-glomerates and consists of ®ne to medium grained cal-careous sandstones, with sedimentary structuresindicative of a high ¯ux regime. They are followed bylithoclastic wackestones with shales and siltstones intabular beds with sharp contacts. These deposits havebivalve and ammonite bioclasts, associated withaligned silici®ed logs. The sequence is interpreted as astorm dominated marine environment, indicative of alow to transitional shore-face, as evidenced by ®negrained sandstones and shales with parallel lamination,and paschinia bioturbation on the bed tops. These ®nelevels alternate with storm beds, characterized by anerosive base covered by shell deposits, that gradeupwards to medium to ®ne sands derived from nearshore sectors. During these intervals of high energy,the benthic fauna is remobilized from the substratumand chaotically redeposited. Based on the preservationof the di�erent kinds of shells, a minimum transport isassumed, with the fauna being partially autochtho-nous, as indicated by the common occurrence ofarticulated bivalves. These facies of sandstones andwackestones correspond to proximal silicoclastic tem-pestites, typical of middle shorefaces. The thick, ®ne-to medium-grained sandstones are interpreted as amal-gamate surfaces formed by storm events, characteristicof middle to low shoreface environments. The shalesand siltstones are interbedded with ®ne-grained sand-stones and are interpreted as distal tempestites ininner-shelf environments.

Based on the distribution of these sedimentologicfeatures, a deepening of the basin is assumed fromPliensbachian to Toarcian times, as the youngersequences were deposited in deeper environments,dominated by distal tempestites. High energy levels arefrequent along the basin border until the Aalenian±Bajocian boundary.

The Bajocian deposits at these latitudes vary fromapproximately 30 to 60 m thick, depending on theirposition within the basin. Localities in the central partof the basin, such as Arroyo de las Vegas, Arroyo dela Cerrada and in the Cordo n de la Ramada, are

Fig. 6. Stratigraphic section of Rancho de Lata Formation at RõÂ o

Colorado locality (Late Triassic±Early Jurassic), based on Alvarez

(1996a).

P.P. Alvarez, V.A. Ramos / Journal of South American Earth Sciences 12 (1999) 17±3124

characterized by o�shore sedimentation, composed ofblack mudstones and shales with high organic content.Levels with carbonate nodules are widely distributed inthis sequence in the eastern belt of exposures, showinga generalized episode of chemical precipitation. Inmost of these localities, condensed sections representhigh sea level stands. Coeval deposits at the basin mar-gin consist of medium to ®ne grained sandstones withlow angle cross-bedding, hummocky, and parallel lami-nations. These facies are interpreted as middle to highshoreface deposits dominated by storm events.

A generalized shallowing of the basin is observed inthe late early and late Bajocian, represented by a low-stand system tract, which persisted until theBathonian. The corresponding sequences are character-ized by the alternating of carbonatic ®ne-grained sand-stones and tu�aceous sandstones, which have tabularbedding with parallel lamination, inter®ngered withlenssoid beds of ®ne to middle conglomerates. This setof lithofacies is developed in a marginal marine near-

shore environment. Shales and mudstones in the in-terior of the basin represent the basinal facies.

The basal lower Callovian sequences are character-ized by conglomerates, which show evidence of remo-bilization and resedimentation of the older facies.These conglomerates are followed by a ¯ooding epi-sode represented by mudstones and wackestones as-sociated with ooidal packstones levels. These levels areinterpreted as a middle to distal carbonate ramp en-vironment.

The uppermost section of Los Patillos Formation, inthe eastern belt of exposures, is characterized by redpackstones of oysters, no thicker than 10 m thick, as-sociated with an open carbonatic ramp environment.

A carbonatic assemblage of lithofacies was recog-nized at the central belt of exposures for the early tomiddle? Callovian. Packstones, oolithic-skeletal grain-stones and equinoid and oysters wackestones charac-terize these facies.

The systematics, along with an analysis of equinoid

Fig. 7. Stratigraphic sections of Los Patillos Formation (Early±Middle Jurassic) based on Alvarez (1996a).

P.P. Alvarez, V.A. Ramos / Journal of South American Earth Sciences 12 (1999) 17±31 25

morphology of the lithofacies and associated fauna, in-dicate warm shallow waters in a high energy platformenvironment associated with frequent storm events.This interpretation is supported by the presence ofwackestones and oolithic packstones.

