Field trip guide: Evolution of the Pampean flat-slab region over the shallowly subducting Nazca plate
Published:January 01, 2008
- PDF LinkChapter PDF
Víctor A. Ramos, 2008. "Field trip guide: Evolution of the Pampean flat-slab region over the shallowly subducting Nazca plate", Field Trip Guides to the Backbone of the Americas in the Southern and Central Andes: Ridge Collision, Shallow Subduction, and Plateau Uplift, Suzanne Mahlburg Kay, Víctor A. Ramos
Download citation file:
This field guide provides an opportunity to examine the central Andes between 31° and 32°S latitude in a segment characterized by flat-slab subduction. The field trip road was chosen to observe the westernmost contact between the basement uplift of Sierras Pampeanas and Precordillera, the early Paleozoic stratigraphy, and the Andean structure of the Precordillera, as well as a complete section of the Frontal and Principal Cordilleras in Argentina and Chile. The trip ends in the Coastal Cordillera along the Pacific margin. This road log discusses a complete early and late Paleozoic history of the central Andes with their typical Famatinian and Gondwanan orogenic rocks and the accretionary evolution of the Pacific margin at these latitudes. Superimposed on this framework, the structure of the Andes is viewed through the examination of the Precordillera and the Aconcagua fold-and-thrust belts, together with the observation of the Andean volcanic history, will allow reconstructing the shallowing of the subduction zone through the Neogene and the final formation of the Pampean flat-slab.
This field trip provides the opportunity to examine the tectonic evolution of the central Andes in one of the most classic sections. The aim is to show the key localities where field data have been obtained and different regional relationships and tectonic models have been developed. The short duration and long distance to be covered does not allow a comprehensive review of the geology of the central Andes.
The main objective of the trip is to view one of the most complete traverses of the Andes, where a noncollisional orogenic belt reaches elevations near 7 km (the highest mountains of the Western Hemisphere). These mountains are in an area with no present volcanic activity, and therefore Late Cenozoic shortening is directly responsible for the present uplift and convergence rates. The route shows the different structural styles of the Precordillera, the Cordilleras Frontal and Principal along the Argentine slope, and the Cordilleras Principal and La Costa on the Chilean side, as indicated on the field trip road map (Fig. 1). The field trip ends in the Pacific coast.
Brief Review of the History of Geologic Exploration and Research
The geologic understanding of the central Andes started with the pioneering explorations of Charles Darwin in 1835, who was the first to describe the marine Mesozoic deposits deformed by faults (Fig. 2) along the present road that crosses the High Andes. The field trip provides the opportunity to examine the profile described by Darwin and the mountain shelters where Darwin stayed during his mountain crossing.
Several German naturalists were sent by the Academia Nacional de Ciencias and the Museo de La Plata to geologically explore the High Andes of San Juan and Mendoza. The early observations of German Burmeister in 1857–1858, the structure outlined by Stelzner (1873, 1885), and the descriptions of Wehrli and Burckhardt in 1898 presented the concept that the Andes were a relatively simple mountain chain without the thrusts and overthrusts known in other mountain chains at that time.
Later, in 1906 and 1907, Walter Schiller, a young geologist and mountain climber working in the Argentine Geological Survey conducted the first reconnaissance of the region (Fig. 3). As a result of his work, many structural complexities and important thrusting were recognized (Schiller, 1907, 1912).
At the same time, the Precordillera was also explored, and its stratigraphy, structural geology, and mineral resources were described, making this area one of the best known regions of the time (Stelzner, 1885; Bodenbender, 1902; Stappenbeck, 1910; Keidel, 1921; Bracaccini, 1946; etc.). Over the past several decades, many studies have provided a large volume of information on different aspects of the sedimentology, structure, and geologic evolution of this part of the central Andes (see Caminos, 2000, for a review).
Major Geological Provinces
This segment of the central Andes has been divided into morphostructural units or geological provinces based on structural styles, geologic evolution, and morphological expression (Fig. 4).
This geological province, located in central Argentina, is characterized by a series of crystalline basement blocks of Precambrian–Early Paleozoic age, which were uplifted and tilted during Tertiary Andean compression (González Bonorino, 1950a) in association with an episode of shallow subduction (Jordan et al., 1983a, 1983b; Kay et al., 1987; Ramos et al., 2002). The resulting structures closely resemble those of the Laramide region of the United States (Jordan and Allmendinger, 1986).
The basement is composed of metamorphic and igneous rocks which correspond to two distinct orogenic cycles. The oldest Brasiliano or Pampean cycle is preserved along the eastern Sierras Pampeanas, and the metamorphic facies and igneous rocks define a north-south–trending belt of Late Proterozoic–Early Cambrian age (600–520 Ma) (Ramos, 1988a). The younger cycle is characterized by outcrops, along the western Sierras Pampeanas, which define the Famatinian orogen, an Early Paleozoic magmatic belt that reached its magmatic climax between 490 and 460 Ma (see reviews in Pankhurst and Rapela, 1998; Ramos, 2004).
This metamorphic basement was partially covered by a series of continental deposits consisting of the Late Paleozoic Paganzo Group and Tertiary synorogenic deposits in alluvial and fluvial facies related to the uplift of the Sierras Pampeanas. Locally, some Triassic and Cretaceous continental sequences were deposited along rift basins developed in the eastern and western margins of the Sierras Pampeanas.
The Precordillera is an Andean fold-and-thrust belt sequence with a typical thin-skinned structure that has developed in an early Paleozoic carbonate platform (Baldis and Chebli, 1969; see overview in Astini and Thomas, 1999). The western edge of the Precordillera coincides with a longitudinal depression known as the Iglesia-Calingasta-Uspallata valley. This tectonic trough is similar to the Canadian Rocky Mountain trench. In both regions, the modern morphology is controlled by the old Paleozoic continental margin (Price, 1981; Baldis et al., 1982).
The Early Paleozoic history is represented by a carbonate platform of Early Cambrian to Middle Ordovician age. A fine biostratigraphic zonation has been defined in these highly fossiliferous deposits (Bordonaro, 1980; Baldis et al., 1982). Clastic marine Middle to Upper Ordovician rocks cover the platform in the eastern and central sectors, while slope and oceanic facies occur to the west. This Early Paleozoic continental margin existed until the Middle to Late Devonian. The marine sequences are mainly turbiditic facies, typical of flysch deposits (González Bonorino, 1975).
A major deformation, known as the Chanic, affected these Early Paleozoic rocks during the Middle to Late Devonian, developing a series of isolated ranges known as the “Protoprecordillera” (Amos and Rolleri, 1965). The previous slope facies were covered by continental Early Carboniferous alluvial deposits in the western Precordillera.
Late Paleozoic rocks are deposited in a foreland basin, mainly littoral marine facies in the western Precordillera and continental fluvial to alluvial deposits in the central and eastern Precordillera. Continental redbeds of the Paganzo Group rocks overlap the eastern Precordillera. In well-known localities such as Barreal and Rinconada among others, excellent outcrops of the Gondwana glacial deposits are preserved in both marine and continental facies (Keidel, 1921; Du Toit, 1927; López Gamundi and Amos, 1986).
Synorogenic Tertiary deposits permit the reconstruction of the Precordillera thrust sequence, which began ca. 18 Ma in the northwestern side and continues until the present day on the eastern side (Jordan et al., 1997).
The Cordillera Frontal is composed of units that formed during the Gondwanides orogeny in the Late Paleozoic to early Mesozoic. These units result from Andean-type subduction, followed by generalized extension. Most of the rocks of this province are Late Paleozoic–Triassic andesitic to silicic magmatic rocks of the Choiyoi Group (Caminos, 1979). During the Andean deformation, the Cordillera Frontal behaved as a rigid block, as shown by the presence of thick-skinned thrusts (Polanski, 1957, 1970).
Scattered exposures of the pre–Late Paleozoic basement of this province occur in Argentina and Chile. They consist of low- to medium-grade metamorphic rocks of latest Proterozoic to Early Cambrian age. The Early Paleozoic is represented by isolated outcrops of Silurian-Devonian marine limestones which are covered by widespread Carboniferous–Early Permian turbidites. This Late Paleozoic facies contrasts with the littoral facies of Precordillera to the east (Caminos, 1979).
Volcanic activity started in the Early to Middle Carboniferous with subduction related andesites, dacites, and rhyolites. A subsequent period of generalized extension from Middle Permian up Early Triassic times resulted in the thick pile of Choiyoi rhyolites and associated granites (Kay et al., 1989; Llambías and Sato, 1990). These volcanic rocks, which reach thickness of up to 2–4 km in the Cordillera del Tigre, unconformably overlie the older rocks. Deformation of the Carboniferous–Early Permian rocks occurred in the middle Permian San Rafael orogenic phase (Ramos, 1988a; Rapalini, 1989).
The boundary between the Cordillera Frontal and the Precordillera was the locus of Triassic rifting that is associated with up to 2 km of synrift deposits, scattered alkaline basalts and associated sag facies. The rift developed in the hanging wall of the suture between Cuyania and Chilenia terranes (Ramos and Kay, 1991). The early sedimentation of the rift was coeval with the Choiyoi volcanism.
The Cordillera Principal, or Main Andes, was the locus of the Andean orogeny during latest Mesozoic and Cenozoic times. Jurassic and Cretaceous marine deposits were deformed in different styles depending on the extent of participation of the basement in the deformation. In the northern sector, a thick-skinned tectonic style is described by Moscoso and Mpodozis (1988), whereas to the south thin-skinned structures such as in the Aconcagua fold-and-thrust belt developed (Yrigoyen, 1976, 1979; Ramos, 1988b; Ramos et al., 1996a, 1996b).
A thick sequence of marine Mesozoic deposits unconformably overlies the Carboniferous flysch and the Choiyoi volcanics of the Cordillera Frontal. Several sedimentary cycles are recognized from the Early Jurassic to the Early Cretaceous (Groeber, 1946; Legarreta and Gulisano, 1989). These cycles begin with black shales, sandstones, and limestones and terminate with thick gypsum levels and continental red beds. Abundant ammonites permitted a biostratigraphic zonation of these deposits (Riccardi, 1984).
Along the continental divide these sedimentary sequences interfinger with volcanic and pyroclastic rocks of Late Jurassic– Early Cretaceous age. The volcanic pile, which can be up to 6 km thick in the Chilean side, has a burial metamorphism typical of that developed in a high thermal gradient during active subsidence (Levi et al., 1982). These volcanic sequences occur along a western inner arc developed between the Cordilleras de la Costa and Principal, and an eastern outer arc along the present international border. An intra-arc basin between the two arcs is filled with shallow marine and continental deposits (Charrier, 1973; Charrier et al., 2005; Rivano et al., 1985; Ramos, 1985b).
Most of the early-middle Mesozoic was dominated by an extensional regime closely linked with the early stages of the opening of the South Atlantic (Uliana et al., 1989). The beginning of the drift phase in the Atlantic during the middle Cretaceous changed the tectonic regime to the present Andean compressional stage. A series of volcanic arcs shifted from the Cordillera de la Costa in the Jurassic to the Cordillera Principal in the Late Tertiary (Ramos, 1988b). The volcanic and volcaniclastic rocks interbedded with alluvial-fan facies.
Glacial deposits from four different glaciations are widespread in the main valleys, representing alpine type glaciations during Pliocene and Quaternary times.
