Field trip guide: Andean Cordillera and backarc of the south-central Andes (~38.5°S to 37°S)
Published:January 01, 2008
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Tomás Zapata, Gonzalo Zamora Valcarce, Andrés Folguera, Daniel Yagupsky, 2008. "Field trip guide: Andean Cordillera and backarc of the south-central Andes (~38.5°S to 37°S)", 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
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The Andes of the Neuquén Mesozoic basin have experienced multiple episodic tectonic events as a consequence of the changes of the plate tectonic boundary configuration. Each episode of deformation has overprinted the previous one, making it difficult to unravel the Andean tectonic history. The first deformation event took place in the uppermost Cretaceous with the formation of the Agrio fold-and-thrust belt. This event was related to the shallowing of the subducting plate recorded by the migration of the volcanic arc toward the foreland. During the late Oligocene–early Miocene, an extensional event, related to the steepening of the subducted plate, affected only the hinterland region causing the opening of the Cura Mallín basin. This basin was closed during the late Miocene, together with the development of a new fold-and-thrust belt that reactivated the previous structures. During the Late Tertiary, two more episodes of extension and compression affected the Andean area. The trip focuses on the field evidence that documents this complex history of evolution by looking at evidence of the sequence of the distinct tectonic events.
The field trip covers the westernmost part of the Neuquén province in Argentina (Fig. 1). Geologically, the trip takes place in the fold-and-thrust belt of the Neuquén Basin between 36° and 38°S on the eastern slope of the Andes. The aim of the trip is to visit key localities where observations have been made in recent years that have improved our understanding of the history of the region. The area has undergone a complex tectonic evolution that is suggested to be linked to changes in the dip of the underlying subduction zone. These slab dip changes have led to the migration and retreat of the volcanic arc and the deformation front. The field guide focuses on the evidence for the Mesozoic to Recent deformational events that affected this part of the Andes.
Brief Review of the History of Geologic Exploration and Research
The first geologic descriptions of the region by Bodenbender in 1891 were of the volcanic rocks of the Cerro Tromen region, which is located just east of the city of Chos Malal (Fig. 1). Evidence for a young eruption was reported. The first geologic transect through this part of the Andes was made by Burckhardt in 1900 (Fig. 2), who crossed the Andes through the Pino Hachado Pass and described the main morphostructural units. In the first half of the last century, a large number of geologists studied the sedimentary sequences (e.g., Gerth, 1928; Weaver, 1931; Groeber, 1929, 1946a, 1946b). In 1946, Herrero Ducloux produced a synthesis of the geology of Neuquén . During the 1970s and 1980s, the area was intensively explored by YPF geologists (the former Argentine state petroleum company) leading to advancements in the understanding of the stratigraphic and structural framework of the region (e.g., Gulisano et al., 1984; Legarreta and Gulisano, 1989). Ramos (1978, 1981, 1998) described the structure of the Agrio fold-and-thrust belt and showed that the belt consisted of an inner western sector characterized by basement inversion and an eastern outer sector characterized by thin-skinned deformation. Llambías and Malvicini (1978) and Llambías and Rapela (1987, 1989) studied the igneous rocks. They dated and described the Collipilli igneous rocks, associating them with different volcanic provinces. Recent studies (Zapata et al., 2002; Zapata and Folguera, 2005; Zamora Valcarce et al., 2006) demonstrate structural inversion of the basement at the frontal part of the fold-and-thrust belt. Recent 40Ar/39Ar ages have shown that this deformation has a minimum age of Cretaceous.
The transect of the main cordillera covered by this field guide has been studied more recently. In the 1960s to the 1980s, there was a tendency for researchers working in the area to correlate the sedimentary and volcaniclastic sequences in the Andes of Neuquén with units outcropping to the east in the Agrio fold-and-thrust belt and to the south along the northern Patagonian Andes. Researchers such as Pesce (1981) mistakenly assigned Paleogene volcaniclastic units in the Neuquén Andes to a Middle Cretaceous volcanic pulse that is broadly exhibited in the northern Patagonian Andes. Nonmarine sedimentary facies inter bedded with these volcanic rocks were correlated with Late Jurassic to Early Cretaceous marine sequences of the Neuquén Basin (Zanettini et al., 1987). Gutiérrez and Miniti (1985) could be credited with the first high-quality study of the Cenozoic Cura Mallín basin within the study area. At the same time, a considerable number of radiometric ages were obtained on the western slope of the Andes between 36° and 38°S in Chile. These ages show that the vol canic units and associated sedimentary facies in Chile accumulated in a late Oligocene–early Miocene basin (Niemeyer and Muñoz, 1983). The studies of Burns and Jordan (1999) and Jordan et al. (2001) subsequently showed that the sequences on the eastern slope of the Andes were of similar Tertiary age, and have been tectonically inverted more recently.
The Andes in this region (36°–38°S latitude) show distinctive deformational characteristics that result from alternation of periods of generalized extension followed by periods of compression. The Agrio fold-and-thrust belt in the eastern part reflects these processes (Fig. 3). Deformation in the Agrio belt started during the Cretaceous and was reactivated in the late middle to late Miocene. The Loncopué trough to the west of the Agrio belt (Fig. 3) is a long depression in the foothills that parallel the Principal Cordillera. The Loncopué trough consists of a complex half-graben system that was produced during the Oligocene and extensionally reactivated in the Pliocene-Pleistocene. The northern part of the Loncopué trough is currently located in the contractional orogenic front.
The nature and volume of arc-related igneous rocks, the location of the volcanic fronts, expansions and retreats of the magmatism, and the associated igneous activity in the foreland and superimposed structural styles provide evidence for the alternating tectonic regimes. The various Jurassic to Recent stages are correlated with changes in the geometry of the Benioff zone. Periods of subduction-zone steepening are associated with large volumes of poorly evolved magmas and generalized extension, whereas shallowing of the subduction zone is linked to foreland migration of more evolved magmas associated with contraction and uplift in the Principal Cordillera.