6. Tectosedimentary history

The contact between Los Patillos and Rancho deLata Formations in most of the sections is conform-able, but in some areas such as the Brazo Sur ofArroyo de las Flores and the Cerrada sections, a clear

angular unconformity between these units is seen(Figs. 8 and 9). The Arroyo de las Flores area pre-viously studied by KuÈ hn (1914) and Stipanicic (1966),is a well exposed section that characterizes the tectonicsetting of the Jurassic sequences. The initial ®ll isexposed in a series of half-grabens, controlled by west-dipping normal faults, ®lled with the Rancho de LataFormation deposits. The highly variable thickness ofthis unit indicates synrift facies that were depositedduring the active faulting of the basin. Above thisbreak-up unconformity, a sequence of shales and sand-stones with tabular development overstep the di�erentgrabens, without being a�ected by the normal faulting.

Fig. 8. The synrift deposits and the sag ®ll along the arroyo Flores. View from the Ramada massif to the south (Ramos and Alvarez, 1996).

Fig. 9. Structural cross-section of Figure 8, with conceptual interpretation beneath the level of exposures.

P.P. Alvarez, V.A. Ramos / Journal of South American Earth Sciences 12 (1999) 17±3126

Fossil content of this sequence permits correlation ofthe Arroyo de las Flores section with the type localityof Los Patillos Formation (see Alvarez et al., 1995).The La Manga Formation limestones cover thissequence.

Two cycles have been identi®ed within the thermalsag phase sequences. The lower one corresponds to theearly sag, where the marine sequences of Los PatillosFormation developed within the initial early Jurassicdepocenters. The upper one corresponds to a late sagand is represented by the middle Jurassic sequences,which have a more generalized subsidence and coverthe entire basin. The relationship between both sagsequences is characterized by a coastal onlap. Bothunits are interpreted as sag facies related to the ther-mal subsidence of the rift system. They have a wideextent and overlap not only the synrift sequences, butalso the prerift sequences of Choiyoi Group in someareas. Locally, as in Arroyo de las Flores, the Jurassicsequence ends with gypsum deposits of the AuquilcoFormation (late Oxfordian±Kimmeridgian) and conti-nental red beds of Tordillo Formation. These conti-nental deposits are related to a new sedimentary cycleof Tithonian±early Neocomian age.

Based on the above description, there is no doubtthat the Rancho de Lata Formation coincides with theperiod of active normal faulting while the Los Patillosand La Manga Formations are associated with the

thermal postrift subsidence of the Triassic±EarlyJurassic basin.

7. The Mercedario rift system

The Mercedario rift system is the ®rst to be ident-i®ed in the Cordillera Principal of the High Andes ofSan Juan and Mendoza. The rift consists of anelongated northwesterly trending trough, with an ap-proximate width of 30 km, and an en echelon easternborder (Fig. 5).

A weakness zone within the basement of theChilenia terrane is assumed, as in most other cases, tocontrol the rift geometry. This zone may correspondto a suture, if Chilenia is interpreted as a compositeterrane (Ramos et al., 1996c). Another feasible alterna-tive is to assume a ®rst order synthetic fault zone re-lated to an intense deformation within Chilenia,developed during collision with the Precordillera ter-rane (Fig. 2). This fault zone should dip to the eastand be synthetic to the suture zone between theChilenia and Precordillera terranes.

A conceptual model for the Mercedario rift is pre-sented in Fig. 10. It corresponds to an asymmetricsimple shear rift as proposed by Wernicke (1985) andWernicke and Tilke (1990), among others. This systembegan to develop during the early Triassic (RodrõÂ guez

Fig. 10. Schematic cross-section of the Mercedario rift system (see Fig. 2 for location).

P.P. Alvarez, V.A. Ramos / Journal of South American Earth Sciences 12 (1999) 17±31 27

Ferna ndez et al., 1995; Ramos, 1996; Cristallini, 1996),

but maximum extension was obtained during the late

Triassic and continued up to the early Jurassic, when

active faulting ceased.

The rift axis has a northwesterly trend, recon-

structed on the basis of the early Jurassic depocenters.

These centers are partially isolated by intrabasinal

highs. The eastern border of the basin is bounded by

two sets of normal faults: north±northwesterly trend-

ing faults dipping to the west and east±west faults.

This last set, which coincides with the east±west re-

gional lineaments that segmented the Cordilleras of

Santa Cruz, La Ramada±Mercedario, Espinacito, and

La Cerrada, has been interpreted as an en echelon

transfer system (Cristallini et al., 1995).