Cordillera de la Costa
Along the present continental margin, Late Paleozoic metamorphic rocks are preserved that represent pieces of an accretionary prism developed in the Late Paleozoic (Hervé, 1988). Emplaced in this metamorphic basement are a series of magmatic belts of Jurassic and Cretaceous age. Most of this region is suspected to have significant latitudinal motion (Forsythe et al., 1986; Mpodozis and Ramos, 1990).
The generalized stratigraphy of the Cordillera de la Costa is as follows. On the eastern flank of the Cordillera de la Costa, Mesozoic marine sequences that developed west of the Mesozoic volcanic rocks are preserved. These marine deposits commonly interfinger with volcanic rocks that can be associated with Cu manto-type deposits. Isolated patches of accretionary prism deposits, which are principally Paleozoic to early Mesozoic in age, crop out along the Pacific coast.
Modern Plate Tectonic Setting
This segment of the central Andes between 28° and 33°S has a distinctive plate tectonic setting. The present convergence rate between the subducted Nazca plate and the South American plate averages ~9 cm per year. Earthquake locations delineate a Benioff zone that is gently dipping to the east defining a shallow subduction zone (see Fig. 5) (Cahill and Isacks, 1992; Pardo et al., 2002). This flat subduction segment is characterized by an almost flat section at ~100 km dept that is flanked to the north and south by steeper segments that dip ~30° eastward.
A corresponding tectonic segmentation exists in the plate above the Benioff zone. The most obvious and consistent correlation is between Quaternary volcanism and the dip of the subducted slab. Quaternary volcanism is absent in the subhorizontal segment.
The development of the Sierras Pampeanas geological province is controlled by flat subduction. Present tectonic shortening is principally concentrated along a narrow belt between this province and the Precordillera. Global positioning system (GPS) displacements analyzed by Brooks et al. (2003) show a strong gradient between both provinces. Intraplate earthquake concentrations have been found in the basement of the eastern Precordillera and the western Sierras Pampeanas, in close coincidence with the superficial neotectonic activity. Focal mechanisms indicate east-west contraction with null to minor strike-slip displacements (Chinn and Isacks, 1983; Pardo et al., 2002).
The origin of this flat subduction segment has been attributed to the approach and collision of aseismic ridges to the Pacific continental margin (Pilger, 1981; Yañez et al., 2001), to changes in the age of the subducted oceanic crust (Wortel, 1984), and to the differential shortening of a previously weakened hot continental crust (Isacks, 1988; Cahill and Isacks, 1992). See recent review in Ramos et al. (2002).
The existence of oceanic rocks separating the Cordillera Frontal from the Precordillera has attracted the attention of geologists since the early work of Borrello (1969). These oceanic rocks have been interpreted as indicating a suture between different continental terranes (Ramos et al., 1984, 1986). Consequently, several other sutures have been identified (Ramos, 1984, 1988a, 1988b; Mpodozis and Ramos, 1990; Astini et al., 1995, 1996, 1999). A map of the suggested terranes in the region after Ramos (1988a) is shown in Figure 6.
The first attempt to explain the presence of olenellid trilobites in South America was made by Ross (1975), who explained them by larval transfer by oceanic currents. The location of this fauna with olenellid trilobites outside of Laurentia was intriguing, mainly because this fauna was only known in the ancestral North American craton and in the northwestern British Isles.
Several years later, the striking coincidence in the subsidence curves of the Appalachians carbonates and the Pre cordillera carbonate platform, induced Bond et al. (1984) to suggest that both regions were conjugate margins and shared a common rift drift transition. The shared carbonate platformal history and faunal provinciality between the Precordillera and the Laurentian margin of the Appalachians led Ramos et al. (1986) to propose that the Precordillera was a far-traveled terrane derived from the northern Appalachians. This proposal was refined by Mpodozis and Ramos (1990), and Astini et al. (1995, 1996), within the tectonic framework of the early Paleozoic basement of the Andes.
Two different models have been proposed to explain the accretion of the Precordillera to the protomargin of Gondwana. One of the models proposed an independent microcontinent or microplate, detached from Laurentia during Early Cambrian time, which collided against Gondwana during Middle to Late Ordovician time (Ramos et al., 1986; Benedetto and Astini, 1993; Astini et al., 1995, 1996). The other model proposed a continent to continent collision between Laurentia and Gondwana during Early to Middle Ordovician time from 487 to 467 Ma. Subsequently, during the Late Ordovician, the separation of the two continents left behind the Precordillera terrane on the Gondwanan side, with the opening of an ocean on the western side of Precordillera (Dalla Salda et al., 1992a, 1992b; Dalziel, 1992, 1993, 1997; Dalziel et al., 1994, 1996). Both proposals required an active early Paleozoic margin in the Sierras Pampeanas, and explained the magmatism and the Ordovician deformation known as the Ocloyic event as the result of a collisional orogeny (Ramos et al., 1986; Dalla Salda et al., 1992a, 1992b).
The microcontinent hypothesis required a second early Paleozoic accretion to close the ocean that bounded the western Precordillera. The accretion of the Chilenia microcontinent produced a second foreland basin and a shifting of the magmatic activity to the Pacific margin (Ramos et al., 1984). This collision occurred either in Late Devonian time (Ramos et al., 1986) or as proposed by Astini (1996) in Early Devonian time. Sedimentological studies of these foreland basins and geochronologic data of peak metamorphism and associated deformation suggest an Early Devonian age for the beginning of the accretion of Chilenia. The challenging pre-Pangea Southwest United States– East Antarctic (SWEAT) reconstruction of the late Proterozoic continents proposed by Moores (1991), complemented by Dalziel's 1991 Laurentian end run, may have provided a mechanism for the transfer of the Precordillera. See recent reviews by Thomas and Astini (1999, 2003) and Ramos (2004).
The basement of the central and western Sierras Pampeanas constitute an independent terrane that was accreted to the Río de La Plata Craton during the Late Proterozoic–Early Cambrian times (Ramos, 1988a; Kraemer et al., 1995). A subduction zone dipping toward the Río de la Plata craton was responsible for the development of a magmatic arc, which is represented by a series of gabbros, tonalites, and granitoids in the eastern Sierras Pampeanas (Lira et al., 1997). These rocks have ages ranging between 700 and 525 Ma (Rapela et al., 1998). Final amalgamation and an uplift of 15 km (Ramos, 1988b) occurred during the Early Cambrian (Rapela et al., 1998). The magmatic rocks exposed along the western border have petrological and geochemical characteristics typical of an active continental margin (Rapela et al., 1992; Pankhurst et al., 1998; Quenardelle and Ramos, 1999). The deformational history and the age of the magmatism are consistent with the western Sierras Pampeanas representing an active margin from the Late Cambrian until the Middle to Late Ordovician (Ramos, 1989, 1993; Pankhurst et al., 1998). The western border of the Sierras Pampeanas is also characterized by a synthetic fold-and-thrust belt with west vergence (Dalla Salda, 1987).
Cuyania Composite Terrane
Petrologic studies and dating performed in the basement of Precordillera and adjacent terranes (Kay et al., 1996; Sato et al., 2000) have demonstrated that Precordillera was part of a composite terrane of Grenvillian age. This terrane, named Cuyania, was accreted as a whole to the Gondwana margin (Ramos et al., 1996b; Thomas and Astini 1996, 1999, 2003; Astini and Thomas, 1999). A map of the composite Cuyania terrane is shown in Figure 7.
In addition to the olenellid trilobites, other faunal data from Cambrian and Ordovician rocks of the Precordillera terrane were examined by Benedetto et al. (1999) to refine their biogeographic relationships with Laurentia and Gondwana. The study, based on benthic organisms such as sponges, bryozoans, brachiopods, bivalves, and ostracods, led to the recognition of four successive stages of Precordilleran biogeographic evolution: (1) Laurentian stage (Cambrian-Tremadoc), (2) isolation stage (Arenig–early Llanvirn), (3) pre-accretion stage (Llanvirn-Caradoc), and (4) Gondwanan stage (Hirnantian-Silurian). Each stage denotes its specific position during the rifting-drifting-collision sequence and each one reflects a different pattern of faunal exchange. During the Laurentian stage, the nearly complete identity with Appalachian faunas supports a close geographic connection between Precordillera and Laurentia. The isolation stage begins when taxa that have not been recorded in Laurentia appear for the first time in the Precordillera basin. Through this stage, the Laurentian faunal influence decreases and a number of endemic Baltic-Avalonian genera correlatively increase. The pre-accretion stage is characterized by a paucity of Laurentian forms, the arrival of Gondwanan taxa, and an unusually high level of endemicity. The latter may reflect a degree of geographic isolation, and may in part be due to biologic factors related to dispersal mechanisms. The Gondwanan stage starts after the accretion of the Cuyania terrane at the end of the Ordovician. The review of faunal data supports the “far traveled microplate” hypothesis which is generally consistent with the geological evidence.
The Precordillera, a part of the Cuyania composite terrane, was interpreted by Astini et al. (1995, 1996), Thomas and Astini (1996, 1999, 2003), and Astini and Thomas (1999), as a fragment of rifted Laurentian continental crust and passive-margin cover. Two separate episodes of extension, ~60–70 m.y. apart, and a contractional event are recorded in the early Paleozoic history of the Precordillera. Crustal extension during the Early Cambrian (possibly starting in the latest Proterozoic) led to asymmetric continental rifting and separation of Precordillera from the Ouachita embayment of southern Laurentia (Thomas and Astini, 1996). Synrift graben-fill successions of the Precordillera are overstepped by latest Early Cambrian carbonates indicating rift-to-drift transition and initiation of passive margin deposition.
Faunal evolution in the uppermost Lower Cambrian through Lower Ordovician passive margin succession suggests isolated drifting of the Precordillera as a Laurentian orphan across the Iapetus Ocean from the Late Cambrian to the Early Ordovician. Subsidence curves are typical of post-rift thermal subsidence on rifted continental margins (see Thomas and Astini, 1999, 2003). A contractional event, interpreted as the docking of the Pre cordillera with Gondwana, is documented by mylonitic fabrics, Ocloyic metamorphic ages (464 Ma) imprinted on Grenville basement rocks (Ramos et al., 1998), and west-directed thrusting of passive-margin limestones in eastern Precordillera. The collision is confined to early Middle Ordovician time. A second extensional episode, during the Middle Ordovician starting in the Llanvirn, is evidenced by irregular distribution of sediments and hiatuses, abrupt changes in lithofacies, and local slope-scarp facies associated with block faults. This extension may have been related to pre-collisional flexural extension (Astini et al., 1996).
Paleomagnetic data on the Early Cambrian rocks of the Precordillera (Rapalini et al., 1999) indicate paleolatitudes of approximately 20°, similar to the Ouachita embayment of Laurentia, as inferred by Thomas and Astini (1996, 1999, 2003).
Pie de Palo is the other terrane that constitutes the Cuyania composite terrane. The Precambrian Grenvillian basement of this terrane is composed of an ophiolitic assemblage (Vujovich and Kay, 1998) of Middle Proterozoic age, and a series of gneisses and amphibolites of similar age. U/Pb ages in zircons, as well as Ar/Ar ages, have demonstrated that this basement was already part of the same terrane as the Precordillera when it collided against the protomargin of Gondwana during Middle-Late Ordovician times (Ramos et al., 1998).