The purpose of this field guide is to show, in chronological order, the evidence for the complex evolutionary history of the Neuquén Andes at this latitude. The first day of the trip travels from Neuquén city to Chos Malal with a few stops to recognize the structural styles. The second and third days are in the Agrio fold-and-thrust belt looking at the evidence for the Cretaceous and Miocene deformation. The fourth day is in the region west of the Agrio fold-and-thrust belt in the foothills of the modern orogenic front. The stops on this day focus on the late Oligocene to early Miocene extensional collapse of the inner sectors of the Agrio fold-and-thrust belt, and the subsequent late Miocene compressional inversion.
The Neuquén Basin is a triangular-shaped basin that contains a Mesozoic-Cenozoic sedimentary succession that is at least 7 km thick. The tectonic evolution of the basin records a continuous subsidence lasting at least 220 million years that extends from the Early Jurassic to the end of the Early Cretaceous. During this time the region was a marine retroarc basin (Ramos, 1989; Vergani et al., 1995). A stratigraphic column summarizing the major units in the region is shown in Figure 4. As a result of the Andean deformation that progressed from west to east, the basin became a foreland basin at the beginning of the Late Cretaceous. Vergani et al. (1995) summarized the main tectonic elements of the basin (Fig. 5) that include a north-south fold-and-thrust belt in the west (in the region of the field trip), the east-west–running Huincul Arch, and an inner basin separated from the fold-and-thrust belt by a structural high that corresponds to the Chihuidos High and a frontal syncline.
The basement (pre-Jurassic) underlying the Neuquén Basin is composed of a rhyolite-ignimbrite suite (Choiyoi Group) of enormous proportions associated with extensional tectonics and molasse deposition (Ramos and Kay, 1992). Uliana and Legarreta (1993) associated these sequences with the extensional collapse of a Permian-Triassic orogenic belt.
From the Late Triassic through the Early Jurassic, basement-involved extensional faults created a series of half grabens (Vergani et al., 1995) that formed the initial Neuquén Basin. During this stage, the pre-Cuyo and Cuyo Groups were deposited in the developing half grabens (Fig. 4). This period of rifting ended with the deposition of the Tábanos Formation evaporites. During the Late Jurassic, general uplift and erosion affected much of the Neuquén Basin and marked a conspicuous change in the sedimentation style from clastic and volcaniclastic deposits to the evaporite-limestone–dominated deposits of the Lotena Group. The end of this phase coincides with a period of regional subsidence that Vergani et al. (1995) attributed to compressional relaxation.
Later deposition of the Mendoza Group (Fig. 4) in the Andico cycle reflects marine encroachment and expansion of the basin (Gulisano et al., 1984). The Mendoza Group starts with fluvial sandstones of the Tordillo Formation, which are overlain by marine shales of the Vaca Muerta Formation that are the most prolific source rock of the basin. This cycle also shows structural inversion events that interrupted marine sedimentation and caused the erosion of older units in the southern and eastern margins of the basin (Vergani et al., 1995). Next is the Mulichinco Formation. Tectonic quiescence resulted in renewed marine transgression and deposition of the Agrio Formation. General shallowing of the basin and evaporitic and clastic sedimentation (Rayoso Group, Fig. 4) marks the culmination of the Andico cycle. Renewed tectonic activity and inversion during the Early Cenomanian reactivated the provenance regions and resulted in accumulation of the continental deposits of the Neuquén Group. This sedimentation persisted until the end of the Cretaceous, when the continental and marine deposits of the Malargüe Group marked the culmination of the Riográndico Cycle (Fig. 4) (Legarreta and Gulisano, 1989).
In late Cretaceous, the Neuquén Basin entered a new phase of deformation and basin subsidence. The north-trending Andean fold-and-thrust belt encroached on the western margin of the basin deforming the Mesozoic prism in front of it. Preexisting structures, as well as the mechanical properties of the Mesozoic sequence, controlled the deformation style in this belt.
Agrio Fold-and-Thrust Belt
The Agrio fold-and-thrust belt is located south of the Corta deras Lineament. The belt is bounded to the east by the Los Chihuidos high and to the west by the Loncopué trough. The structure of the Agrio fold-and-thrust belt is characterized by a combination of thin-skinned and thick-skinned structures (Fig. 6), which show good examples of detachment folds. The geologic sections show north-northwest–trending structures with a difference in structural style between the western and eastern parts. To the west, the structure is characterized by thick-skinned structures with broad anticlines, which are the product of basement inversion (Ramos, 1998). To the east, the Agrio fold-and-thrust belt is composed of large doubly-plunging anticlines detached in the Auquilco evaporites. The anticlines are separated by broad synclinoria with general rhombic shapes that reflect the existence of basement blocks at depth (Ramos, 1978; Viñes, 1985; Zapata et al., 1999; Zapata et al., 2002). Due to the differences in structural style, the Agrio fold-and-thrust belt has been divided into two regions (Fig. 6). The western part or inner sector exposes folds related to the inversion of a Mesozoic extensional structure called the Tres Chorros extensional system (Vergani et al., 1995). The eastern or outer sector (Ramos, 1977; Ramos and Barbieri, 1989) is composed of tight, axially extended anticlines that bound deep basement blocks (Zapata et al., 2002). This part of the fold-and-thrust belt has experienced several episodes of deformation from the late Cretaceous to the Miocene (Zapata et al., 2002, 2005; Cobbold and Rosello, 2003; Zamora Valcarce et al., 2006), which are recorded by volcanic rocks and syn orogenic deposits. In the last compressional pulse, which could have begun in the middle Miocene, the whole fold-and-thrust belt was thrust toward the foreland. The thrusts probably used the preexisting Jurassic detachment (Zapata et al., 2002) that was inherited from the extensional period of the Neuquén Basin. The related Miocene synorogenic deposits of the Agrio fold-and-thrust belt are buried in the Bajo de Añelo area to the east of the field trip region (Ramos, 1999).
