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The Agrio fold-and-thrust belt is located between 37°S and 38°S latitude in the eastern part of the Neuquén Andes. The belt can be divided into a western inner sector and an eastern outer sector. The inner sector is characterized by a thick-skinned deformation style. The dominant structures are large anticlines produced by the inversion of half-grabens formed during the Triassic-Jurassic extension that initiated the Neuquén Basin. The outer sector is characterized by thin-skinned structures; recent studies have shown that these structures have been reactivated in a thick-skinned style. A long-standing question has been whether the deformation in this belt occurred in a continuous pulse or in two independent pulses. The analyses of synorogenic deposits, crosscutting relationships between magmatic rocks and sedimentary formations, and new single-crystal 40Ar/39Ar ages from volcanic rocks presented here indicate a minimum age of 102 Ma for the beginning of deformation in this belt and that deformation occurred in at least two pulses, one during the Lower to Middle Cretaceous, and a second one in the middle Miocene, with different degrees of propagation into the foreland.

INTRODUCTION

The age and temporal history of deformation in the Agrio fold-and-thrust belt have been topics of long discussion in the literature on the Neuquén Basin and the south-central Andes (Ramos, 1978, 1998; Zapata et al., 2002, 2003; Repol et al., 2002; Cobbold and Rossello, 2003). The presence of Miocene synorogenic deposits has been argued to constrain the age of the youngest deformation (Ramos, 1998; Zapata et al., 2002, 2003), but the age of initial deformation and the number of distinct deformational pulses are still widely debated. The purpose of this paper is to present new single-crystal 40Ar/39Ar ages from volcanic rocks, field relationships between igneous and sedimentary rocks, data from seismic lines and exploration wells, and geochemical data on magmatic rocks that constrain the initial age of deformation and confirm the existence of two deformational pulses.

GEOLOGIC AND TECTONIC SETTING

The study area is located in the northwestern sector of Neuquén province in Argentina between 37° and 38°S latitude (Fig. 1). The region of interest lies in the western part of the Neuquén Basin and is part of the Neuquén fold-and-thrust belt in the Principal Cordillera. The Agrio fold-and-thrust belt, as defined by Bracaccini (1970), consists of a series of fault-bounded basement blocks and large double-plunging anticlines developed in a Mesozoic sedimentary sequence.

Figure 1. Regional location map of the Agrio fold-and-thrust belt, where the main morphostructural units of the Neuquén Basin are displayed.

Figure 1. Regional location map of the Agrio fold-and-thrust belt, where the main morphostructural units of the Neuquén Basin are displayed.

Based on morphostructural characteristics, Ramos (1978) divided the Agrio fold-and-thrust belt into an internal sector in the west and an external sector in the east. The internal sector is dominated by a thick-skinned deformational style, whereas the external sector is dominated by large anticlines associated with detachment folds. To the west, the Agrio belt is bounded by the extensionally active Loncopué trough, which is filled by a thick pile of Quaternary volcanic rocks. The eastern limit of the Agrio belt corresponds with the broad and gently dipping anticline of the Los Chihuidos high. To the north, the Agrio fold-and-thrust belt is separated from the Chos Malal trough and the Cordillera del Viento by the Cortaderas lineament. The Cortaderas lineament has a subtle expression on the surface but can be recognized as a prominent basement structure in seismic profiles.

The Agrio fold-and-thrust belt lies in the Neuquén Basin, the history of which can be considered in three stages: (1) a Triassic to Early Jurassic prerift and rift stage, (2) a Late Jurassic to Cretaceous subsidence stage, and (3) a Tertiary to Holocene modification stage punctuated by magmatic events. Tectonic summaries (Kozlowski et al., 1993; Uliana and Legarreta, 1993; Manceda and Figueroa, 1995; Vergani et al., 1995) emphasize the first two stages. The Tertiary magmatic and tectonic evolution is the least understood and little discussed up to this point. This final stage of the evolution of the Neuquén Basin is a consequence of expansion and retreats of the magmatic arc (see summaries in Kay et al., this volume, chapter 2; Ramos and Kay, this volume, chapter 1; Ramos and Folguera, 2006).