Several deformation phases have been recorded in the eastern border of Precordillera during the Late Ordovician and Silurian (Baldis et al., 1984). Particularly spectacular are the tightly deformed flysch deposits of the Late Ordovician–Silurian Rinconada Formation that contain olistoliths of Early Ordovician rocks. Most of this deformation lasted until the Ordovician-Silurian boundary, at the time when the Precordillera was in the final stages of amalgamation with the Sierras Pampeanas. The Cambro-Ordovician magmatic belt of the western Sierras Pampeanas also records a series of episodes that can be matched to the sedimentary evolution of the Early Paleozoic sequences of Precordillera. A coeval evolution can be integrated in a single simple model (Ramos, 2004). The deformation recorded in the sedimentary sequences also matches the metamorphic episodes that are indicated by 40Ar/39Ar ages at Pie de Palo (Ramos et al., 1998).
The western border of the Precordillera was the continental margin during most of the Early Paleozoic, as shown by the sedimentary facies in the Sierra de Tontal (Cingolani et al., 1989). The reconstruction of this continental margin is based on sedimentologic and paleontologic evidence (Baldis et al., 1982), as well as geochemical characteristics of the ophiolitic assemblages developed along the western border of Precordillera (Kay et al., 1984; Haller and Ramos, 1984; Davis et al., 1999). Imbricated structures in the Ordovician and Siluro-Devonian rocks indicate a strong deformation during the Middle to Late Devonian. The western vergence of these structures can be seen in the Sierras de Cortaderas and Sandalio (Cortés, 1989). This deformation, known as the Chanic event, has been interpreted as the result of the collision of the Chilenia terrane against the western Pre cordillera continental margin (Ramos et al., 1984, 1986).
The basement of this large terrane underlies the Main Andes of Argentina and Chile, and is exposed in a series of minor erosional windows or is preserved as roof pendants in some of the large granitic batholiths. The basement is known in the Cordón del Plata and Cordón del Portillo (Caminos, 1965), and in the La Pampa gneisses (Mpodozis and Ramos, 1990). The metamorphic rocks have been dated at Las Yaretas in the Cordillera Frontal where U/Pb ages in zircon indicate 1069 ± 36 Ma (Ramos and Basei, 1997). This age, combined with the absence of Brasiliano deformation, suggests Laurentian affinities for the Chilenia terrane. The large amount of upper Paleozoic granitoid on both slopes of the Andes in Argentina and Chile has been also taken as evidence for the continental nature of the basement by Nasi et al. (1985). Isotopic (initial 87Sr/86Sr ratios) and geochemical characteristics of these magmatic rocks are consistent with a crustal component of Precambrian age in the source of these magmas (Mpodozis and Kay, 1990). The Early Paleozoic cover is scarce and different from Precordillera. Accretion to Gondwana is postulated during Middle-Late Devonian times (Ramos et al., 1986). Final amalgamation occurred in Early Carboniferous times, where a continental to shallow marine deposits overstepped both terranes.
Carboniferous marine units were the first sedimentary rocks amalgamated to the Chilenia terrane. Continental deposits are widespread in the Sierras Pampeanas and Eastern Precordillera; nearshore marine facies occur in the Western Precordillera and estuarine to turbiditic facies are present along the Cordilleras Frontal and Principal. An important subduction-related magmatic activity is recorded during the Late Paleozoic. This magmatic belt, when compared with the location of the Early Paleozoic magmatic belt, was shifted more than 300 km toward the ocean (Ramos et al., 1986). These entire Carboniferous to Early Permian units were strongly deformed during the San Rafael orogenic phase (Ramos, 1988b). Some authors have attributed the cessation of subduction-related magmatism and the consequent middle Permian deformation to the collision of an unidentified terrane (terrane Equis of Mpodozis and Kay, 1990). Other authors interpreted the cessation of magmatism as being related to and episode of flat subduction during the middle Permian that produced the strong San Rafael deformation (Martínez et al., 2006).
Soon after the San Rafael deformation, a generalized extension took place associated with a period in which the Gondwana plate was stationary with respect to the South Pole, as shown by the polar wandering path (Valencio et al., 1983; Ramos, 1988b). Magmatism was widespread during this extensional period (Zeil, 1981), and as a consequence, batholiths like the Colangüil were emplaced between 264 and 247 Ma (Llambías and Sato, 1990). Magmatic activity in this period is represented by rhyolitic volcanism within the Paleozoic accreted terranes, which are presently exposed in the Cordillera Frontal, western Sierras Pampeanas, and Precordillera. This felsic volcanism known as the Choiyoi province (Kay et al., 1989) was interpreted as evidence of generalized extension during Triassic times (Zeil, 1981) and post-collisional volcanism associated with slab breakoff in the final stages of the amalgamation of Pangea (Mpodozis and Kay, 1992). These rhyolitic rocks have alternatively been related to the steepening of a shallow subduction zone starting in the Late Permian (Martínez et al., 2006).
The Cenozoic sedimentary history records the eastward migration of the orogenic front (see Fig. 8). Thick sequences of continental deposits (Santa María Conglomerates of Schiller, 1912) unconformably overlie the Mesozoic rocks in the Cordillera Principal. The angular unconformity is clearly seen east of Cerro Aconcagua and west of Cerro Penitentes. Those conglomeratic deposits are interpreted as alluvial fan sediments interfingered with the volcanics of the Farellones Formation (25–10 Ma, Munizaga and Vicente, 1982). A minimum age of 8.6 Ma was obtained in the continental deposits based on K/Ar dating of pyroclastic rocks interbedded in the uppermost section of the Santa María Conglomerates (Ramos et al., 1996b). The Tertiary deposits farther east of the High Cordillera are represented by distal fluvial facies partially synchronous with the Santa María Conglomerates and the volcanism of the Farellones Formation. The foreland Tertiary Andean sequences at these latitudes (30°–33°S) contain several tuff layers, which attest to cordilleran volcanic activity at that time. An unconformity separates La Pilona beds exposed in the Uspallata valley and the Cacheuta area from older Tertiary beds. This unconformity was produced in the late Miocene prior to the 10–8 Ma age tuffs (Ramos et al., 1996a, 1996b), and it was also coeval with the uplift of the Cordillera Frontal. The Late Miocene and Pliocene deposits of the Uspallata and Cacheuta regions were folded and thrust at that time with the subsequent deposition of the alluvial fan deposits of the Mogotes Formation during the Plio-Pleistocene.
Therefore, the Tertiary sedimentary facies show a migration of the coarse alluvial fan facies from: (a) the inner area of Cerro Penitentes in the High Andes between 20 and 10 Ma to (b) the Uspallata valley and Cacheuta between 10 and 5 Ma, to (c) the outer foothills of the city of Mendoza between 2 Ma and the present active front. Even the Plio-Pleistocene fanglomerates of the Mogotes Formation cropping out west of the city of Mendoza (Cerro de la Gloria) and other younger alluvial fans have been deformed by neotectonic activity in the Mendoza region.
The seismic sections of the plains located eastward of the Precordillera clearly show that the present orogenic front is composed of a set of imbricated thrusts. This structural style was corroborated by drilling, and has somewhat similar characteristics to the previous fronts (Fig. 9). The thrust front in the eastern side of the Precordillera is still active. Intense compressive deformation, as seen in Sierra de las Peñas (see Cortés, 1990) and, as inferred from earthquake focal mechanisms and escarpments on the alluvial fans, is continuing today. The Andean structure of the central Andes is the result of a combination of several tectonic mechanisms. There is a striking coincidence between the increase of plate motion rates, the cessation of magmatism, and the compressive deformation at the orogenic fronts.
The Andean history begins in the late Oligocene (Ramos, 1999). Most of the Oligocene was quiescent with localized extension (Godoy et al., 1999) coincident with the volcanic rift along the axis of the Cordillera. This extension ended at ca. 20 Ma when the volcanic activity of the Farellones Formation started (Munizaga and Vicente, 1982). Two interrelated tectonic features demonstrate that a change in the geometry of the Benioff zone followed: (a) eastward migration of the subduction-related magmatic centers from a position ~180 km from the present trench at ca. 25 Ma, to 700 km away from the trench at 2 Ma at the Central Sierras Pampeanas in the latest Pliocene (Ramos et al., 1991; Kay et al., 1987, 1991; Kay and Mpodozis, 2002); (b) geochemical characteristics indicate a thickening of the continental crust, between 18 Ma and the present, related to the tectonic stacking of the Andean Cordillera (Kay et al., 1987; 1991); and (c) eastward shifting of the orogenic deformation during the past 20 Ma: 275 km from the trench at 20–10 Ma, 325 km at 10–5 Ma, and 365 km at 2 Ma. This implies an average propagation rate of 2.5 mm/yr of the orogenic front in the past 20 m.y., although shifting was probably episodic.
The causes of the change in the Benioff zone geometry and segmentation are probably complex and multifaceted: (a) The breakup of the Farallon plate into the Cocos and Nazca plates, which occurred at 25 Ma, seems to mark the beginning of a period of higher convergence rates (Handschumacher, 1976); this age coincides with the initiation of Farellones magmatism and is a milestone in the geodynamic evolution of the area; (b) the increase in plate convergence from 25 to 26 Ma up to 10 Ma as defined by Pilger (1984) and Pardo Casas and Molnar (1987) when the present deceleration began; and (c) several authors have explained the present segmentation of the subducted Nazca plate as being controlled by the collision of aseismic ridges (Pilger, 1981). In Pilger's interpretation, the buoyancy effects produced by subduction of those ridges, combined with the younger age of the subducted slab, contribute to diminish the angle of the Benioff zone. The effects of the subduction of the Juan Fernández hot spot trace began ca. 15 Ma according to Pilger (1984). At the present latitudes evidence of shallowing of the subduction zone shows a similar trend of southward migration, which can be correlated with the southward shift of the Juan Fernández ridge collision along the trench (von Huene et al., 1997; Yañez et al., 2001).
South of the region of flat subduction, there is an age decrease in the subducted slab and even with a higher thermal gradient there than in the flat-slab segment, the present angle of subduction is ~30° (Isacks et al., 1982). Isacks' model (1988) indicates that the present dip of the different slab segments is controlled by the width of previous weakened zone of the upper plate. This zone is related to the size of the asthenospheric wedge between the oceanic and the continental plates. A greater amount of shortening occurred in the central segment because the weakened area was the widest, as indicated by the extension of the magmatic activity in the Puna Altiplano. In the analyzed segment (30°–31°S), a relatively narrow weakened zone produced the overriding of the Nazca plate by the South American plate associated with relatively minor shortening.
Field Trip Log
The road map of Figure 10 shows the location of detail geologic maps of the different stops of the fieldtrip guide. This map should be complemented with a road map such as the Atlas Vial of the Automovil Club Argentino or similar commercial maps.
DAY 1—ACTIVE TECTONICS OF EASTERN PRECORDILLERA
From Mendoza to San Juan, Argentina
The present setting of the Precordillera within the Pampean flat-slab segment of the central Andes records an intensive shortening along the eastern foothills (Fig. 11). Although the geometry of the cover is characterized by thin-skinned thrusting, important shortening is recorded in the middle-upper crust, where the basement is deformed as indicated by earthquakes at ~15 km depth. The migration of the thrust front reached the eastern Pre cor dillera during late Pliocene times, as shown by the proximal facies of the synorogenic deposits of Los Mogotes Formation.