Structure of the Inner Sector
The Inner Sector of the Agrio fold-and-thrust belt includes the southern extension of the Cordillera del Viento basement uplift. This uplift belongs to the Tres Chorros extensional system, which is composed of northwest-trending, inverted half grabens forming broad anticlines. One structure is the Cerro Mocho anticline, a doubly vergent basement uplift that resembles a “pop-up” structure. Two-dimensional seismic data (Fig. 7) show that the eastern limb of the structure is affected by a deep fault that cuts across the sedimentary sequence through the Auquilco Jurassic evaporites and transfers >6 km of shortening to the thin-skinned structures of the Outer Sector (Zapata et al., 2002). An anomalous thickness of Jurassic synrift sequences recorded in borehole data is interpreted to be associated with an extensional half graben that was inverted during the Andean orogeny.
Structure of the Outer Sector
The structure of the Outer Sector is composed of thin-skinned tight folds associated with deep faults. Borehole and two-dimensional seismic data show that the deep faults are detached from the Jurassic Auquilco evaporites and propagate up through the Mesozoic sequence (lower and upper Mendoza Group) until they reach the Cretaceous Huitrín evaporites (Rayoso Group) where they form fault-bend fold structures (Fig. 6). The upper stratigraphic units of these structures, which correspond to the Agrio Formation (upper Mendoza Group), have been locally deformed by flexural folding adding a detachment folding component (Zapata et al., 2002; Zamora Valcarce, 2007). Seismic information also shows that the external structure of the Agrio fold-and-thrust belt is characterized by a refolded triangle zone bounded on the eastern side by a backthrust that produces a fault-related fold geometry (Zapata et al., 2002; Zamora Valcarce, 2007).
DAY 1—TRIP FROM THE CITY OF NEUQUÉN TO CHOS MALAL
The stops on Day 1 provide an introduction to the stratigraphy and structural style of the external zone of the Agrio fold-and-thrust belt in this region of the Andes (Fig. 8).
Stop 1: Rio Agrio Anticline
Location: Agrio River near town of Bajada del Agrio at 38°20′47″S; 69°58′3″W; 607 m.
The Agrio anticline is located at the thrust front zone of the southern termination of the Agrio fold-and-thrust belt. At first glance, the geometry can be described as a simple, doubly plunging structure (Fig. 9) with a north-south axis characterized by a smooth crest and steeply dipping flanks with dips up to 40°–60°. The anticlinal limbs are part of broad synclines that separate the Agrio anticline from the unexposed Quili Malal–Esquinero anticline to the east and Cordón del Salado anticline to the west. The core of the anticline, where the Agrio Formation crops out, is affected by a series of strike-slip and minor thrust faults that offset the key bed horizon of the Avilé Member. These structures are interpreted as part of the crestal extension due to the buckling process of folding.
This is also an historical stop, since the first well of the Agrio fold-and-thrust belt, the RA.x-1, was drilled here in 1935 by the Standard Oil Company. The geological studies that led to the drilling proposal were based on the presence of oil seeps, following the classical “anticline” play (surface anticline with associated oil seeps). Later, in 1949, YPF (the former Argentine national oil company) re-drilled the play reaching a final depth of 3220 m below ground level and ending in the Jurassic Tordillo Formation. The results in both cases were dry holes.
Stop 2: Cordón del Salado
Location: 38°16′10″S; 70°03′28″W; 758 m.
The Cerro La Mula–Naunanuco anticline, which is the longest structure of the Agrio fold-and-thrust belt, extends for more than 60 km. The structure is an axially extended, doubly plunging anticline, whose axis is broken by lateral ramps that may accommodate local differences in shortening, acting as transfer zones. The oldest unit exposed at the core of the anticline is the Vaca Muerta Formation. The outcrop pattern shows that these are geometrically complex structures that result from multi ple décollement horizons producing internal disharmonies (Fig. 10). The anticline has steeply dipping to overturned flanks, which are affected by minor thrusting and back thrusts that accommodate the internal layer-parallel shear deformation.
Stop 3: Pampa del Salado, Road to Chos Malal
Location: 38°01′53″S; 70°02′54″W; 993 m.
The purpose of Stop 3 is to review the Lower Cretaceous stratigraphy of the upper section of the Agrio Formation and the individual members of the Huitrín Formation. The Huitrín Formation is divided into three members: (1) Lower Troncoso, which is composed of a lower fluvial and an upper eolian section; (2) Upper Troncoso, which is constituted by evaporites; and (3) La Tosca Member, which is formed by interbedded limestones and shales.
DAY 2—CRETACEOUS DEFORMATION AND THE VOLCANIC ARC
For the next three days, evidence for the different deformational events affecting the Neuquén Andes at this latitude will be examined (Fig. 11). The stops on Day 2 focus on the inner part of the Agrio fold-and-thrust belt where evidence for the Cretaceous deformation event is displayed. Evidence for the timing of this deformation comes from 40Ar/39Ar ages (Zamora Valcarce et al., 2006), crosscutting relationships, and paleomagnetic data.
This region is partially covered by the Upper Cretaceous–Paleocene Collipilli volcanic field. Magmatic rocks in the Collipilli consist of volcanic facies that are associated with cones, lava flows, dikes, sills, and laccoliths. Llambías and Malvicini (1978) were the first to describe these volcanic rocks. Later, Llambías and Rapela (1987) included them in the Neuquén-Mendoza Volcanic Province, which encompasses the units between 38°30′S and 34°S (Groeber, 1946a, 1946b; Yrigoyen, 1972; Bettini, 1982; Kozlowski et al., 1987; Haller et al., 1985). The intrusive series of the Collipilli Formation was emplaced as laccoliths and associated sills. These laccoliths intruded the contact between the Agrio and the Rayoso Formations, filling the spaces left by the evaporites of the Huitrín Formation during the folding process (Llambías and Malvicini , 1978; Llambías and Rapela, 1989). The volcanic rocks in the Collipilli area include extrusive domes, different types of breccias and vol canic agglomerates, pyroclastic flow deposits, and massive lava flows. These volcanic rocks unconformably cover the Agrio, Huitrín, and Rayoso Formations that are folded in the Collipilli syncline. Therefore, the area was already uplifted (and partially eroded) before or at least simultaneously with the magmatic event (Zamora Valcarce et al., 2006).