The western margin of the basin is on the Chilean

side of the Cordillera Principal and was not examined

in the ®eld. The boundaries have been identi®ed from

the geologic map of Illapel (Rivano and Sepu lveda,

1991). The Estero del Cenizo graben is interpreted as

the boundary of the west-dipping faults. West of this

graben, the normal faults dip to the east, as seen in

the Pocuro, Llimpo, and Maquegua faults recognized

by Rivano and Sepu lveda (1991).

Based on the Thematic Mapper images, several re-

gional lineaments have also been recognized, comple-

menting the outline of the rift geometry.

Therefore, di�erent direct and indirect criteria were

used to identify the rift system developed north and

south of Cerro Mercedario, at the southern sector of

Cordillera Principal of San Juan. The structural frame-

work was built based on the following evidence:

1. Direct observation of normal faulting and synrift

deposits, such as in the Espinacito Pass (Alvarez et

al., 1995), Arroyo Flores (Ramos et al., 1993) and

Cie naga del Gaucho (Ragona, 1993; Blengino,

1994).

2. Indirect interpretation of basement control of the

Andean structures. For example, at Rõ o Volca n and

Los Teatinos, younger deposits cover the early

Mesozoic structures. However, the structural in-

terpretation indicates that the thrust plate geometry

was modi®ed by a series of lateral and oblique

ramps that produced a series of interferences

between the eastward transport and the basement

geometry (see for discussion Cristallini et al., 1995).

Based on this evidence, a set of northwesterly trending

half-grabens can be identi®ed. This system is similar to

the one proposed by Charrier (1979), and explains the

en echelon pattern of the early to middle Jurassic ex-

posures, oblique to the main north-south Andean

trend (Ramos et al., 1993).

8. Rift architecture

The complexity of continental rift systems is stronglyin¯uenced by the preexisting fabric. A set of isolatedsegments is developed at an early stage, connected byoblique or transverse jumps that transfer the fault dis-placements among segments. These conspicuous jumpshave been identi®ed in the Mercedario rift system asorthogonal structures, parallel to the extension direc-tion. The strain concentrates at the surface along theorthogonal structures, outlining the transfer faulttraces with an en echelon pattern, similar to thatdescribed by Morley (1995). The transfer zones ident-i®ed in the region are synthetic (sensu Morley, 1995),as they separate faults of similar dips. Fig. 5 illustrateseast±west transfer zones, which join the di�erent riftsegments. The surface pattern of the di�erent blocks iscomplemented by zigzag faults, transverse strike-slipfaults and transfer zones (Alvarez, 1997), although thetectonic inversion may have a�ected the original struc-tures.

8.1. Rift polarity

The polarity of the master detachment in theMercedario rift system is based on stratigraphic andstructural surface observations. The maximum exten-sional throws are found in the eastern margin of therift, where the largest synrift thicknesses have beenmeasured, corresponding to the c zone of Wernicke(1985).

The isopachs of the sag-phase deposits of the LosPatillos Formation follow a similar trend, with a gentlepinch-out toward the west. The thermal subsidence hasbeen larger in the eastern sector, indicating an east-dipping master detachment. This implies a larger sub-crustal attenuation in this sector with a higher heat¯ux. As partial evidence of this assumption, a bimodalvolcanic activity associated with the initial stages hasbeen identi®ed only in the eastern sector (Alvarez,1997). The volcanic activity is concentrated in the cand d zones of the upper plate, according toWernicke's model, where the maximum heat ¯uxoccurs.

All the observations indicate an east-dipping po-larity of the master detachment, similar to otherTriassic rifts found in Central Argentina (Uliana andBiddle, 1988; Ramos, 1992).

9. The magmatism at the continental margin

Di�erent authors have emphasized that an import-ant extension dominated the transition between theTriassic and Jurassic periods along the magmatic arc

P.P. Alvarez, V.A. Ramos / Journal of South American Earth Sciences 12 (1999) 17±3128

in the 308±338 segment of the Coastal Cordillera ofChile.

According to Parada et al. (1991) and Gana (1991),who studied the geochemistry and the petrology of thegranitic plutonism and the associated ma®c volcanism,the plutonic history of Central Chile between 308 and338 S shows two distinctive stages for the Mesozoic(Fig. 1).

Leucogranites and ma®c dikes that result from par-tial melting of a quartz-feldspathic source characterizethe ®rst stage, probably Upper Paleozoic plutonicrocks (see Parada et al. 1991). This upper Triassic±lower Jurassic bimodal plutonic suite occurs along theCordillera de la Costa in northern Chile. It consists ofleucogranites of the Tranquilla±Millahue Unit, andmonzodiorites to gabbros of the Talinay Unit (Gana,1991). The Sr87/86 ratio is high, between 0.7048±0.7134(Parada 1990). The age of these rocks can be con-strained between 220 and 191 Ma (Parada et al.,1991).