Two thrust systems are seen in the segment considered here (see Fig. 12). The western system is characterized by a series of imbricated thrusts with east vergence detached in Early Paleozoic rocks, as shown in the Cerro La Cal, Divisadero Largo (Fig. 11), and Sierra de las Peñas sections. The thrust sheets are placing Triassic red beds over the distal and proximal facies of the Cenozoic strata. This system records an important seismotectonic activity as observed east of Cerro La Cal, in Río de las Peñas, and Cerro de la Gloria.
The eastern system has west vergence at these latitudes and involves the Proterozoic–Early Paleozoic basement of Sierras Pampeanas. Miocene deposits are overridden by these metamorphic rocks. Farther north in the San Juan area, the present shortening is recorded by backthrusts with west vergence belonging to this eastern system (see Day 2).
Stop 1-1: Cerro de la Gloria
Cerro de la Gloria is a small hill located along the western border of the city of Mendoza (Fig. 11). Important earthquakes destroyed the city of Mendoza in 1861 and partially destroyed it in 1920 and 1985. The first earthquake is related to small displacements on the Cerro La Cal thrust and had an estimated magnitude of Ms = 7.0 (Mingorance, 2004). The fault scarp can be seen across Las Heras Avenue, a few meters west from where it intersects Perú Street. The railroad seen here was built along the fault scarp a few years after the 1861 earthquake that destroyed the city of Mendoza. The 1985 earthquake had a hypocenter located a few kilometers southwest of the city (Ms = 5.75, depth 14 km), with a compressive focal mechanism. The most probable active fault plane was a reverse fault dipping 56° to the west (Triep, 1987).
A series of east-verging thrusts imbricate Paleozoic rocks (C–Cambrian, Or–Ordovician, and Dv–Devonian), Triassic rift deposits (Tr), and Tertiary synorogenic deposits (Tp– Oligocene, Tm–Miocene, and TQm–Late Pliocene) with (Qt) Pleistocene alluvial fan deposits (see Fig. 11). The dominant north-south trend of the different thrusts along tens of kilometers is abruptly modified by an east-west deflection near the city of Mendoza. This deflection corresponds to a lateral ramp that is a reactivation of an east-west transfer fault of the Triassic rift system. North of this transfer fault, the main rift depocenter is to the east whereas to the south, the main depocenter is in the western side of the rift basin.
The flat-lying to gently-dipping Pleistocene deposits (0° to 25°E) seen along the road show evidence of progressive unconformities. As we move to the west, dips steepen until reaching subvertical in the conglomerates of the Mogotes Formation (TQm–Late Pliocene). These sequences indicate the strong neotectonic activity of the thrust front during the Quaternary. The coarse conglomerates correspond to mass wasting flow and proximal alluvial fan facies.
From the summit of Cerro de la Gloria, there is a general view of the thrust front and the different Quaternary terraces related to the Precordillera uplift. Some of the terraces are cut by younger Quaternary faults. The general geometry of the faults is controlled by the tectonic inversion of the Triassic rift system.
A subvolcanic dacitic body of Tertiary age (Tv in Fig. 11) is one of the easternmost evidences of the Oligocene–Early Miocene volcanic arc at these latitudes.
Stop 1-2: Borbollón Anticline
The next stop is reached by taking Route 40 north just past the Mendoza airport (Fig. 13). The orogenic front in this region is formed by two actively growing structures, the Borbollón and Capdeville anticlines, bounded to the west by the Cerro La Cal Fault. Both structures have an east vergence. The hypocenter of the 1861 Mendoza earthquake was located along this fault in the segment north of Cerro La Cal.
The Mendoza airport, located in the northern end of the city is sited along the axis of an actively growing anticline (Fig. 13). Seismic sections across the Borbollón anticline show progressive unconformities since at least late Miocene times (ca. 6 Ma, Olgiati, 2002). 40Ar/39Ar ages of 27.9 ± 0.6 k.y. for the white ash-fall tuffs in the Borbollón anticline and 16.2 ± 0.6 k.y. in the Capdeville anticline confirm a Late Quaternary age. Both ashfall tuffs are derived from the active volcanic arc located to the southwest, south of the Tupungato volcano.
At this stop, the western exposed limb of the anticline is dipping at ~3°W in contrast with late Miocene deposits that dip more than 25° at depth on the seismic line.
Drive to the City of San Juan
The drive is to the north from the city of Mendoza along National Highway 40 for ~150 km. The road goes along the boundary between distal alluvial fan and playa-lake deposits. On the western side of the road, the Sierra de las Peñas can be seen. This is one of the most active uplifting areas in Quaternary times in the region. Synorogenic deposits are cut by several fold scarps next to the southern plunge of the main structure and several progressive unconformities occur. Las Peñas thrust is one of the most spectacular fault scarps with more than 60 m of throw on Quaternary gravels.
The alluvial fan deposits show several active fault scarps parallel to the mountain front. North of Río de las Peñas, a west-verging system intersects the mountain front. This west-verging system is a thick-skinned belt controlled by deep basement faults as depicted in the seismic lines and is characteristic of the adjacent Sierras Pampeanas. The minor uplifts could represent the still buried westernmost basement blocks of the Sierras Pampeanas.
Stop 1-3: Cerro Valdivia
Stop 1-3 is along Highway 40 a few kilometers before arriving in San Juan (see Fig. 10). This stop shows the Grenville-age basement rocks of the exotic Cuyania block that collided against Gondwana in Late Ordovician times. An U-Pb age in this sector yielded ages ca. 1100 Ma. The protoliths of these metamorphic rocks are island-arc deposits and a sedimentary cover similar to that in El Llano uplift exposures in Texas. Isotopic analysis, geochemical data, and paleomagnetic studies indicate that the Cuyania block was adjacent to the Ouachita embayment during Early Cambrian times.
The Andean structure shows an east-dipping, basement-involved thrust overriding the Permian and Tertiary continental deposits. The Cerro Valdivia is a small hill with its lithology and structure characteristic of the Sierras Pampeanas (Fig. 14). Farther east, Cerro Barbosa, another Grenville-age basement block, has a similar tectonic setting. Both uplifted blocks represent the westernmost exposures of the Sierras Pampeanas basement at these latitudes.
Stop 1-4: Eastern Precordillera at Sierra Chica de Zonda
The drive is from the San Juan city to the west on Provincial Route 12 in the direction of the town of Zonda, which is some 10 km from the city of San Juan (see Fig. 10). This stop provides the opportunity to examine the Cambrian sequence of limestones bearing the Early to Middle Cambrian trilobites that have been correlated with the Olenellus fauna. The Olenellus fauna of the Eastern Precordillera of San Juan, together with the northwestern Scotland and Spitsbergen Island faunas, are the only ones outside of North America. Paleomagnetic, paleoclimatic, isotopic, geochemical, and geochronological data indicate that the limestones and their basement are exotic to Gondwana (see review in Ramos, 2004). The biostratigraphic control and the faunal affinities clearly indicate that an original Laurentian fauna was gradually isolated in Arenigian times, and eventually became endemic by the mid Ordovician, as proposed by Benedetto and Astini (1993) and Benedetto et al. (1999), among others. Gondwanan forms arrived by the Llanvirnian and became dominant in Caradocian to Ashgillian times during the glaciation that dominated this part of Gondwana (see discussion in Benedetto, 2004).
Stop 1-4 also allows a regional view of the boundary between the Eastern and Central Precordillera (Fig. 15). This boundary is characterized by a thick-skinned triangle zone (Zapata and Allmendinger, 1996). The thin-skinned, east-vergent Central Precordillera is thrust on synorogenic valley fill of Miocene age. The highly deformed Early Paleozoic rocks of the Eastern Precor dillera are uplifted by a basement wedge. These basement structures correlate with a zone of important seismic activity with compressive focal mechanisms at ~14 km depth.
The present Juan Pobre valley is an abandoned landscape where the Río San Juan used to flow in Pleistocene times. The latest Quaternary uplift of the Zonda fault displaced the river to the north.
Stop 1-5: Cerro Blanco
Farther to the west, after passing 8 km the town of Zonda in the direction to Calingasta is Stop 1-5. The Cerro Blanco dacite has been emplaced synchronously with the uplift and stacking of the Precordillera thrust sequence (Fig. 15). K-Ar dating of these arc-related dacites yielded 6.3 ± 0.7 Ma (whole rock). The abundant basement xenoliths are one of the few indirect evidences of the Grenville-age metamorphic basement directly under the Precordillera. They have been dated by U-Pb in zircons and yielded ages of ca. 1.1 Ga (Kay et al., 1996).
The Cerro La Sal dacite exposed in the northern margin of the river, west of Punta Negra (Fig. 15), corresponds to an older volcanic complex with ages around 16 ± 2.9 Ma. The strata of Albarracín Formation (Ta) contain proximal pyroclastic flows that have been produced by explosive collapse of an actively growing dome. Fission-track dating of these tuffs indicates ages from 18 to 7.8 Ma (Vergés et al., 2001). Several intrusive contacts between the sedimentary rocks and the deformed dacites are visible along the road.
An eastward pattern of increasingly more depleted signatures is seen toward the east in the Precordillera dacitic magmas. The magmas erupted through crust which, to the east, was less modified by deep crustal thickening and wedge processes. The dacitic melts were related to the Cuyania depleted basement (Kay and Abbruzzi, 1996).
The Tertiary deposits record distal synorogenic facies at the base dominated by shales and fine grained sandstones of Middle Miocene age. Magnetostratigraphic studies recognized an abrupt change in sedimentation rates in the middle to upper section in the Albarracín creek. The strong increase in sedimentation rates at ca. 9 Ma, from 0.09 to 0.44 mm/yr, coincides with a rapid deposition of conglomerates (Vergés et al., 2002). This change is related to the renewed activity in the thrust to the west, and consequent deposition of proximal conglomeratic synorogenic deposits.
The distribution and composition of Tertiary magmatism have been related to a shallowing subduction zone and a thickening crust. Isotopic data combined with trace element data from several regions of the shallow subduction zone permit to infer differences in crustal thickening patterns in the development of the modern seismic zone (Kay et al., 1991). Along the Cordillera Principal the thickening and expansion of the magmatic arc is detected prior to 16 Ma, and the end of the subduction-related magmatism occurred at 7 Ma. The Precordillera of San Juan records magmatic activity between 18 and 6 Ma, while in the Sierras Pampeanas volcanism ended at 1.9 Ma (Ramos et al., 1991).
Stop 1-6: Synorogenic Deposits in the Ullum Dam
The northern valley of Río San Juan downstream from the Ullum dam exposes Miocene synorogenic deposits along the road to the town of Ullum, 10 km west of the city of San Juan (Fig. 16). A change from distal to proximal facies in these deposits, similar to those previously described, is seen at this stop. The distal facies have been deposited between 8.5 and 4.0 Ma, and the intercalated tuffs have fission track ages of ca. 7 Ma. The change to conglomeratic proximal facies occurs at ca. 4 Ma and indicates an important shifting of the thrust front during Pliocene times. Similar trends have been found in the northern Precordillera by Jordan et al. (1988, 1993, 1997). The Tertiary deposits depict a series of minor normal faults that are related to the lateral ramp that segments the Zonda fault at these latitudes.