Llambías and Rapela (1987, 1989) used geochemical analy ses and K/Ar whole-rock ages to correlate the Collipilli region magmatic rocks with Paleogene units mapped in the Molle Formation in the Andean Cordillera. They proposed subdividing the Molle Group into two formations: (1) a sub-volcanic facies called the Collipilli Formation, with ages ranging from 50 to 45 Ma (early and middle Eocene) (Llambías and Rapela, 1989), and (2) a middle Eocene volcanic facies called the Cayanta Formation (Rapela and Llambías, 1985) with one K/Ar age of 39 ± 9 Ma (Llambías and Rapela, 1989). They noted that the volcanic rocks mapped in the Molle Formation to the north had yielded K/Ar whole-rock ages of 71.5 ± 5 Ma (Llambías et al., 1978), showing that not all of the volcanic rocks in the Molle Formation could be of Eocene age.
Recently, Zamora Valcarce et al. (2006) used new 40Ar/39Ar ages ranging from 73 to 65 Ma from the Collipilli volcanic sections to reassign these magmatic rocks to the end of the Cretaceous. These authors also found evidence for a previous magmatic event, not previously recorded, with 40Ar/39Ar ages of 100 Ma (see below). Geochemical studies (Zamora Valcarce et al., 2006) show a strong arc to backarc signature (La/Ta = 66; Ba/La = 21; Ta/Hf = 0.10) and relatively flat rare earth element (REE) patterns (La/Yb = 10; La/Sm = 5.5; Sm/Yb = 2.2). The subvolcanic rocks of the Cerro Naunauco area are less evolved than those from the Collipilli region. Even though the igneous rocks from Collipilli and the Cerro Naunauco localities show different degrees of differentiation, the trace elements and incompatible rare earths indicate a similar source.
Stop 1: Cerro Naunauco Laccolith
Location: Near the town of Naunauco at 37°38′44″S; 70°10′11″W; 1100 m.
The Cerro Naunauco is a laccolith that intruded into the northern termination of the La Mula–Naunauco anticline (Fig. 11) between the contact of the Agrio Formation and Huitrín Formation (Fig. 12). The rocks at this stop show an unconformable relationship between the igneous units of the Naunauco laccolith and the sedimentary rocks of the Mendoza Group. The new 40Ar/39Ar dates recorded for the Naunauco laccolith (Zamora Valcarce et al., 2006; Zamora Valcarce, 2007) have given an age of 65.50 ± 0.46 Ma. The crosscutting relationship puts an Upper Cretaceous age constraint on this deformational event.
Stop 2: Sills of the Collipilli Formation
Location: East of the town of Tralalhué at 37°39′14″S; 70°14′47″W; 1068 m.
A sill dated at 56 Ma cuts the backlimb of the La Mula–Naunauco anticline at this stop (Zamora Valcarce et al., 2006). This sill, which belongs to the Collipilli Formation, intrudes the Agrio Formation that dips ~65° to the west on this limb of the structure. Paleomagnetic studies permit the structural evolution at the time of the sill intrusion to be reconstructed (Fig. 13). When the sill is restored to horizontal, the attitude of the paleopole differs from the regional Paleocene pole (Fig. 13). To match this difference, the sedimentary sequence needs to be tilted 25° to the west to restore its position prior to the intrusion (Fig. 13). This shows that the backlimb of the La Mula–Naunauco structure already existed in the Paleocene at 56 Ma, and that the sill intruded an already deformed sequence (Zamora Valcarce, 2007; Zamora Valcarce et al., 2007). A subsequent event tilted the stratigraphic sequence, together with the sill, to the present 65° dip.
Stop 3: Synorogenic Tralalhué Conglomerate
Location: East of the town of Tralalhué at 37°37′53″S; 70°16′45″W; 1113 m.
The Tralalhué conglomerate was first described by Ramos (1998) to describe conglomerate deposits on the western flank of the Cerro Naunauco anticline. These deposits have andesite, sandstone, and limestone clasts. Internally, the sequence is composed of several 1–2 m thick cycles that both thicken and thin upward, and show an upward decrease in grain size. The cycles have erosive bases with stacked channel patterns.
The conglomerates overlie an angular unconformity of 18°–20° on the Naunauco igneous rocks and onlap the Rayoso Formation (Fig. 14). Restoring the Tralalhué conglomerate to horizontal shows that the Rayoso Formation was deformed at the time of Tralalhué deposition (Zamora Valcarce, 2007; Zamora Valcarce et al., 2007). These observations are in accord with those at Stop 2 (Fig. 13) where the Collipilli sills intrude a deformed sedimentary sequence (Agrio Formation) with a similar dip. Farther west, the Tralalhué deposits dip 8°–12° west, and in the western border of the piggy-back basin, the dip is up to 70° east.
A late Miocene age for the Tralalhué conglomerate is based on mammal fossil remains (Repol et al., 2002). The clastic composition of the conglomerate records an unroofing sequence related to the uplift of the La Mula–Naunauco anticline.
Stop 4: Panoramic View of the Collipilli Volcanic Arc
Location: Near the town of Collipilli at 37°45′37″S; 70°19′26″W; 1108 m.
Stop 4 shows a panoramic view of the expanse of the Collipilli volcanic field. The main components of the Collipilli volcanic field are extrusive domes and laccoliths like the Cerro del León. 40Ar/39Ar ages (Zamora Valcarce et al., 2006) indicate these rocks are upper Cretaceous to lower Paleocene. The Collipilli volcanic field unconformably overlies Upper Cretaceous units that form a broad synclinorium called the Collipilli syncline. The ages and crosscutting relationships impose a minimum age of Late Cretaceous for one of the deformational events. The geochemical signatures of the volcanic rocks show clear arc-like characteristics (Zamora Valcarce et al., 2006) (La/Ta = 49; Ba/La = 223; Ta/Hf = 0.10) and relatively flat REE patterns (La/Yb = 12.2; La/Sm = 2.26; Sm/Yb = 5.39).
Stop 5: Extrusive Rocks
Location: Near the town of Collipilli at 37°45′37″S; 70°19′26″W; 1108 m.