The Mesozoic and Paleogene sedimentary deposits of the Neuquén Basin constitute a sequence that can be up to 8000 m thick. These deposits are locally covered by Neogene synorogenic deposits. The Agrio fold-and-thrust belt occurs in the basin depocenter (Legarreta and Gulisano, 1989) where Jurassic to Upper Cretaceous sedimentary strata are well represented (Fig. 2).

Figure 2. Geological map of the study area.

Figure 2. Geological map of the study area.

The basement of the Neuquén Basin crops out in the Cordillera del Viento, north of the study area (Zollner and Amos, 1973). It is composed of volcanic and volcaniclastic deposits of the Choiyoi Group of Permian-Triassic age, which are associated with crustal extensional on a regional scale (Ramos and Kay, 1991), and the sedimentary and volcanic rocks of the Carboniferous Andacollo Group. The oldest basement outcrops in the study area are marine deposits of the Cuyo and Lotena Groups. They are found in the proximity of the town of Loncopué (Fig. 2; Zavala et al., 2002), and they have been documented in several oil exploration wells. The top of these marine sequences corresponds to the evaporites of the Auquilco Formation, which constitutes the basal décollement of the fold-and-thrust belt (Viñes, 1985).

Overlying the basement are Kimmeridgian to Early Cretaceous sedimentary sequences (see Fig. 3). These sequences start with the marine deposits of the Mendoza Group, which conformably overlie the older rocks. The continental sandstones of the Tordillo Formation (Gulisano Gutierrez Pleimling, 1994) comprise the base of this group. Above them is the Vaca Muerta–Quintuco Formation (Fig. 3), which is a basinal deposit characterized by a finely stratified alternation of black and gray shales, calcareous micritic limestones, and bituminous marls. Because of its high content in organic matter, it is an excellent source of hydrocarbons and the most prolific source rock in the Neuquén Basin. The next unit is the Mulichinco Formation, which corresponds to a sandy platform deposit (Zavala, 2000). A new transgressive sequence, corresponding to the Agrio Formation, overlies it. The intercalated Avilé Member represents a sea-level lowstand sequence. The Huitrín Formation overlying this unit is composed of the eolian and river sandstones of the Lower Troncoso Member and evaporites of the Upper Troncoso Member. These evaporites, which are widely distributed across the basin, constitute the regional upper décollement of the Agrio fold-and-thrust belt (Ploszkiewicz, 1987). The evaporites mark the desiccation of the basin (Legarreta and Gulisano, 1989) that terminated with the deposits of the La Tosca Member, which record the last marine event in the basin.

Figure 3. Schematic stratigraphic column (without scale) that shows the main lithologic units of the region, modified from Brissón and Veiga (1998).

Figure 3. Schematic stratigraphic column (without scale) that shows the main lithologic units of the region, modified from Brissón and Veiga (1998).

Subsequently, deposits of evaporites and clastic sedimentary rocks corresponding to the Rayoso Group were deposited as the basin shallowed. They represent the culmination of the Andean Cycle (Vergani et al., 1995). Recent studies of the Rayoso Group in the study area have documented a discordance within the group (Zavala et al., 2002). Continental red sandstones, conglomerates, and shales of the Neuquén Group cover these beds and mark the continentalization of the basin. They are exposed in the eastern part of the study area, where they begin with basaltic conglomerates levels and rhyolitic tuffs that may be derived from the Cordillera del Viento to the northwest.

A series of Neogene deposits that fill small basins is well exposed in the study area. They have been recognized in two areas. The first is on the west side of Cerro Naunauco (Fig. 2), where Ramos (1998) described the Tralalhué Conglomerate, which was interpreted as part of a Miocene piggyback basin. This unit consists of a conglomeratic sequence composed of angular clasts from andesites, dacites, and calcareous sub-rounded clasts. Recent mammal fossils confirm its Miocene age (Repol et al., 2002). Field work in this study area shows that the Tralalhué Conglomerate unconformity overlies the Cretaceous Rayoso Group and the igneous rocks of Cerro Naunauco. The second area is in the Pampa de Agua Amarga (Fig. 2), where the Puesto Burgos Formation occurs. This unit is composed of primary and reworked pyroclastic deposits and tuffs (Leanza and Hugo, 2001). These gently folded deposits unconformably overlie the Neuquén Group. On the basis of mammal fossils, Zapata et al. (2002) argued that these synorogenic deposits are of middle Miocene age. These deposits are onlapped by the Rincón Bayo Formation, which marks the end of deformation.