Stop 1-7: Late Quaternary Lake Deposits
Stop 1-7 is 3 km northwest of the previous stop along the road to the town of Ullum (Fig. 16). The lake deposits of the Valentín Formation exposed here are dated at 6500 yr (14C ages) clearly showing that the shore of the Holocene natural lake was even larger than the present lake produced by the dam. A series of neotectonic features are observed along the Zonda fault trace. The Pleistocene terraces have been analyzed by cosmic-ray exposure dating, which has yielded ages of 18.7 and 6.8 10Be ka for the abandonment of the surfaces (Siame et al., 2002). These data indicate a minimum shortening rate of more than 1 mm/yr in the past ~20 k.y., which illustrates the rapid uplift of the Sierra Chica de Zonda (Fig. 16). Displacements along this basement fault may have originated the 1944 San Juan earthquake.
DAY 2—FROM SAN JUAN TO BARREAL, ARGENTINA
Stop 2-1: La Laja Fault
Drive north along National Highway 40 heading to the town of Albardón, some 10 km north of San Juan city (Fig. 17). Continue to the La Laja thermal baths heading north for 5.5 km until the first stop. Continue in the direction of the La Laja thermal baths heading north for 5.5 km to reach Stop 2-1 at the La Laja fault.
The epicenter site of the large 1944 earthquake which destroyed San Juan city corresponds to a reactivation of La Laja backthrust. This fault places Mio-Pliocene deposits in contact with Pleistocene gravels. The fault has a length of 8 km, a total throw of 18 m, and is part of a system of backthrusts located in the eastern side of the Sierras de Villicum and Rinconada. The fault system extends over 150 km along the eastern border of Precordillera (Bastías, 1990).
The 1944 earthquake of magnitude 7.4 produced a reverse displacement of 50 cm on the fault, and the scarp was traceable north of the road affecting recent soils. Most of the thrust system is partially coated by extremely porous travertine carbonates deposited from groundwater activity related to the recent faults.
Stop 2-2: Sierra de Villicum
Return to the town of Albardón and take National Highway 40 in the direction to Jachal (Fig. 17) for 6 km. to reach Stop 2-2. A major backthrust system is exposed in the southern end of the Sierra de Villicum at this stop. This south-plunging structure has an imbricated backthrust that overrides Cambro-Ordovician limestones over Miocene strata. The backthrust is the northern segment of the Zonda fault (see Figs. 15, 17, and 18). This succession of Cambrian limestones was the first locality where numerous trilobites of the Early Cambrian Olenellus fauna with North American affinities were found (Borrello, 1963).
South of the road there are several abandoned surfaces with ages varying from 6 to 20 ka, dated by Siame et al. (2002).
Drive across the Matagusanos Valley
Continue north on Route 40 and then head west to the Baños de Talacasto along Provincial Route 436 (see Figs. 10 and 17). The Matagusanos valley is the northern extension of the Zonda triangle zone. It is bounded by the Sierra de Villicum to the east, where a series of west-verging backthrusts of Eastern Precordillera are overriding the synorogenic deposits. Along the valley some active growing structures are seen with west vergence. A wildcat drilled in one of these structures, the Matagusanos anticline, went through more than 6000 m of synorogenic deposits directly overlying the Ordovician San Juan Limestones.
The western side is bounded by a series of east-verging imbricated thrusts of the Central Precordillera that are detached in Lower Ordovician limestones, repeating the Early Paleozoic sequence. Figure 18 shows the structure of the triangle zone.
Stop 2-3: Sierra de Talacasto Front
The thrust front of the Central Precordillera shows three west-dipping forethrusts around the Baños de Talacasto (Fig. 19). Ordovician San Juan Limestones are imbricated over Miocene deposits in the first; Ordovician limestones are over Late Ordovician shales in the second; and Ordovician limestones are over Silurian shales in the third. Accommodation structures such as minor faults and second-order chevron folds are seen in the main thrust sheets.
At this stop K-bentonite levels can be observed. These K-bentonites were very significant in the correlation of the Laurentia-derived Cuyania block and Gondwana (Cingolani et al., 1997). Sensitive high-resolution ion microprobe (SHRIMP) ages of zircons in bentonites near this locality yielded ages similar to the magmatic arc rocks of the Famatina region farther east in the Sierras Pampeanas (Fig. 20). The K-bentonites here are older than the typical K-bentonites in the Appalachians.
Stop 2-4: Western Sierra de Talacasto
Stop 2-4 is a brief stop a short distance northwest of Stop 2-3 to see the thrust to the south that puts Ordovician limestones on Silurian shales (Fig. 19). The stratigraphic separation decreases to the north, where limestones are thrust on similar limestones, indicating out-of-sequence thrusting. Although there is no precise dating on the age of thrusting in this locality, by correlation with adjacent thrusts to the north and south, the timing of deformation is interpreted to be between 10 and 8 Ma (Vergés et al., 2002). The timing of thrusting in the Precordillera shows not only a migration of the thrust front to the east, but also a wave of deformation that youngs to the south. This wave of deformation progressing to the south is also seen in the Sierras Pampeanas uplifts farther east (Ramos et al., 2002), and both are probably linked with the shifting of the Juan Fernández ridge collision along the trench as proposed by Yañez et al. (2002).
Drive along the Talacasto Valley and Quebrada de las Burras
A few kilometers from Talacasto, leave Route 436 to take a paved road to the southwest toward the town of Calingasta. The road crosses all of the main stratigraphic marine units of the Early Paleozoic of the Central Precordillera (Fig. 21). The 350-m–thick sequence of platform limestones of the San Juan Formation ranges from latest Tremadoc to Early Llanvirnian (Peralta, 2003). An erosional unconformity separates this unit from the clastic deposits of Las Chilcas Formation (uppermost Ordovician– Early Silurian). The Ordovician-Silurian boundary has been recognized south of Baños de Talacasto in Las Chilcas Formation. The yellowish sandstones and shales of the La Chilca Formation (~110 m thick) of Early to Middle Silurian age unconformably overly the San Juan Limestones in the Quebrada Ancha. The Los Espejos Formation of Late Silurian age is composed of green siltstones and fine sandstones (~250 m thick). Both Silurian units represent the marine foreland distal deposits associated with the collision of the Cuyania terrane with Gondwana.
West of the Quebrada Poblete Norte, a complete section of the Talacasto Formation overlies the Silurian deposits. This is the type section of this unit and is represented by olive green sandstones and shales, between 100 and 200 m thick, of Early Devonian age. A thick section of turbidites and shales of the gray color Punta Negra Formation represents a reactivation of the deformation during Middle to Late Devonian times. A deep incised valley of glacial origin cuts the Punta Negra Formation and preserved glacio-lacustrine coarse conglomerates of Late Paleozoic age.
Miocene synorogenic deposits unconformably overlie the Early Paleozoic rocks (Fig. 21), a few kilometers west of the junction to Pachaco and Calingasta along the Quebrada de las Burras. This Miocene unit is truncated by the Sasso thrust that repeats Devonian deposits.
Stop 2-5: Northern End of Sierra de la Cantera
This stop is located in the drainage divide between the Río San Juan drainage basin and the Quebrada de las Burras (commonly dry) river. It is at 2215 m above sea level and corresponds to an active thrust zone related to the La Cantera thrust. This is one of the few active faults in Central Precordillera, where important earthquakes occurred in 1924, 1964, and 1984 (Mingorance, 1998), along many other neotectonic features. This fault which coincides with the Cuesta del Tambolar in the valley of the Río San Juan was interpreted as an out-of-sequence thrust farther south where it truncates overturned Late Paleozoic deposits.
The regional view from this point shows the San Juan limestones in the hanging wall of the Pachaco thrust and the Devonian Punta Negra Formation represented by foreland distal facies.
Stop 2-6: The Cerro Blanco de Pachaco
After crossing the bridge across the Rio San Juan, turn east toward the small town of Pachaco (Figs. 21 and 22). The Pachaco thrust exposes the westernmost edge of Ordovician platform limestones in the Precordillera. The limestones override Tertiary mudstones and fine sandstones of the Pachaco Formation (Milana et al., 1993). These deposits are distal synorogenic facies probably related to the uplift of the Main Cordillera west of Calingasta at 18–16 Ma (Milana, 1991).
The abnormal thickness of the Ordovician San Juan limestones exposed north of the river was explained by Heim (1952) as a splay that repeated the San Juan Limestone (Fig. 22). East of the main peak, with proper sun lighting, a tight syncline which corresponds to the fault trace can be seen. Thick-bedded limestones of the lower member (Late Tremadocian to Early Arenigian) of San Juan Formation are thrust on the thin bedded fossiliferous limestones of the upper member (Late Arenigian to Early Llanvirnian). The competent nature of the limestones contrasts with the highly deformed Silurian shales that are exposed west of Cerro Blanco de Pachaco.
Stop 2-7: Ordovician Slope Facies and Olistoliths
Continue west along the road paralleling the Rio San Juan in the direction of the town of Calingasta to Stop 2-7 (Fig. 22).
The first Ordovician slope facies are represented by the Alcaparrosa Formation, which corresponds to slope and bathyal facies of the Early Paleozoic platform. These rocks host a series of olistoliths up to several hundred meters in diameter of Cambrian and Early Ordovician rocks. At this stop, the olistoliths are mainly limestone blocks of Middle Cambrian age that were deposited by gravitational sliding from the carbonate platform. The size of the olistoliths ranges from a few meters up to a couple of kilometers. There are many of them along the edge of the Early Paleozoic platform in this region.
Stop 2-8: Sierra de Tontal
Continue west along the road paralleling the Rio San Juan in the direction of the town of Calingasta to Stop 2-8 (Fig. 22).
Most of the Sierra de Tontal is composed of the Alcaparrosa Formation. These slope facies vary from coarse proximal turbidites to more distal ones. The graywackes and shales bearing scarce graptolites are tightly folded and developed an incipient cleavage that increases to the west. The dense debris flows with resedimented conglomeratic blocks seen at this stop were derived from the synrift facies. Some pebbles in equivalent deposits farther to the north have been dated at 1367 ± 5 and 1370 ± 2 Ma (U-Pb in zircons). These ages are in concordance with the 1.3–1.4 Ga ages of the Granite-Rhyolite province west of the Grenville Front in the northwestern corner of the Ouachita embayment (Thomas et al., 2000). Mafic rocks are interbedded with this sedimentary sequence on the western side of the Sierra de Tontal.
As a whole, this sequence has been interpreted as a klippe of western facies thrust on to the platform edge. The Andean age west-dipping Los Ratones thrust fault at this stop (Fig. 22) is covered by Quaternary deposits.
Drive from Western Precordillera to Barreal
The road continues to the west along the southern margin of Rio San Juan heading to the town of Calingasta ~38 km farther west. Turn south at Calingasta to spend the night in Barreal.
The western section of the San Juan valley is characterized by a complex series of thrusts, which repeat the different Early Paleozoic slope deposits that are tectonically interfingered with more oceanic facies. The valley widens at Calingasta, in the junction of Los Patos and Castaño rivers. There, the Uspallata-Calingasta valley, a north-trending depression, probably corresponds to an unbroken piggy-back basin, similar to the one described farther north in this depression by Beer et al. (1990). Along the road isolated patches of Triassic synrift deposits are observed. These deposits are controlled by northeast-trending half grabens developed in the hanging wall of the suture between Cuyania and Chilenia, which at these latitudes coincides with the boundary between Precordillera and Cordillera Frontal. This depression has many similarities with the Rocky Mountain trench of North America.