A typical volcanic succession of the Collipilli Formation (as defined at Stop 2) is exposed at Stop 5 (Fig. 15). Walking from the road to the small cone in a southeast direction, a series of fluid lava flows with intraclasts can be observed. A few volcanic bombs and clastic lava flows are exposed in the middle part of the section. Proximal volcanic agglomerate crops out around the extrusive dome.
Stop 6: Unconformity of the Collipilli Formation (as Defined at Stop 2)
Location: Next to the town of Collipilli at 37°46′19″S; 70°20′49″W; 1145 m.
This stop on the western limb of the Collipilli syn clinorium shows the unconformity between the Collipilli Formation (as defined at Stop 2) and the Mendoza Group. To the north, the magmatic rocks overlie the Huitrín Formation, whereas to the south, they overlie the Huitrín and the Rayoso Formations (Fig. 15). The Collipilli igneous rocks are restricted to the syncline. The difference in erosional characteristics has produced an inversion of relief in the region because the syncline is at a higher elevation than the anticline.
Stop 7: Cerro Mocho Dikes
Location: Two stops close to each other. The first is next to Cerro Mocho, east of Coihuico at 38°04′39″S; 70°14′01″W; 1241 m. The other is at 38°09′33″S; 70°09′08″W; 849 m.
Leanza and Hugo (2001) mapped the dikes in the Cerro Mocho region as a subunit of the Collipilli Formation. This series of east-west–trending dikes was emplaced along preexisting structures. The most distinctive is a 19-km–long, east-west dike that cuts the Cerro Mocho anticline (Fig. 16). This dike is better seen on the Landsat image than in the field due its lack of topographic expression. In detail, the dike is not a single unit, but rather a series of dike segments with chilled margins.
Repol et al. (2002) mapped the Cerro Mocho dikes as the Pichaihue andesite, and assigned them a Miocene age based on field relationships and correlations with other Miocene volcanic rocks in adjacent regions (Rovere and Rossello, 2001). However, two single-crystal 40Ar/39Ar analyses have yielded ages of 101.99 ± 0.69 Ma and 90.00 ± 4.06 Ma (late Early Cretaceous–Albian; Zamora Valcarce et al., 2006). These ages are older than those obtained in the Collipilli and Naunauco igneous rocks and can be interpreted as being related to a 90–100 Ma magmatic event, not previously reported in this region.
As discussed by Zamora Valcarce et al. (2006), the Cerro Mocho dikes are subalkaline basalts with relatively high Nb/Y ratios that vary between 0.3 and 0.4 indicating only a weak convergent continental margin signature. Their small Nb anomalies with respect to Th and Ce and high Ti and Y contents are transitional between those of arc and mid-oceanic ridge basalts. Both the high heavy rare earth and incompatible element contents require a distinct origin for the Cerro Mocho dikes compared to the arc-like Naunauco and Collipilli magmas. As such, both the geochemistry and age of the Cerro Mocho volcanic rocks are distinct from those of the Collipilli Formation (as defined at Stop 2) (Zamora Valcarce et al., 2006).
DAY 3—MIOCENE DEFORMATION AT THE FRONTAL PART OF THE AGRIO FOLD-AND-THRUST BELT
The stops on Day 2 were focused on the inner sector of the Agrio fold-and-thrust belt where it was possible to see clear evidence for the Cretaceous deformation that affected this part of the Andes. The only evidence for Miocene reactivation was the Tralalhué synorogenic deposits and paleomagnetic data.
The stops on Day 3 focus on the outer sector of the Agrio fold-and-thrust belt (Fig. 11) where the structural deformational style is dominated by thin-skinned structures reactivated by basement inversion. The main evidence for Miocene deformation in this region comes from synorogenic deposits on the backlimb of the Pichi Mula anticline, south of the Cortaderas lineament, and from deformed lower Miocene igneous rocks in the Huantraico syncline, north of the Cortaderas lineament.
Stop 1: Panoramic View of the Cordillera del Viento, Chos Malal Fold Belt, and the Tromen Volcano (North of Cortaderas Lineament)
Locations: View at two stops close to each other—one east of the town of Chos Malal, south of Tromen Volcano (Route 52) at 37°20′03″S; 70°08′35″W; 1342 m; the second near Laguna Auquilco (Route 52) at 37°21′29″S; 69°59′39″W; 1476 m.
From west to east, the main components of the Cretaceous to Miocene Andean fold-and-thrust belt can be observed in a regional view from this stop. To the west is a large, north-south–extending ridge known as the Cordillera del Viento. This is a basement-related structure (Fig. 17) that is mainly composed of the volcaniclastic Choiyoi Group and underlying Carboniferous metasedimentary rocks. Immediately to the east of the Cordillera del Viento is a narrow deformed belt known as the Chos Malal fold belt. Toward the east, the Chos Malal fold belt terminates against a series of large basement structures known as the Tromen synclinorium and La Yesera and Pampa Tril anticlines. These folds are characterized by flat crests and steeply dipping flanks. The insertion of the basement structures within the sedimentary sequence has produced complex triangle zones and elongated deformed fold belts.
Stop 2: Frontal Syncline and Magmatic Rocks
Location: East of Laguna Auquilco at 37°23′07″S; 69°55′19″W; 1235 m.
The Huantraico syncline (Fig. 11) north of the Cortaderas Lineament constitutes the eastern limit of the Agrio fold-and-thrust belt. From this stop, it is possible to observe the early to middle Miocene lavas overlying the Mesozoic–lower Tertiary sedimentary sequences that fill the northern and central part of the Neuquén Basin (Fig. 18). These lavas erupted in a backarc position (Kay and Copeland, 2006) relative to the ca. 24–20 Ma Cura Mallín Formation volcanic and sedimentary sequences that formed in an intra-arc basin (e.g., Suárez and Emperán, 1995; Jordan et al., 2001; Burns, 2002; Burns et al., 2006). The backarc lavas erupted north of or along the Cortaderas lineament (Ramos, 1978), which generally marks the southern extent of post–Oligocene backarc magmatic activity in the Neuquén Basin. The significance of the Cortaderas boundary, which lies along a persistent northeast-trending regional structural grain and projects into an offset in the modern Southern Volcanic Zone arc has been unclear. Kay et al. (2006) argue that this lineament marks the southern boundary of a transient Miocene shallow subduction zone.