AGRIO FOLD-AND-THRUST BELT

The Agrio fold-and-thrust belt, which was called the Agrio trough by Bracaccini (1970), is mainly characterized by detachment folds. The geological map of the study area (Fig. 2) shows N-NW–trending structures, with a difference in structural styles between the western and the eastern parts. Broad anticlines, products of basement inversion, characterize the structure to the west (Ramos, 1998). To the east, the fold-and-thrust belt is characterized by large double-plunging anticlines separated by broad rhombic-shaped synclines, reflecting the presence of basement blocks at depth (Ramos, 1978; Viñes, 1985; Zapata et al., 1999a, 1999b, 2002).

Traditionally, the Agrio fold-and-thrust belt has been divided into three regions (Ramos, 1998): the Loncopué graben, and the inner and outer sectors, structured by thick- and thin- skinned structure, respectively. Recent studies have demonstrated that the entire Agrio fold-and-thrust belt has undergone thick-skinned deformation (Zapata et al., 1999a, 1999b, 2002, 2003). This fold-and-thrust belt is bounded to the north by the Cortaderas lineament, and to the east by the Chihuidos high (Fig. 4).

Figure 4. Regions of the Agrio fold-and-thrust belt and its limits to the east and to the north.

Figure 4. Regions of the Agrio fold-and-thrust belt and its limits to the east and to the north.

The following analysis of the subsurface structure is based on surface geology derived from geological maps by YPF (Yacimientos Petroleros Fiscales) geologists and new observations, and on the interpretation of seismic sections (Figs. 7–9) that cover the transition between the inner and the outer sectors. They show excellent examples of the basement inversion and the Miocene synorogenic deposits.

Loncopué Fault System

The Loncopué trough is a 300-km-long and 30–40-km-wide depression that bounds the western side of the Agrio fold-and-thrust belt. Ramos (1978) defined it as an extensional system. Numerous monogenic basaltic cones and Tertiary volcaniclastic and taphrogenic series fill it, but its initial fill is unknown. Because the thickest section of the volcanic rocks in the Eocene Cayanta Formation are found in this graben, it is assumed that the Loncopué trough existed in the Paleogene (Llambías and Rapela, 1989). Because geophysical data are lacking, there is no consensus on the geometry of the boundary between the Loncopué through and the Agrio fold-and-thrust belt. Eisner (1991), Lesta et al. (1985), Ramos (1998), and Cobbold and Rossello (2003) favor an eastward-dipping thrust, whereas Ramos (1978), Zapata et al. (1999a, 1999b), and Jordan et al. (2001) propose a west-dipping normal fault.

Inner Sector

The inner sector corresponds to the western part of the Agrio fold-and-thrust belt (Fig. 4). Vergani et al. (1995) interpreted the basement faults in this region as part of the Tres Chorros extensional system, which controlled the Jurassic depocenters of the Neuquén Basin. This zone likely correlates with the Cordillera del Viento (Zapata et al., 1999a, 1999b; Zapata and Folguera, 2006), which is related to the inversion of another Jurassic depocenter.

The analysis of folding and associated faults, as well as their morphological expression in the area, has allowed the recognition of a series of basement highs and lows (Viñes, 1985; Ramos, 1998; Zapata et al., 1999a, 1999b, 2002) with N-NW axes (Fig. 5). These axes can be related to basement inversion, probably associated with older Jurassic depocenters (Vergani et al., 1995). The most representative of these structures in the study area is the Cerro Mocho anticline (Figs. 5 and 6). Even though there is no direct evidence for such a depocenter, the fact that the CMO X-1 exploration well, emplaced on the eastern side of the Cerro Mocho anticline (Fig. 6), penetrated more than 1500 m of black shales from the Los Molles Formation documents the presence of an anomalous thickness best interpreted as a Jurassic depocenter (Zapata et al., 2002).