DAY 3—FROM BARREAL TO USPALLATA, ARGENTINA
The Barreal area has many classic localities studied by Du Toit, Keidel, and others, since the beginning of the last century. In the area, there are excellent exposures recording the Late Paleozoic Gondwanian glaciation with glaciated pavements, dropstones, striated clasts, till, and outwash deposits. These deposits were used as a confirmation of the importance and extension of the Late Paleozoic ice cap (Keidel, 1916; Du Toit, 1927). The evidence of this Late Paleozoic glaciation described by Keidel (1916) was one of the first-order pieces of geological evidence used by Wegener (1929) to hypothesize the continental drift.
The drive from Barreal to Calingasta provides an opportunity to observe the magnificent scenery of snowy mountains along the eastern margin of the Cordillera Frontal. If weather conditions permit, the northern side of the Mount Aconcagua far to the south can be seen from the road.
Stop 3-1: Km 114 Triangle Zone
This stop is in the region of Km 114 on the road west from Calingasta in the direction of Pachaco. The stop and the important features at this stop are shown on the map in Figure 22. In a distance of ~5 km along the road, two west-verging thrusts (Tontal thrusts) on the eastern side of an Andean age triangle zone and two east-vergent thrusts (western Agua de los Pajaritos thrust and eastern Carrizal thrust) on the western side can be seen. A short walk east of the Carrizal thrust and west of the IMSA building provides access to an important angular unconformity between heavily deformed Devonian flysch and less deformed Carboniferous foreland deposits. These Carboniferous continental deposits bear an abundant Early Carboniferous Rhacopteris flora typical of the Gondwanian realm. They also have rhyolitic and granitic pebbles derived from the Chilenia terrane. This unconformity is related to the docking of the Chilenia terrane against the protomargin of Gondwana during the Late Devonian times.
Stop 3-2: Low-Grade Metamorphic Rocks
Follow the road west along the Rio San Juan to Stop 3-2 on Figure 22. The low-grade metamorphic rocks at this stop belong to the Don Polo Formation, a unit that has been traditionally considered as the oldest exposed in the area. As it is in tectonic contact with other Ordovician rocks, its age is in dispute between Late Proterozoic (?) and Ordovician. The sequence consists of highly deformed graywackes and shales. To the south, the unit grades to anchimetamorphic to low-grade metamorphic facies. This assemblage has been correlated with the Taconian allochthon of the Appalachians by Nullo and Stephens (1996).
Stop 3-3: Ordovician ophiolites
Continue west along the Rio San Juan to Stop 3-3 on Figure 22. Beautiful basaltic pillow lavas crop out on the south side of the road near where the road bends and the high cordilleras are first seen to the west. The basalts are at the base of the Alcapa rrosa Formation, and upward the slope facies are interfingered with them. Locally, they are associated with fine-grained cherts. This stop shows some of the best preserved Ordovician pillow lavas in the Precordillera. A detailed observation of the pillows shows the smectitic “bread” crust, amygdules, and radial cleavage. Although the outcrop here only shows pillow lavas and breccias, farther to the east and to the south there are outcrops of gabbros, peridotites, and sills typical of an ophiolitic assemblage. This assemblage is exposed along a belt of more than 1000 km that represents the suture between the Cuyania and Chilenia terranes.
Secondary mineralization is due to both sea water alteration and later tectonic metamorphism. Their geochemistry indicates the strong oceanic affinities of an enriched mid-oceanic ridge basalt (E-MORB) setting, which is similar to other oceanic tholeiites of abnormal ridges. Geochemical and isotopic characteristics are similar to the Newfoundland ophiolites (Kay et al., 1984; Haller and Ramos, 1984). This stop provides a cross section of the pillows. If time allows, an optional stop on the west of the bridge at Calingasta Bridge (Fig. 22) provides a plane view of the ophiolites.
Stop 3-4: El Alcázar
This stop is located south of the city of Calingasta, ~17 km north of Barreal (Fig. 23) near the locality of Hilario. The small outcrop of Triassic sag phase deposits at this stop is characterized by tuffs, reworked tuffs, siltstones, and fine-grained sandstones of lacustrine and fluvial facies. These sediments were deposited along the western and passive ramp of a half-graben system (López Gamundi and Astini, 1992). The active margin is located to the east and is characterized by coarse conglomerates and red sandstones similar to the synrift deposits of the Río Mendoza Formation near Potrerillos dam (see chapter 3, this volume).
Although the age of the deposits in this locality is poorly constrained, other outcrops along the Río de los Patos valley have abundant flora and palynomorphs indicating a Late Triassic age.
Drive along the Calingasta-Uspallata Valley
The long depression that separates the Western Pre cordillera from the Frontal Cordillera coincides with an old tectonic feature–the Early Paleozoic western continental margin of first Cuyania and then Gondwana. This depression is known as the Iglesia-Calingasta-Uspallata valley and extends for over 300 km (see Fig. 23).
This drive from Calingasta to the south for 147 km to the town of Uspallata shows the clear differences between the Precordillera and the foothills of the Cordillera Frontal. On the western side, the black tones of the Carboniferous deposits contrast with the light pink colors of the Gondwanian granitoids. The structure is homogeneously massive in comparison with the different thrust sheets observed in the Precordillera. Along the eastern side of the road, slope facies of Ordovician and Silurian age are interbedded with basaltic pillow lavas. Alum mines exploit secondary minerals associated with the oceanic facies. Along the eastern side of the road, outstanding white outcrops of Triassic rocks are composed of siltstones and tuffs containing abundant flora remains of Late Triassic age. The sequence comprises thick red conglomerates and sandstones of the synrift facies at the base, and sag deposits with siltstones, shales, and tuffs in the upper section. All of these small basins are developed on the hanging wall of the suture between the Cuyania and Chilenia terranes.
South of the flat basin of the Barreal de Leoncito south of Barreal, the valley is deflected to the east with an echelon pattern, and shows a more complex broken structure. Several isolated exposures of Early Paleozoic rocks are seen in the valley axis. Some of these rocks are imbricated Siluro-Devonian strata and mafic rocks with west vergence in the Sierra de Sandalio (Cortés, 1989). The Sierra de Sandalio is located west of Sierra de Cortaderas, where the main outcrops of ophiolites are exposed (east of Stop 4-1 on Fig. 23). On the western side of the valley, thick sequences of Choiyoi deposits are inter bedded with sedimentary lacustrine facies (Cortés, 1985). Several half-graben systems have been identified along this border of Cordillera Frontal. Important Permian magmatism is responsible for the porphyry copper system present near Yalguaraz and known as the San Jorge prospect.
Upon arriving in the town of Uspallata, the surface of the uplifted peneplain of the Cordillera del Tigre is seen at 4500–5000 m high all along the western margin of the valley.
Stop 3-5: Agua Hedionda
Stop 3-5 is a little more than half way to Uspallata from Calingasta (Fig. 23); the latitude and longitude are 32°01′15″S and 69°21′10″W. Highly deformed Devonian rocks of the Ciénaga del Medio Group are exposed at this stop. These graywackes and shales represent distal turbiditic sequences characteristic of the slope facies that are interbedded with pillow lavas and mafic dikes farther south. The geo chemistry and isotopic signature of the magmatic rocks indicate an oceanic source. Plant remains point to a possible Silurian or Devonian age for these deposits (Cortés, 1993; Cortés and Kay, 1994). These rocks are either part of the accretionary prism of the eastern edge of the Chilenia terrane or part of the subduction complex of the Cuyania terrane.
Optional Cultural Stop: Tambillos
In the foothills of Cordillera Frontal, isolated segments of the Camino del Inca (Inca trail) can be observed as in Tambillos. In this locality just north of Uspallata, there are preserved ruins of a small setting built by the Incas in the fifteenth century, to be used as lodging for the messengers that crossed up and down the empire. There are several of these places along the Río Mendoza valley set apart at ~50 or 60 km distance. Picheuta and Ranchillos are among them. After this stop, continue south to the town of Uspallata.
Day 4—Uspallata Region and West to Puente Del Inca
The first two stops on Day 4 are to see the Miocene volcanic rocks and Triassic basalts and sedimentary sequences east of the town of Uspallata.
Stop 4-1: Darwin Forest
From the town of Uspallata, proceed north on old Route 7 in the direction of Villavicencio and the city of Mendoza. Stop 4-1 is a few kilometers east of the Agua de la Zorra (Fig. 24) and can be located on the south side of the road fragments of a broken sign. This is the classic locality where Charles Darwin found the first fossil floras of southern South America in 1835. He reported a fossil forest with vertical silicified stems of “Araucarites” in life position near Agua de la Zorra (Fig. 24). Electron microscope studies performed in recent years confirm that the trunks belong to Araucarioxylon (Brea, 1997). The fossil horizon is in the Potrerillos Formation of Late Triassic age. The red sandstones and siltstones of the Potrerillos Formation are interfingered with within-plate alkaline basalts of middle Triassic age (235 Ma, Ramos and Kay, 1991). Columnar jointed basaltic sills are well seen along the road near Agua de la Zorra (see horizontal striped pattern on map in Fig. 24) and can be observed on the way to Stop 4-2.
Stop 4-2: Cerro Colorado Dacite
Continue east on old Route 7 (Fig. 24). The volcanic center of Cerro Colorado is part of a series of dacitic and andesitic bodies of early Miocene age, which are widespread along the Precordillera of San Juan and Mendoza. They erupted prior to the shallowing of the Benioff zone in Miocene times at these latitudes (Fig. 24). Cerro Colorado has been dated by K-Ar whole-rock analyses yielding an age of 18.4 ± 0.7 Ma (Kay et al., 1991; Ramos et al., 1991). Geochemical and isotopic studies indicate a mixed mantle and crustal source with low La/Yb ratios indicating magma genesis prior to the thickening of the crust at these latitudes (Kay et al., 1991).
Drive West from the Town of Uspallata into the Frontal and Main Cordilleras
The drive to the west of the town of Uspallata town along National Highway 7 for 80 km provides the opportunity to cross the Cordillera Frontal along the northern and southern margins of the Rio Mendoza Valley. The drive features the relationship between Late Paleozoic subduction and the deformation known as the Gondwanide orogeny since the early works of Keidel and Du Toit, as well as the subsequent generalized extension responsible of the Choiyoi Permo-Triassic magmatic province (Kay et al., 1989). These features are clearly seen across this sector of the Andes along the boundary between Precordillera and Cor dillera Frontal. The Cordillera Frontal is the locus of Late Paleozoic tectonics and is where the Choiyoi province exposes the subduction related volcanics, the San Rafael orogenic phase, widespread rhyo litic vol canism, and coeval Triassic Cuyo rift deposits. The Cor dillera Frontal uplift was one of the first consequences of the colli sion of the Juan Fernández ridge at this latitude and the development of the present Pampean flat slab. The late Miocene peneplain is still preserved in the Cordillera Frontal. The following stops supplement those along this route in chapter 3 of this volume.