Stop 3: Cerro Rayoso Anticline
Location: Two stops along Provincial Route 9 as indicated below.
Stop 3a: Panoramic View at 37°38′11″S; 69°58′41″W; 844 m
Cerro Rayoso (Fig. 11) is a doubly plunging anticline that has an elongated, north-south–trending axis extending for more than 22 km (Fig. 19). The maximum fold width is 6 km. The lower units of the Agrio Formation crop out in the core of the anticline. There is a dramatic increase in dip from the crest toward the flanks of the anticline, with flank dips reaching 70°. The hinges are narrow and show a chevron-like structure. According to Cristallini and Allmendinger (2000), the structure is quite symmetrical based on the distribution of strain indicators. These indicators show evidence of layer-parallel shear toward the crest of the structure (a consequence of the parallel folding process). The crest is perpendicular and oblique to the actual bedding. The proposed kinematic model that explains the geometry of the structure is attributed to either detachment folding or to a trishear folding mechanism, with the former being the more probable.
Stop 3b: Panoramic View at 37°38′11″S; 69°58′41″W; 844 m
At the western flank of the Cerro Rayoso anticline is the broad Pichi Neuquén syncline, which affects the uppermost units of the Rayoso Formation and the overlying Neuquén Group deposits (Fig. 20A). The syncline overlies the southern termination of the Loma Rayoso anticline. The contact between the two structures is marked by a regional fault. This fault zone, which represents the upper detachment of the Agrio fold-and-thrust belt structures, is located in the Rayoso Formation evaporites. The detachment level is so efficient that it also detaches the Loma Rayoso anticline from the Pichi Neuquén surface syncline at depth. The surface geometry relationship is imaged on the seismic lines in Figure 20B.
Stop 4: Puesto Burgos and Rincón Bayo Formations
Location: 38°02′6″S; 69°58′12″W; 1173 m.
From this stop, a white and red stratigraphic sequence is seen near the Neuquén River (Fig. 21). These deposits are located in the frontal part of the fold-and-thrust belt in the Pampa de Agua Amarga. The red and white unit is composed of primary and reworked pyroclastic deposits and tuffs (Leanza and Hugo, 2001) that are gently folded. They unconformably overlie the Neuquén Group with an onlap relationship (Fig. 21). Zapata et al. (2002) argued that these synorogenic deposits are of middle to upper Miocene age on the basis of mammal fossils. These deposits are onlapped by the Rincón Bayo Formation, which marks the end of the deformation (Zamora Valcarce et al., 2006).
DAY 4—LATE MIOCENE TO RECENT DEFORMATION IN THE HIGH ANDES (37°30′S)
Introduction to Day 4
On Days 1–3, we visited the Agrio fold-and-thrust belt where middle Cretaceous to late Miocene contractional deformation is responsible for the observed structures (Fig. 22). Today, a complex deformational history, which began with the collapse of the inner sectors of the Agrio fold-and-thrust belt during the late Oligocene–early Miocene to form the intra-arc Cura Mallín basin (Folguera et al., 2006b), will be observed. The Oligocene to Miocene outcrops displaying this deformation are located west of the Agrio fold-and-thrust belt. The westernmost sector of the Agrio deformed belt lies beneath thick piles of volcanic rocks in the modern arc and westernmost retroarc. The main inversion of the late Oligocene–early Miocene Cura Mallín basin and its final incorporation into a fold-and-thrust belt occurred in the late Miocene (Folguera et al., 2006b), synchronous with the stacking of the external part of the Agrio fold-and-thrust belt seen on Day 3. During the final closure of the Cura Mallín basin, the Paleogene and Neogene fill was stacked to the east over the previously denuded western sector of the Agrio fold-and-thrust belt.
The contraction in the inner sectors of the cordillera is considered to be related to a separate belt called the Guañacos fold-and-thrust belt that is west of the Agrio belt (Figs. 22–24). The Guañacos belt shows some important differences from the Agrio belt. First, even though the Guañacos belt started to deform during the last phases of contraction to the east in the Neuquén Basin, its evolution continued into the Late Pliocene and Quaternary. By this time, compressive deformation to the east was completely fossilized. Second, the deformation in the Guañacos belt only involves Tertiary rocks at the surface. Third, shortening in the Guañacos belt is accommodated by two mechanisms: (1) inversion of late Oligocene to early Miocene extensional structures, and (2) thrusting of the sag facies of the Cura Mallín basin. The inversion of Late Triassic structures at depth that are mechanically linked to younger rift basins could also be important; this is the main mechanism of deformation in the Agrio fold-and-thrust belt.
After an initial phase of deformation in the Guañacos fold-and-thrust belt, a renewed period of extensional collapse was partially superimposed on the inverted Cura Mallín basin. This collapse produced a narrow, almost 200-km–long half-graben system called the Loncopué trough. The trough formed in an extensional phase that began in the early Pliocene with reactivations occurring into the early Quaternary. To the south of the Day 4 traverse, the Plio-Quaternary normal fault bounded trough accommodates nearly 1000 m of intra-arc sequences super imposed on the main depocenter of the Cura Mallín basin. Extension associated with the Loncopué trough in this part of the Andes occurred between 5 and 1 Ma. At the latitude of the Day 4 traverse, the Loncopué lowland is of lesser magnitude, consisting of a narrow zone of Plio-Quaternary lava flows whose extensional control has been largely modified by Late Pliocene to Quaternary deformation. This late episode of contraction is now inverting the northern section of the Loncopué trough. As a result, the Guañacos fold-and-thrust belt has started an eastward progression into the foreland cannibalizing the synrift sequences that were previously in the Loncopué trough. While the Loncopué extensional stage seems to be aborted at these latitudes (37°–37°30′S), farther south (38°–39°S), neotectonics is governed by extension related to transtensional deformation. In this southern region, there are still indications of ongoing extension and related lower crustal attenuation (Yuan et al., 2006; Folguera et al., 2007).