Figure 5. Landsat TM image of the Agrio fold-and-thrust belt showing the main fold axis and the morphostructural elements in the study area.

Figure 5. Landsat TM image of the Agrio fold-and-thrust belt showing the main fold axis and the morphostructural elements in the study area.

Figure 6. East-west structural cross section through the Agrio fold-and-thrust belt (see location in Fig. 5). The structure of the Agrio fold-and-thrust belt combines thin-skinned and thick-skinned deformation (modified from Zapata et al., 1999a, 1999b).

Figure 6. East-west structural cross section through the Agrio fold-and-thrust belt (see location in Fig. 5). The structure of the Agrio fold-and-thrust belt combines thin-skinned and thick-skinned deformation (modified from Zapata et al., 1999a, 1999b).

Figure 7. Seismic sections showing the basement faults being inserted into the Auquilco evaporites and transferring the shortening to the outer sector. See location in Figure 5. TWT—two-way traveltime.

Figure 7. Seismic sections showing the basement faults being inserted into the Auquilco evaporites and transferring the shortening to the outer sector. See location in Figure 5. TWT—two-way traveltime.

The oldest rocks of the region crop out near the core of the basement-related structures, as documented by the Cuyo Group rocks found on the western flank of the Cerro Mocho anticline (Fig. 2). The basement-related faults do not crop out; they can only be recognized on seismic sections. Figure 7 shows two seismic lines (time sections) located on the eastern boundary of the Cerro Mocho anticline. The first package of reflectors corresponds to the Mendoza and the Rayoso Group, drilled by the PDS X-1 exploration well, and serves as a primary input to tie the geology with the seismic information. Below these reflectors, a series of west-dipping inverted faults can be easily recognized. They are interpreted as the inversion of previous Jurassic half-grabens. These faults do not cut upsection. Instead, they are inserted into the evaporite levels of the Auquilco Formation (documented by the PDS X-1 exploration well), transferring horizontal shortening to the frontal part of the Agrio fold-and-thrust belt.

Outer Sector

Interactions between adjacent anticlinal domes, such as those between the Pichi Mula, La Mula–Naunauco, and Chorriaca anticlines (Figs. 5, 6, and 8), are common in outer sector structures. Although there are few places to analyze whether a fault or a squeezed syncline separates the anticlines, field work could corroborate the existence of a fault between the adjacent anticlines for stratigraphic levels older than the evaporite of the Upper Troncoso Member. The seismic sections show a basal detachment near the top of the evaporite of the Auquilco Formation (Late Jurassic; Viñes, 1985; Figs. 7 and 8) and an upper detachment level in the Huitrín Formation (Early Cretaceous) (Fig. 8). The resulting geometry of the structures in this area is a typical fault-bend fold, with some internal detachment folds, known in the area as the “Chorriaca fold type” (see Figs. 6 and 8; Groeber, 1946a; Herrero Ducloux, 1946). In the triangle zone shown on Figure 8, the Pichi Mula anticline could not evolve to a typical Chorriaca fold type, because it grew as a fold propagation fault through the eastern limb of the Cerro La Mula–Naunauco anticline, which was formed first.

Figure 8. Detail of east-west cross section of the outer zone of the Agrio fold-and-thrust belt showing the two detachments levels and basement reactivation fault (modified from Zapata et al. 2002). See location and legend in Figure 6.

Figure 8. Detail of east-west cross section of the outer zone of the Agrio fold-and-thrust belt showing the two detachments levels and basement reactivation fault (modified from Zapata et al. 2002). See location and legend in Figure 6.

When the basement inversion of the frontal part of the Agrio fold-and-thrust belt occurred later, deep basement faults cut the previous folds (Fig. 8). At this time, some of the detachment folds evolved to either a fold-bend fault or a fold-propagation fault geometry, as seen in the Pichi Mula triangle zone (Fig. 8; Zapata et al., 2002). Such refolded triangle zones characterize the external structure of the Agrio fold-and-thrust belt. This inversion corresponds to a second deformation stage that produced the uplift of the triangle zone and the synorogenic deposits at the back limb of the Pichi Mula anticline (Puesto Burgos Formation). These deposits have been analyzed in the Pampa de Agua Amarga area (Fig. 2), where an onlap relationship can be observed between the Puesto Burgos and the sandstones and shales of the Neuquén Group (Fig. 9A). Figure 9B shows a seismic line (time) crossing the external part of the Agrio fold-and-thrust belt, where the same relationship can be observed. The age of these units constrain the timing of the deformation to the middle Miocene (Zapata et al., 2002; Zamora Valcarce et al., 2005).