Stop 4-3: Southern End of the Uspallata Valley
At this stop along Route 7 (31°36′53.5″; 69°23′54.1″; 1874 m asl), the southern end of the Calingasta-Uspallata valley can be observed (Fig. 25). The Cordillera Frontal at these latitudes is thrust over the Precordillera. A series of imbricate thrusts repeats Choiyoi volcanic rocks on top of Miocene continental foreland basin strata. Based on the magnetostratigraphic study performed in the synorogenic deposits related to the Cordillera Frontal uplift, these thrust initiated at ca. 9 Ma (Irigoyen et al., 2002). This is immediately after the collision of the Juan Fernández ridge against the trench.
Stop 4-4: Picheuta Rhyolitic Porphyry
This stop is in the middle of the Choiyoi province at these latitudes (Fig. 25). Thick piles, up to 2–4 km thick, of pyro clastic and volcanic rocks of rhyolitic composition represent a widespread extensional period. Several rhyolitic domes north of Río Picheuta are emplaced in the pyroclastic sequences. These domes are recognized by their homogeneous texture and their columnar jointing. At this stop section, most of the ignimbritic and pyroclastic flows are of Triassic age. Similar rhyolitic domes have been dated at 203–205 Ma farther west (Latest Triassic).
Stop 4-5: Polvaredas Normal Fault
This stop provides a good example of the Gondwanian tectonics (location on map in Fig. 25). A normal fault puts the western facies of the Carboniferous deposits in contact with the pyroclastic sequences of the Choiyoi Group. The fault trace was intruded by a rhyolitic dike of Late Triassic age, which excludes an Andean reactivation of the fault. The well-stratified pyroclastic deposits of the Choiyoi Group have been dated here at 235–238 Ma by K-Ar (Pérez and Ramos, 1996), which indicates a Middle Triassic age. Note that the rhyolitic volcanism of this region is synchronous with the alkaline basalts at the Agua de la Zorra (235 Ma). The extensional regime that controlled the Choiyoi rhyolites was coeval with the half graben developed farther to the east in the modern foreland area.
Stop 4-6: San Rafael Orogenic Phase
The angular unconformity that separates the Late Carboniferous and Early Permian turbidites from the Choiyoi Group represents the San Rafael diastrophic phase of Middle Permian age (location of stop on map in Fig. 25). The sedimentary rocks have contact metamorphism produced by granitoids of Early Permian age (ca. 287 Ma). The Cordillera Frontal between Punta de Vacas and the Uspallata valley has a simple Andean structure, characterized by a large uplifted block of basement. All the Late Paleozoic–Triassic rocks of the Cordón del Plata, which is the southern expression of the Cordillera Frontal (see section in Fig. 26), have been passively transported to the east in a detachment located on the Carboniferous shales.
Stop 4-7: Río Colorado Granite
This stop shows the roof of the Lower Permian granite intruded in the Carboniferous sequence (Fig. 25). The granite was first described by González Bonorino (1950b) to illustrate the stopping mechanism in different stag in the emplacement of blocks of Carboniferous rocks. There are blocks partially detached falling into the granite and blocks partially absorbed. There are several other granitic bodies between this stop and Punta de Vacas, all of which are preserved at shallow levels with their roofs partially exposed. These granitoids are subduction-related and were part of the Early Permian magmatic arc.
Stop 4-8: Volcán Tupungato
At this stop (Fig. 25), the Tupungato volcano (6800 m) can be seen looking to the south along the Rio Tupungato. This is the first Quaternary volcano south of the flat-slab segment of the central Andes, and is located ~40 km south of Punta de Vacas. The last activity recorded in this volcano is a pyro clastic flow in its northern slope of 0.7 ± 0.3 Ma. The Tupungatito volcano located nearby the main center is active and has fumaroles. The linear trend of Río Tupungato is also controlled by a Gondwanian fault.
Optional Cultural Stop: Punta de Vacas Shelter
This is one of the four shelters on the Argentine side that are preserved from the late eighteenth century. They were built by the Correos Reales de España (Spanish Royal Mail) in 1765. In this shelter, Charles Darwin overnighted during his crossing to Paso de la Cumbre in 1835 (see Fig. 2, and also Darwin  for a picturesque description of the place).
Stop 4-9: Cruz de Caña Granite
This stop shows a subduction-related granitoid of Early Permian age, emplaced in Late Carboniferous black shales and graywackes, and truncated by extensional faults (Fig. 25). These normal faults do not affect the Middle Jurassic limestones that unconformably overlie the Carboniferous rocks.
DAY 5—FROM PUENTE DEL INCA, ARGENTINA, TO VIÑA DEL MAR, CHILE
One of the outstanding features of the Mesozoic succession is the development of many Mesozoic marine sequences controlled by Pacific transgressions and regressions. Those sequences are grouped into four sedimentary cycles which are separated by regional first-order unconformities. These sequences are clearly depicted in the Neuquén Basin farther south where a Liassic to Neocomian marine retroarc to foreland basin developed is well developed behind the magmatic arc in the eastern foothills of the Cordillera. The Neuquén basin is linked to the north with the Aconcagua basin, which has a different paleogeography with a larger participation of volcanic rocks, but similar stratigraphic cycles in the middle Jurassic to Cretaceous evolution.
There are interesting parallels between the evolution of the basin and the magmatic arc history. The main periods of regional unconformities in the foreland are coincident with the times at which the magmatic arc migrated eastward. The intermittent nature of the magmatic activity as well as the spatial variation of the volcanic front in the Andes appears to be closely related to changes in plate motion controlled by variation in the spreading velocities of the Pacific oceanic ridges. Forearc subduction erosion cannot explain by itself the magnitude of the shifting of the Neogene volcanic front.
Most of the Chilean and the westernmost Argentine Mesozoic basins of the Cordillera Principal are intra-arc basins controlled by the development of two distinctive arcs: an inner, more active arc along the Cordillera de la Costa in Chile, where the main andesitic activity was concentrated, and an outer arc that produced rock suites mainly of andesitic to bimodal composition. Several authors have proposed that an extensional regime in the arc massif region mainly during the Early Cretaceous was responsible for the intra-arc basin development (Fig. 27). This process has been explained by the negative rollback velocity of the South American plate previous to the opening of the South Atlantic Ocean. The intra-arc basins were active until Early Aptian times when an important eastward migration of the main magmatic arc occurred, together with a sea-level lowstand in the retroarc and intra-arc basins and the development of a single and expanded central arc.
The retroarc easternmost basin was exclusively continental from this time on, and the Pacific seas no longer reached the eastern side of the cordillera. This important paleogeographic change is closely linked with the beginning of one of the periods of higher spreading rates within the South Atlantic and the change to a positive rollback velocity of the South American plate.
The maximum sea-floor spreading rate at the Pacific and the South Atlantic spreading centers was reached during the Late Cretaceous (ca. 80–110 Ma) and may be responsible for a new stage in the Andean evolution. At this stage (Fig. 27), a mountain chain was built and the deformation of the strata on the eastern flank of the orogen formed a fold-and-thrust belt. As a consequence, a foreland basin developed at the leading edge of the deformation, due to tectonic loading of the adjacent thrust belt. At the final stage of compression, several granitoid stocks were emplaced in the arc massif.
As a concluding remark on the orogenic cycle, it is necessary to emphasize that the different paleogeographic settings, the major depositional sequences and the successive tectonic regimes seem to be controlled by the subduction dynamics as inferred by the changes in the tectonic regime and from the migration of the volcanic front. The latest change in the subduction geometry was controlled by the docking of the Juan Fernández ridge (Pilger, 1984, and subsequent works).
Stop 5-1: Cerro Penitentes Thrust Front
This is the thrust front of the Aconcagua fold-and-thrust belt (Figs. 28–31). Middle to Late Jurassic limestones override the thick Santa María conglomerates of early to middle Miocene age. These conglomerates unconformably overlie marine and continental Jurassic strata. The Penitentes thrust dips from 50° to 22° to the west. Erosion is almost forming a klippe on the top of Cerro Visera. Figure 29 illustrates the 22° west-dipping footwall ramp that this thrust developed in the Santa María Conglomerates in the southwestern slope of Cerro Penitentes (not seen from the road).
Stop 5-2: Puente del Inca
This is the most classic section of the High Andes. Compare the evolution of thought since Darwin and Schiller by comparing their sections (Figs. 2 and 3) with the present interpretation (Figs. 28 and 31). Figure 31 shows the autochthon represented by Carboniferous hornfels, thin pyroclastic deposits of Choiyoi Group, and Jurassic limestones and conglomerates of proximal facies, and small outcrops of marine Early Cretaceous deposits. The first thrust sheet is composed of Middle to Late Jurassic marine and continental deposits and Early Cretaceous continental to transitional marine deposits. The second thrust sheet is represented by Late Jurassic gypsum at the base and Titho-Neocomian continental and marine strata.
All these sequences are intruded by thick Miocene dikes of trachyte (15 Ma). In the upper thrust plates, basaltic and andesitic lenses can be seen interbedded into the sequence. These volcanic rocks are more abundant in the westernmost thrust sheets.
Stop 5-3: Rio Horcones and Cerro Aconcagua
Continue west along National Highway 7 heading west until the Parque Provincial Aconcagua. At this stop (Figs. 30 and 32), there is a magnificent view of the Pared Sur (south wall) of Cerro Aconcagua (6967 m asl). The wall is formed by volcanic and breccia flows of andesitic composition of the Aconcagua Vol canic Complex (15–9 Ma, Ramos and Yrigoyen, 1987; Godoy et al., 1988). On the western side of the valley, there is an imbrication of Jurassic continental red beds and Early Cretaceous limestones. On the eastern side, the diapiric effects of the Late Jurassic gypsum of the Auquilco Formation produced the complex structure of Cerro Panta. To the south, the imbrication of the second to fourth thrust sheet repeating the different Titho-Neocomian units can be observed.
Stop 5-4: Quebrada Navarro
At this altitude, the Quebrada de Navarro thrust, which dips more than 80° to the west can be seen (Fig. 32). This is the result of the rotation of the different thrust sheets. It was previously interpreted as an original high angle fault. Based on that interpretation, Zeil (1979) characterized the Andes as being formed by typical upthrusts (high-angle thrusts). A detailed examination of the cut-off angles in the different thrusts points out to lower angles (~20°–22° at maximum).
Optional Cultural Stop: Paramillos de las Cuevas Shelter
An optional stop can be made to visit a colonial shelter built in 1765. The Spanish Royal Mail constructed those shelters to make it possible to cross the Main Andes during the winter, because at that time it was very unsafe to cross the Magellan strait due to the war with the British. The Paramillo de las Cuevas shelter (“casucha”) was built by the Spaniards as one of eight shelters across the Andes. It has a prolific history with many outstanding visitors in the nineteenth century, such as D.F. Sarmiento and the San Martin army during the cross of the Andes during the independence times. Several famous travelers described the penuries related to the winter and the snow storms and strong winds (see Darwin stories in his famous trip across the Andes in 1835).
Stop 5-5: Las Cuevas Rock Avalanche
The Neocomian limestones in the thrust seen here override Late Jurassic–Early Cretaceous red beds. The thickness of the continental and volcaniclastic deposits exceeds several times the normal thickness of these units, indicating their proximity to the volcanic arc. The limestones bear Olcostephanus sp. of Late Valanginian age (Aguirre-Urreta and Rawson, 1997). A spectacular rock avalanche was produced from one of the volcaniclastic members of the Mesozoic deposits.