Drive across the Cordillera del Viento to Andacollo
The Cordillera del Viento is part of the innermost sector of the Agrio fold-and-thrust belt north of 37°30′S. Along with the Domuyo and Tromen massifs, the Cordillera del Viento is one of the highest regions in the province of Neuquén. The range had an initial and major phase of uplift ca. 69 Ma (Burns, 2002; Burns et al., 2006) followed by minor reactivation in the late Miocene. Higher degrees of shortening in comparison with the inner part of the Agrio fold-and-thrust belt to the south (Day 2) have exposed the basement of the Neuquén Basin in this region. This metamorphic basement, which is of unknown age (Zappettini et al., 1986), is unconformably covered by fossil-bearing Carboniferous marine sequences, which are in turn unconformably covered by the extensive rhyolites, andesites, and volcanic breccias that form the Choiyoi Group.
The structure of the Cordillera del Viento has been traditionally interpreted as a major basement structure, although the style of deformation has been interpreted differently through time. Initially, this basement cored structure was thought to be connected with a system of ramps and flats that transported the basement horizontally. These structures were thought to involve large amounts of shortening and to have produced deformation in the Jurassic to Cretaceous sequences of the Neuquén Basin to the east (Kozlowski et al., 1996). More recent models relate the uplift to tectonic inversion of normal faults associated with Late Triassic–Early Jurassic depocenters (Fig. 17) (Zapata et al., 1999, 2002). These faults would transport less shortening into the fold-and-thrust belt to the east than the faults considered in previous models. In detail, the Cordillera del Viento is now considered to be a back thrust, associated with a décollement that resulted from the inversion of a Late Triassic detachment (Fig. 17). Older, deeper sequences are exposed on the western slope, whereas synrift sequences and sag facies of the Neuquén Basin are seen on the eastern side. Early Jurassic sequences that form part of the synrift association (Vergani et al., 1995) are commonly grouped into the Cuyo Group and locally described as the Chacay Melehue Formation (Llambías et al., 1978). They are exposed along the southern tip of this range.
Stop 1: Late Triassic to Early Jurassic Depocenters of the Inner Agrio Fold-and-Thrust Belt
Location: South of Cordillera del Viento at 37°17′08″S; 70°33′21″W; 1807 m.
This stop is located at the southern extreme of the Cordillera del Viento, north of the Neuquén canyon. From this stop, we can see almost the complete section that has been denuded in the core of the Cordillera del Viento since the emplacement of the basement back thrust. The sequence shows the Upper Permian to Triassic volcaniclastic Choiyoi Group covered by the Lower Jurassic marine sediments of the Cuyo Group and the Middle Jurassic to Lower Cretaceous regressive and transgressive cycles of the Neuquén embayment. The Upper Triassic to Lower Jurassic sequences were deposited during the synrift stages of a north-south–elongated rift system that developed in the western part of the Neuquén embayment during the initial stages of its evolution. At Stop 1, unusual Lower Jurassic strata will be seen that have been interpreted as the product of a laharic mass wasting phenomena that advanced through coastal environments (Llambías et al., 1978). Evidence for gravitational deformation that affected breccias, pyroclastic flows, and shore clastic wedges on steep slopes at the time of intraplate volcanism during the first phases of Gondwana breakup will also be seen.
Drive across the Loncopué Trough
Few examples of retroarc extensional basins are well preserved in the southern central and Patagonian Andes. The less than 5 Ma extensional stage of the western sectors of the Andean fold-and-thrust belt at these latitudes is represented by the Loncopué trough, which has been being mildly inverted in the past 2 m.y. in its northern section (Figs. 25 and 26). Little information exists as to the amount of collapse on the Cretaceous to Miocene structures, although Lower Jurassic strata on the western side of the Andes can be interpreted to indicate that Jurassic strata are present beneath the Loncopué trough. Extensional faults with young morphological expression border the eastern flank of the trough (García Morabito and Folguera, 2005). These faults cause eastward tilting of the early Pliocene synrift sequences that in turn produce a Quaternary half graben, which has been partially filled by a synrift depocenter. Other minor extensional and transtensional depocenters to the west, superimposed over the Guañacos fold-and-thrust belt, indicate that extension affected the whole western sector of the cordillera.
Stop 2: A Quaternary Extensional Basin Developed in the Retroarc Zone between 37°10′ and 37°30′S
Location: West of the town of Andacollo at 37°13′35″S; 70°42′50″W; 1074 m.
At Stop 2, the upper section of the Loncopué trough fill consists of well-exposed, early Quaternary andesitic lavas. The lower Pliocene section of the basin is missing at this point but is present in depocenters to the west and south where the complete section is represented. The flat geometry of the sequences here contrasts with the highly deformed older sequences to the east and west in the Agrio and Guañacos fold-and-thrust belts. The andesitic lavas here were emplaced in a depression between two deformational belts characterized by relatively higher relief.
Drive South Crossing the Guañacos River and Discussion of the Regional Picture
The Cura Mallín Basin in the Guañacos River
The late Oligocene to early Miocene Cura Mallín basin is characterized at these latitudes by an important asymmetry that is reflected in the sedimentation pattern. To the west, both along the western slope of the Andes and near the continental drainage divide, contractional deformation has exhumed synrift packages. These packages are identified as synextensional because the main thicknesses are associated with reverse faults with normal relationships at the surface indicating that they are inverted normal faults. The sediment packages are composed of volcaniclastic materials and minor fluvial sequences. The easternmost part of the basin is superficially formed by sag sequences composed of lacustrine sediments and low-energy fluvial systems forming uniform blankets, cropping out in the upper Lileo fluvial basin (Figs. 27 and 28). These sequences were detached during the main inversion phase of the Cura Mallín basin that produced the thin-skinned deformation in the easternmost part of the Guañacos fold-and-thrust belt. Those sequences were dated as early Miocene by Sarris (1964) with palynomorphs, and related to a lacustrine environment coeval with volcanism in the Cura Mallín basin. Their lateral continuity as well as facies homogeneity makes them a good marker throughout the basin.