Figure 9. (A) View to the north of a field example of the synorogenic deposits of the Puesto Burgos Formation; note the onlap relationship with the sediments of the Neuquén Group. See location in Figure 8. (B) Seismic section (time) in the Pampa de Agua Amarga showing the unconformity of the synorogenic deposits over the Neuquén Group sediments. See location in Figure 5. TWT—two-way traveltime.

Figure 9. (A) View to the north of a field example of the synorogenic deposits of the Puesto Burgos Formation; note the onlap relationship with the sediments of the Neuquén Group. See location in Figure 8. (B) Seismic section (time) in the Pampa de Agua Amarga showing the unconformity of the synorogenic deposits over the Neuquén Group sediments. See location in Figure 5. TWT—two-way traveltime.

MAGMATIC ROCKS

Little information is available on the chemistry and ages of Mesozoic igneous rocks at the latitude of the study area. Data from these rocks are important in establishing the age of uplift and the timing of deformation in the area. Volcanic and subvolcanic rocks in the study area occur in the inner part of the Agrio fold-and-thrust belt, where they are mainly found in the Collipilli volcanic field and the Cerro Naunauco laccolith (Figs. 2 and 10). In addition, a series of E-W–trending basaltic dikes cut the Cerro Mocho anticline (Figs. 2 and 10). The Collipilli Group and the Cerro Mocho dikes were included in the same unit by Leanza and Hugo (2001), but the new single-crystal 40Ar/39Ar ages in Figure 11 show that the Cerro Mocho dikes have an age of ca. 100 Ma, and the Collipilli volcanic rocks are from 76 to 63 Ma in age. Crosscutting relationships of volcanic units with sedimentary rocks, along with the 40Ar/39Ar ages, constrain the deformation age of the Agrio fold-and-thrust belt.

Figure 10. (A) Outcrop in the Collipilli area showing the western limb of the Collipilli syncline. Note that the Collipilli volcanic rocks are restricted to the core of the syncline. (B) Field example showing the Cerro Mocho dikes cutting the Agrio Formation.

Figure 10. (A) Outcrop in the Collipilli area showing the western limb of the Collipilli syncline. Note that the Collipilli volcanic rocks are restricted to the core of the syncline. (B) Field example showing the Cerro Mocho dikes cutting the Agrio Formation.

Figure 11. Age spectra and isochron plots for the 40Ar/39Ar single-crystal analysis from the samples in this study: (A) andesitic volcanic rock from Naunauco, (B) andesitic sill, (C) volcanic bomb from Collipilli, and (D) two basaltic dikes from the Cerro Mocho. MSWD—mean square of weighted deviates.

Figure 11. Age spectra and isochron plots for the 40Ar/39Ar single-crystal analysis from the samples in this study: (A) andesitic volcanic rock from Naunauco, (B) andesitic sill, (C) volcanic bomb from Collipilli, and (D) two basaltic dikes from the Cerro Mocho. MSWD—mean square of weighted deviates.

Collipilli Volcanic Area

Magmatic rocks in the Collipilli area consist of volcanic facies associated with cones, lava flows, dikes, sills, and laccoliths. Llambías and Malvicini (1978) were the first to describe Collipilli area volcanic rocks. Later, Llambías and Rapela (1987) included them in the Neuquino-Mendocina volcanic province, which encompasses units between 38°30′S and 34°S (Groeber, 1946a, 1946b; Yrigoyen, 1972; Bettini, 1982; Kozlowski et al., 1987; Haller et al., 1985). Domes and laccoliths characterize these series. The intrusive series of the Collipilli Formation were emplaced as laccoliths and associated sills. These laccoliths intruded the contact between the Agrio and the Rayoso Formations, filling the space of 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 volcanic agglomerates related to extrusive domes, pyroclastic flow deposits, and massive lava flows. These volcanic rocks unconformably cover the Agrio, Huitrín, and Rayoso Formations, which are folded in the Collipilli syncline (Figs. 2 and 10). Therefore, the area was already uplifted (and partially eroded) before or at least simultaneously with their intrusion. In several places, there are conglomeratic sandstones and volcanogenic deposits intercalated between the sub-volcanic volcanic rocks. There is also evidence of younger deformation, as indicated by small thrust faults that cut these rocks. These thrusts may be a consequence of the Miocene regional uplift discussed next.