Gabbros, diorites, tonalites and minor granites ofmiddle to upper Jurassic age, represent the secondstage. This hornblende-bearing suite has lower Sr87/86

ratios (0.7033±0.7056) and geochemical a�nities oftypical subduction related magmatism. The beginningof this second stage can be placed at up to 171 Ma(late Bajocian) (Parada et al., 1991).

Magmatism in the Coastal Cordillera was thereforecontrolled by extension during 220±200 Ma (Parada etal. 1997) while the ®rst orogenic calcalkaline granitoidsand associated subduction are only 170 Ma old(Bathonian).

The paleogeographic evolution of the PrincipalCordillera closely matches the magmatic history of theChilean continental margin. Therefore, the beginningof subsidence on the taphrogenic basin of Rancho deLata Formation was approximately dated as Rhetian(about 190 Ma). The subsidence of this tectonic basinwas driven by normal faults, which are still preservedin spite of the severe tectonic inversion of the Andesduring the Cenozoic, along the international border ofArgentina and Chile.

10. Concluding remarks

The analysis of the tectonic history along the conti-nental margin of Chile, when combined with the maintectonic events recorded in the High Andes of SanJuan shows a striking correlation.

Important extension is recorded in Triassic timesalong the Coastal Cordillera and in the PrincipalCordillera after the amalgamation of Pangea. Thebimodal sequence of granitoids and ma®c dikes alongthe continental margin is coeval with active faulting inthe Mercedario rift system (Fig. 11). The synriftsequences of Rancho de Lata Formation were depos-ited at the same time as the emplacement of theChilean Talinay and Tranquilla±Millahue synexten-sional magmatic units. The bimodal volcanism at thetop of Rancho de Lata coincides with the end of activefaulting and the emplacement of the extensional mag-matic suites in Chile (Ramos and Alvarez, 1996). Thebasement fabric controlled the rift architecture. As

Fig. 11. Main geologic events along both sides of the Andes (308±338 S).

P.P. Alvarez, V.A. Ramos / Journal of South American Earth Sciences 12 (1999) 17±31 29

result of that polarity, the master detachment pro-duced maximum throws in the eastern sector, indicat-ing a regional east dipping detachment.

There is a time interval when no magmatism is regis-tered on the Chilean side at these latitudes. The maintectonic activity is the thermal subsidence of LosPatillos Formation, driven by the lithospheric coolingof the Mercedario rift system. The thermal sag subsi-dence started along the axis of the rift duringPliensbachian times, continued along the Toarcian inthe depocenter of the rift, to ®nally cover nearly thewhole area during Aalenian and Early Bajocian times.Onlap relationships are evident between the deposi-tional center and the peripheral parts of the basin,showing maximum regional subsidence during thisperiod. The age of the marine transgression was con-strained by the di�erent ammonite assemblages.

Based on the available ages of subduction-relatedmagmatism along the Chilean border, active subduc-tion started 170 my ago, at the beginning of theBathonian and has continued since then. This stage ischaracterized by di�erent sedimentary cycles, probablyrelated to eustatic sea-level changes, partially con-trolled by the local tectonics of the CordilleraPrincipal. The Middle Jurassic carbonate platform ofLa Manga Formation was replaced by the low-standfacies of Auquilco gypsum and by coarse clastic conti-nental sequences by the end of the Jurassic. This factclearly indicates the progression of the volcanic frontto the foreland, as shown by the volcaniclastic prove-nance of the Tordillo red beds.

A comprehensive tectonic evolution of the continen-tal margin was reconstructed based on two di�erentdata sets representing the sedimentary history of theMercedario rift system in Argentina and the coevalmagmatic episodes at the Chilean continental margin.

Based on the data presented here, the previouslyunknown Mercedario rift system is now incorporatedinto the tectonic framework and paleogeography ofthe Late Triassic±Early Jurassic of the High Andes ofSan Juan.

Acknowledgements

The authors especially acknowledge critical reviewsby Dr Estanislao Godoy, Dr M.B. Aguirre-Urreta andDr Suzanne Kay on an early version of this paper.Thanks are also extended to Dr Reinaldo Charrier andDr Toma s Zapata for their comments and ideas. Thework was carried out with partial funding ofCONICET (PIP 4162) and UBACYT (TW87).

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