The Contractional Orogenic Front of the Andes at 37°30′S
Middle to late Miocene deformation associated with inversion of Paleogene extensional structures formed the easternmost part of the Guañacos fold-and-thrust belt. After 2–1.5 Ma, the Guañacos fold-and-thrust belt suffered an in-sequence reactivation with respect to the late Miocene deformational phase. The main activity of this last phase of deformation is considered to be early Quaternary as 1.7 Ma lavas are deformed and 1.4 Ma lavas unconformably overlie the structure. Scarps developed on soils as well as progressive unconformities in Late Quaternary sediments suggest that minor deformation on these structures persists.
Only the eastern 30 km of the Guañacos fold-and-thrust belt displays evidence for Quaternary shortening. In this region, the faults show different slip sense depending on their orientation. A northeast-trending set mainly absorbed right lateral components imposed by the oblique convergence between South America and Nazca plate. North- and northwest-trending structures show reverse offsets with subordinate strike-slip displacements. These faults (Antiñir-Copahue fault system or the orogenic front of the Guañacos fold-and-thrust belt) show a curved geometry in plan view, which is delimited by two regional lineaments associated with the Neuquén and Cura Mallín basins (Fig. 29). These are the Mandolegüe lineament to the south and the Cortaderas lineament to the north.
Stop 3: Folding and Thrusting in Lower Quaternary Retroarc Volcanics and Upper Quaternary Sediments: The Antiñir-Copahue Fault System at the Orogenic Front of the Guañacos Fold and Thrust and the Main Phase of Inversion of the Cura Mallín Basin
Location: Reñileuvú River at 37°20′39″S; 70°48′52″W; 1177 m.
At this stop on the Reñileuvú River, one of the clearest expressions of the orogenic front at the latitude of 37°30′S is visible. Here, 1.7 Ma lavas are folded in an anticline (Fig. 30). This anticline is offset by a reverse fault in the section seen along the Guañacos River section to the north. An east-dipping monocline formed at the structural front affects Upper Quaternary sediments, which are associated with an outwash fan related to Pleistocene glaciation. In the frontal syncline, a narrow tongue of synorogenic deposits thickens to the east into the fold axis.
Late Oligocene collapse of the inner sectors of the fold-and-thrust belt at these latitudes was followed by mild inversion between 20 and 17 Ma, and final closure of the intra-arc basin at 10–8 Ma. This deformation can be observed in the inner western sectors of the cordillera on a regional unconformity between synrift sequences and early Pliocene plateau successions. In contrast, the contractional deformation that reinitiated in the Quaternary along the easternmost sector of the Guañacos fold-and-thrust belt exhibited at Stop 3 has progressed to the east with no substantial deformation to the west. This eastward progression explains why lower Pliocene sequences in the high cordillera associated with the extensional stages of the Loncopué trough have not been contractionally deformed.
Stop 4: Main Unconformity between Lower Miocene Synrift Sequences and Pliocene Units
Location: Along the Reñileuvú River at 37°20′35″S; 70°52′50″W; 1223 m.
Stop 4 is located in the westernmost Argentinean side of the Andes at 37°30′S. At this stop, synrift sequences of the Cura Mallín basin, which are mainly composed of volcanic breccias, lacustrine sediments, and subordinate lava flows, are unconformably overlain by Early Pliocene plateau lavas (Fig. 31). Nearly flat-lying Pliocene sequences show that contraction in this area was over before the Early Pliocene.
Stop 5: Mass Wasting Phenomena Associated with the Orogenic Front and Geological Hazards Associated with Seismic-Triggered Avalanches in the Moncol Area
Location: Nera Moncol at 37°21′42″S; 71°00′06″W; 1467 m.
An effect of the young tectonic activity throughout this region is the alignment of avalanche deposits along traces of neotectonic faults. The amount of mass wasting is anomalous compared to other parts of the Andean fold-and-thrust belt due to the position of the Guañacos fold-and-thrust belt. The migration of the orogenic front toward the west during the latest Pliocene rather than toward the foreland as in most of the rest of the Andes has produced young faults. These faults are intercepting the deeply carved, glacial-fluvial basin and reactivating a few of the late Miocene faults such as the El Moncol fault in the region of this stop (Figs. 28 and 32). This situation has produced steep slopes on the Pleistocene glacial morphology that have become gravitationally destabilized by both erosion and crustal earthquakes.
The innermost neotectonic faults in the Moncol area are associated with profuse mass wasting phenomena (Fig. 32). Some associated features of the cordillera in this region need to be highlighted: (1) a large magnitude of mass wasting that has profoundly altered the valley morphology; (2) a minor post-glacial fluvial incision that contrasts with sectors to the east where fluvial processes have carved a glacial morphology; and (3) changes in valley morphology that are spatially related to neotectonic faults along the orogenic front of the Guañacos fold-and-thrust belt.
Drive Back to Chos Malal through the Cortaderas Lineament (Neuquén Canyon)
The stops on the last part of Day 4 on the way back to Chos Malal are again in the westernmost Agrio fold-and-thrust belt. Along this part of the route, the Middle to Upper Jurassic continental sequences that extensively cover the synrift deposits of the Neuquén Basin are well exposed. After a short period of evaporation during the Middle Jurassic when extensive salty environments developed in relation to a fall in sea level, fluvial systems dominated the Neuquén Basin.
Stop 6: Late Jurassic Depocenter at the Inner Part of the Agrio Fold-and-Thrust Belt
Location: Neuquén River to Chos Malal at 37°22′44″S; 70°30′38″W; 1167 m.
Passing through El Mollar along the southern margin of the Neuquén River, the southernmost extreme of the Cordillera del Viento is crossed south of Stop 1 on Day 4. Here more than 1000 m of Late Jurassic continental Tordillo Group sandstones cover synrift sequences of the Neuquén Basin (Fig. 33). They are anomalously thick as can be achieved only in a basin. These Late Jurassic continental sandstones represent a low stand of sea level that is seen throughout the basin. The sands accumulated on erosional surfaces like those in deep channels. The Cortaderas lineament that runs through the southern extreme of the Cordillera del Viento is thought to have had a strong influence on differential fluvial incision. END OF TRIP.
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.