Llambías and Rapela (1987, 1989) used geochemical analyses 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 subvolcanic facies called the Collipilli Formation, for which known ages ranged from 50 to 45 Ma (Llambías and Rapela, 1989), and (2) an Eocene volcanic facies called the Cayanta Formation (Rapela and Llambías, 1985), which had one K/Ar age of 39 ± 9.11 Ma (Llambías and Rapela, 1989). They noted that volcanic rocks mapped in the Molle Formation farther north yielded K-Ar whole-rock ages of 71.5 ± 5 Ma (Llambías et al., 1978), so that not all volcanic rocks in the Molle Formation could be Eocene in age.

More problems for assigning an Eocene age to Molle Formation volcanic centers arise from recently reported ages, such as the Miocene 40Ar/39Ar age of 11.7 ± 0.2 Ma for the Cerro Negro center, west of Chos Malal (Kay, 2001; Kay et al., this volume, chapter 2), the 40Ar/39Ar age of 60.1 ± 1.6 Ma from igneous rocks of Cerro Nevazón (eastern flank of the Cordillera del Viento; Franchini et al., 2003), and the K/Ar ages of 60.7 ± 1.9 Ma in Campana Mahuida (south of the study area; Franchini et al., 2003). Sillitoe (1977) reported ages of 74.2 ± 1.4 Ma for flat-lying andesitic porphyry intruded into previously folded Jurassic sediment in Campana Mahuida.

New 40Ar/39Ar single-crystal analyses (Table 1) yield well-defined plateaus (Figs. 11A and 11B) that indicate ages of 65.5 ± 0.46 Ma for the Cerro Naunauco laccolith, 72.83 ± 0.83 Ma for a volcanic bomb from Collipilli region, and 56.64 ± 0.44 Ma for an andesitic sill (Fig. 11C) emplaced in the Agrio Formation in the Collipilli region. The first two ages overlap the 65–75 Ma age range that has been reported for andesitic volcanic rocks uncomformably overlying the Tordillo Formation on the western flank of the Cordillera del Viento anticline (Linares and Gonzalez, 1990; Franchini and Schalamuk, 1999).

TABLE 1. DETAILS OF AR/AR DATING

Geochemistry

Chemical analyses of the Collipilli and Naunauco samples (Table 2) show that they are subalkaline low-K andesites with Nb/Y ratios that vary between 0.2 and 0.3 (Fig. 12). The analyses fall in the volcanic arc field, which is typical of the Andean margin, on trace-element discriminative diagrams; this is represented by their arc- to backarc-like signature (La/Ta = 66; Ba/La = 21; Ta/Hf = 0.10; Fig. 13). Other characteristics include relatively flat rare earth element (REE) patterns (La/Yb = 10; La/Sm = 5.5; Sm/Yb = 2.2). Even though the rocks of Collipilli and the Cerro Naunauco have different degrees of differentiation, the trace elements and incompatible rare earth elements indicate a common source (Table 1; Fig. 13).

TABLE 2. MAJOR- AND TRACE-ELEMENT ANALYSES OF VOLCANIC ROCKS

Figure 12. Geochemical classification of Winchester and Floyd (modified by Pearce, 1996).

Figure 12. Geochemical classification of Winchester and Floyd (modified by Pearce, 1996).

Figure 13. (A) Ba/Ta versus La/Ta ratios for the samples analyzed. (B) Th/Hf versus Ta/Hf ratios for the samples analyzed. La/Ta, Ba/La, Ba/Ta, and Th/Hf ratios can be used as relative measures of the importance of a source component associated with a subducting slab. Relative fields for values commonly observed in intraplate, backarc, and arc magmas are indicated. MORB—mid-ocean-ridge basalt.