Stop 5-6: Matienzo Valley (Argentina)
A complex out-of-sequence thrust folded and thrust the red beds and volcaniclastic rocks of the Early Cretaceous Cristo Redentor and Juncal Formations over the Neocomian limestones. The thick pile of Early Cretaceous rocks is deformed in an anticline structure. To the north, along the Cordillera del Límite, interbedded limestones and volcanics are representing the Neocomian deposits in the westernmost thrust sheet.
The Cordillera Principal at the Chilean Slope
The drive after crossing the international tunnel to Chile along the highway in the direction of the city of Viña del Mar will show the western slope of the Andes. The Cordillera Principal corresponds to the highest Andean peaks along the Chile-Argentina border. The Chilean side (see Figs. 26 and 33) is made up of gently folded Mesozoic (Early Cretaceous) and Tertiary (Paleogene and Neogene) volcano sedimentary formations intruded by Tertiary (Miocene) granodioritic stocks. The whole sequence is thrust eastward over the Jurassic–Early Cretaceous sedimentary formations of the Aconcagua fold-and-thrust belt between Las Cuevas and Puente del Inca. The Pocuro fault (Fig. 26) is a major structural discontinuity that separates the Cordillera Principal from the Cordillera de la Costa.
The main components of the geological history of the Chilean part (Fig. 33) of this segment of the Andes (Segment B from 27° to 33°S of Mpodozis and Ramos, 1990) are as follows: (1) emplacement along the coast of the Pichidangui suspect terrane; (2) development in the Jurassic of a subduction related magmatic arc (Cordillera de la Costa) and of a retroarc sedimentary sequence toward the foreland (Aconcagua platform); (3) during the Early Cretaceous, a large subsiding volcanic zone, the Central Chile “aborted” marginal basin or intra-arc basin that formed behind the inner arc; (4) this basin tectonically collapsed during the Late Cretaceous; (5) in the Cretaceous-Tertiary, the area witnessed a progressively eastward migration of magmatic activity and deformation; and (6) magmatism vanished in the Late Miocene as a consequence of the shallowing of the subduction angle.
The Pichidangui terrane of unknown size seems to have been emplaced by margin-parallel, strike-slip motion in the Late Triassic–Early Jurassic (Forsythe et al., 1986). Strike-slip motion along the Chilean coast was accompanied in Argentina, by the development of Triassic rift basins (Ramos and Kay, 1991). They succeeded Choiyoi magmatism at the end of the Gondwanide history of this Andean segment.
The oldest in situ post-Pichidangui sequences in the Coast Cordillera are the Ajial and Cerro Calera Formations that consist chiefly of rhyolitic and pyroclastic rocks. They erupted during an interval of Early Jurassic volcanism (Piracés, 1977). Volcanic eruptions were partly submarine as indicated by the presence of interbedded Bajocian limestones (Fig. 32). At the same time, large gabbroic to granitic plutons were emplaced in the coastal region (Rivano et al., 1985). These plutons have yielded K/Ar ages ranging from 191 to 138 Ma (Munizaga and Vicente, 1982; Rivano et al., 1985). In the retroarc region to the east, a Liassic to Dogger marine transgression initiated as a clastic-carbonate platform that developed over the Late Paleozoic igneous basement of the Cordillera Frontal.
Uplift during the Late Jurassic caused produced subaerial conditions, which are indicated by the Horqueta Formation in the Cordillera de la Costa (Piracés, 1977). In the retroarc region, an Oxfordian marine regression is suggested by thick evaporite units such as the Auquilco gypsum. These units are overlain by upper Jurassic continental red beds that were derived from erosion of the volcanic rocks of the Cordillera de la Costa.
The sea advanced over the Aconcagua platform during the Early Cretaceous (Ramos, 1985a, 1985b). This Early Cretaceous event was followed by: (a) a noticeable decrease of plutonism in the Cordillera de la Costa, where only small stocks are found just to the east of the Jurassic magmatic belt (Nasi, 1984; Rivano and Sepúlveda, 1986); (b) a progressive volcanic change from andesites to basalts, interbedded with marine carbonate rocks (Lo Prado and Veta Negra Formations) having low 87Sr/86Sr initial ratios (Piracés, 1976; Levi and Aguirre, 1981; Aberg et al., 1984); and (c) eastward expansion of volcanism into the Aconcagua platform in which the Early Cretaceous Los Pelambres Group lavas interfinger with Neocomian limestones along the Chilean-Argentine border. Aberg et al. (1984) and Levi and Aguirre (1981) interpreted the widespread Early Cretaceous volcanic activity as taking place in a short-lived “aborted” marginal basin of Hauterivian-Albian age. The term aborted was used by Aberg et al. (1984) to denote a zone where large volumes of basalts and andesites were erupted through a thin, attenuated continental crust which did not evolve into an oceanic crust floored basin. In central Chile, a high thermal gradient associated with subsidence caused pervasive “burial” metamorphism of the volcanic pile. At the end of the Early Cretaceous, the basin was filled with coarse red conglomerates interbedded with lavas and limestones (Las Chilcas Formation, Rivano et al., 1985).
Sometime during the Late Cretaceous, the basin collapsed and subsequent deformation led to the beginning of the thrusting of the Cretaceous volcanic pile toward the east over the sediments of the Aconcagua platform. Large Late Cretaceous batholiths were emplaced along the axis of the volcano-tectonic rift. Despite the lack of a clear record of Late Cretaceous sediments or volcanism, a belt of Paleocene granitoids was emplaced along the western foothills of the Cordillera Principal. An even younger belt (Miocene) of granodioritic stocks intruded farther to the east, linked to the Miocene volcanics of the Farellones Formation (Mpodozis and Ramos, 1990). This unit, which overlies the Early Cretaceous volcanics of the Pelambres Group, represents the youngest volcanism recorded in the Cordillera Principal. Miocene shallowing of the subduction angle resulted in the shut-off of the magmatism (Jordan et al., 1983a, 1983b; Kay et al., 1987) and beginning of deformation in the Pre cor dillera and Sierras Pampeanas.
The route in Chile (Figs. 26 and 33) goes down the Aconcagua valley crossing the Cordillera Principal, the Pocuro fault, and the Cordillera de la Costa (Portillo to Los Andes to San Felipe), and then heads to La Calera and ends at Concón and Viña del Mar along the Late Paleozoic to Jurassic coastal batholith in the Papudo-Valparaíso area. The stops locations are shown on Figure 33.
Stop 5-7: Laguna del Inca
View to the north of the lake (Laguna del Inca) and the steeply dipping strata of the Juncal Formation (infill of the Early Cretaceous central Chile intra-arc basin) along the international boundary between Chile and Argentina. On the eastern side, Neocomian limestones are interbedded with volcaniclastic and volcanic sequences of the Pelambres Complex (Morata and Aguirre, 2003). Farther to the west, a Miocene diorite intrudes the Juncal Formation. To the south, the Alto de Juncal anticline can be seen. This large structure affects the Early Cretaceous volcanic upper plate of the Aconcagua fold-and-thrust belt. Large-scale folds are typical of this region (Fig. 33), but the intensity of folding decreases rapidly toward the west.
Stop 5-8: Los Caracoles
The Juncal Formation intruded by sigmoidal dikes and sills can be seen to the west at this stop. These dikes of dacitic composition were emplaced during Miocene thrusting (Godoy et al., 1999). Note the typical U shape of the glacial valley of the upper Río Aconcagua.
Stop 5-9: Río Aconcagua Valley
This stop shows a Miocene granodiorite emplaced into the Tertiary Abanico Formation (Charrier et al., 2005). The narrow gorge at Salto del Soldado is the result of a massive slide from the northeast. The stop provides an impression of the thick pile of andesitic flows, pyroclastic breccias, and tuffs that constitute the Abanico Formation that is mainly Oligocene in age. This unit has been deposited in a volcanotectonic rift episode (Godoy et al., 1999).
Stop 5-10: San Felipe Quarry
Several quarries exploit algal (lagoonal?) limestones that are interfingered with the conglomerates of the Early Cretaceous Las Chilcas Formation. These limestones have typical palynomorphs of Neocomian age that can be correlated with the marine Neocomian limestones of the retroarc basin (Arévalo, 1992). A reverse fault and a related drag fold affect the sedimentary strata of Las Chilcas Formation, which is intruded by a 96 Ma granite. This unit is probably an age-equivalent of the Pelambres Complex.
The Cordillera de la Costa
The basement of the Cordillera de la Costa is formed by a displaced terrane (Pichidangui terrane). It was probably emplaced by margin-parallel, strike-slip movements in the Lower Jurassic. The basement is covered by a thick, autochthonous, east-dipping Mesozoic volcano-sedimentary terrane, and intruded by Jurassic and Cretaceous batholiths (Gana and Wall, 1997; Gana and Zentilli, 2000). The Central valley that separates the Cordillera de la Costa from the Cordillera Principal in much of Chile is absent at this latitude (33°S) and over the flat-slab segment to the north. Together, with the lack of active volcanism, these morphological attributes are typical of the modern “flat-slab” region of the Chile-Argentine Andes which extends between 27° and 33°S.
Stop 5-11: Ocoa Quarry
East-dipping outcrops of andesites (Ocoa member) of the Veta Negra Formation (Lower Cretaceous). The local term “ocoite” is used for a rock type with phenocrysts of plagioclase exceeding 2 cm in length. Radiometric dating (Rb/Sr) yielded an age of 105 Ma for this unit (Rivano et al., 1985; Morata and Aguirre, 2003). The Veta Negra Formation is the main volcanic (Aptian-Albian) unit of the Early Cretaceous central Chile volcano-tectonic rift. Note the association of alteration minerals (epidote-chlorite-actinolite-zeolite-albite). Originally attributed to “burial metamorphism,” this assemblage may represent the effects of low-temperature hydrothermal fluids circulating in the Early Cretaceous extensional environment.
Stop 5-12: Papudo
Gneissic tonalites (157–170 Ma) of the Jurassic Coastal Batholith are the oldest of the eastward younging series of “Andean” batholiths. Paleomagnetic studies have found no evidence of large displacement for this or younger rocks in the Aconcagua region.
Stop 5-13: Coast between Concón and Valparaíso
Late Carboniferous tonalites (299 ± 31 Ma, Rb/Sr) in a narrow belt intruded to the east by Jurassic granitoids. K/Ar ages have been reset to the Jurassic (Hervé et al., 2000).
Figures & Tables
Field Trip Guides to the Backbone of the Americas in the Southern and Central Andes: Ridge Collision, Shallow Subduction, and Plateau Uplift
The geologic field guides in this volume to the Andes of Argentina and Chile were written for the five field trips accompanying the 2006 Backbone of the Americas conference in Mendoza, Argentina, which was sponsored by the Geological Society of America and the Asociación Geológica Argentina. The meeting was organized around three processes influential in the evolution of the western margin and cordilleras of the Americas—ridge collision, shallowing and steepening subduction zones, and plateau and orogenic uplift. Designed for use in the office or the field, the field guides are to regions that highlight these themes and present up-to-date overviews with references. The trip in chapter 1 to southern Patagonia highlights the ridge-trench collision theme; the next three to different regions of the south-central Andes examine temporal and spatial issues related to shallowing subduction; and the trip in the last chapter to the central Andean Puna plateau highlights plateau uplift in the context of steepening subduction and lithospheric delamination.