Figure 13. (A) Ba/Ta versus La/Ta ratios for the samples analyzed. (B) Th/Hf versus Ta/Hf ratios for the samples analyzed. La/Ta, Ba/La, Ba/Ta, and Th/Hf ratios can be used as relative measures of the importance of a source component associated with a subducting slab. Relative fields for values commonly observed in intraplate, backarc, and arc magmas are indicated. MORB—mid-ocean-ridge basalt.

The Collipilli and Naunauco volcanic rocks analyzed here are geochemically similar to those analyzed by Llambías and Malvicini (1978). The main difference is that the new Ar-Ar ages indicate that these Collipilli igneous rocks need to be reassigned to an older igneous event. The Collipilli Group is proposed here to designate these rocks. The differences in the Ar/Ar ages presented here and the previous K/Ar ages could indicate two different events, but they also might be interpreted as a long-lasting igneous cycle that extended from ca. 70 Ma until 39 ± 9 Ma (age from Cerro Mayal; Cobbold and Rossello, 2003). More data are needed to define the Eocene magmatic event in this area of the Andes.

Dikes of Cerro Mocho

Leanza and Hugo (2001) mapped the dikes in the Cerro Mocho region as a subunit of the Collipilli Formation. These dikes are exposed as a series of E-W dikes emplaced along pre-existing structures. The most distinctive is a 19-km-long dike that cuts E-W through the Cerro Mocho anticline. It is better seen on the Landsat image (Figs. 5 and 10) than in the field due its lack of topographic expression. In detail, it is not a single dike, but a series of dike segments (Fig. 10) with chilled margins.

Repol et al. (2002) put the Cerro Mocho dikes into 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 Rosello, 2001). Two samples selected for single-crystal 40Ar/39Ar ages (Table 1) yielded the plateaus shown in Figure 11D. The first, sample M3-9 yielded a well-defined plateau indicating an age of 101.99 ± 0.69 Ma (late Early Cretaceous–Albian). A second sample does not show a well-defined plateau because of its alteration, but an age older than those obtained from the Collipilli and Naunauco igneous rocks can be inferred (around 90 ± 4.06 Ma), and an alteration event at 51 Ma might have affected the sample. These ages can be interpreted as an older magmatic event in the area, near 90–100 Ma, not reported before.

Geochemistry

The Cerro Mocho dikes are geochemically distinct from the Collipilli and Naunauco samples because they are subalkaline basalts (Table 3) with relatively higher Nb/Y ratios (0.3–0.4; Fig. 12). They have an intermediate signature (La/Ta = 49; Ba/La = 223; Ta/Hf = 0.10; Fig. 13) and a relatively flat REE pattern (La/Yb = 12.2; La/Sm = 2.26; Sm/Yb = 5.39) showing mantle affinity.

TABLE 3. MAJOR- AND TRACE-ELEMENT ANALYSES OF CERRO MOCHO DIKES

The dikes of the Cerro Mocho area are distinctive, even though they maintain characteristics associated with a convergent continental margin. They have a minor Nb anomaly with respect to Th and Ce; the high Ti and Y contents also outline a transitional pattern between arc basalts and mid-ocean-ridge basalts (MORBs). The high content of heavy rare earth and incompatible elements (Table 3) indicates a different source than the Naunauco and Collipilli volcanic rocks. It seems that the Cerro Mocho volcanic rocks present clear differences in geochemical and chronological analyses and cannot be considered as part of the Collipilli Group.

Tectonic Setting and Correlations of Magmatic Rocks

Figure 14 shows a schematic map with the locations of the main igneous rocks for the Upper Cretaceous through Paleogene. The 40Ar/39Ar ages (65.5 ± 0.46; Fig. 11A) of igneous rocks from Collipilli and the ages of the younger sills (56.64 ± 0.44; Fig. 11) are similar to the radiometric ages obtained by others in the surrounding areas. North of the study area, Franchini et al. (2003) reported K/Ar ages of 56.0 ± 1.7 and 40Ar/39Ar ages of 60.1 ± 1.6 Ma for igneous rocks from Cerro Nevazón, and K/Ar ages of 60.7 ± 1.9 Ma in Campana Mahuida. Domínguez et al. (1984) reported a K/Ar whole-rock age of 67 ± 3 Ma for Los Maitenes–El Salvaje tonalite stock, and J.I.C.A./M.M.A.J. (2000) gave a K/Ar whole-rock age of 64.7 ± 3.2 Ma for the Varvarcó tonalite stock. These Paleocene ages are older than those obtained for a Collipilli intrusive (49.9 ± 3.2 and 48 Ma, whole-rock K/Ar age; Llambías and Rapela, 1989) and the microdiorite stock of Cerro Caicayén (44.7 ± 2.2 Ma, whole-rock K/Ar age; Llambías and Rapela, 1989). However, they are younger than those obtained in samples from the Cerro Naunauco (72.83 ± 0.83 Ma; Table 1; Fig. 11C) and the andesite dikes of the Campana Mahuida district according to the hydrothermal biotite age (74.2 ± 1.4 Ma; Sillitoe, 1977). Nevertheless, all these radiometric ages are younger than those obtained during this study from the Cerro Mocho dikes (101.99 ± 0.69 and 91.97 ± 4.06 Ma; Table 1; Fig. 11D). The alteration event seen in one of the samples at an age of 51 Ma (Fig. 11D) correlates with the middle Eocene radiometric ages of Cerro Caicayén (Llambías and Rapela, 1989) and the Collipilli sills sampled in this study (56.64 ± 0.44 Ma; Fig. 11B).

Figure 14. Map of the Collipilli igneous rocks and Cerro Mocho dikes in the framework of the Upper Cretaceous–Paleogene intrusive and extrusive rocks of the northwestern Neuquén province. See text for sources of age dates.

Figure 14. Map of the Collipilli igneous rocks and Cerro Mocho dikes in the framework of the Upper Cretaceous–Paleogene intrusive and extrusive rocks of the northwestern Neuquén province. See text for sources of age dates.

Based on the available ages, Franchini et al. (2003) grouped the igneous rocks into three different magmatic cycles: Late Cretaceous (Campana Mahuida), Paleocene (Campana Mahuida and Cerro Nevazón), and middle Eocene (Caicayén and Collipilli). With the new data presented in this study, the Collipilli igneous rocks should be assigned to the Maastrichtian-Danian (Cretaceous-Paleocene boundary) magmatic event. There was magmatic activity during the middle Eocene at the Collipilli area, as indicated by the sills and the alteration event registered on one of the samples of the Cerro Mocho dikes. In addition, an older Middle Cretaceous magmatic event is recorded by the Cerro Mocho dikes. This event has not been recorded before for this part of the Andes, and because very few samples have been dated, new radiometric ages are needed to establish a detailed chronology for this event.

According to field relationships, the Collipilli and Cerro Mocho magmas (Figs. 2 and 10) intruded the already formed structures of the Agrio fold-and-thrust belt. In addition, the igneous rocks from the Collipilli and Caicayén volcanic units have been interpreted as having been intruded close to the end of the main deformational event (Llambías and Malvicini, 1978; Minniti et al., 1986; Llambías and Rapela, 1989; Franchini, 1992). This consideration together with field observations and the ages presented here show that the Agrio fold-and-thrust belt was already deformed by the end of the Cretaceous.

STRUCTURE AND MAGMATISM

In order to develop a comprehensive understanding of the changes in the structural style and time of deformation in this sector of the Neuquén Andes, it is necessary to combine all of the available information.

Lower to Middle Cretaceous (First Compressional Stage)

According to Ramos (1981), most of the inversion of the half-graben systems or basement blocks took place during the Late Cretaceous (Cenomanian). Zapata et al. (2002) confirmed this interpretation based on field observations. Cobbold and Rossello (2003) showed evidence for a Late Cretaceous deformation, something that the first geologists working in the basin (e.g., Groeber, 1929; Keidel, 1925; Wichmann, 1934) already postulated. North of the study area, Manceda and Figueroa (1995) inferred ages of 94 Ma for tectonic inversion on the Chilean side from the subsidence curve.

The new 101 Ma Ar-Ar age from the Cerro Mocho dikes in this study further confirms a Middle Cretaceous deformational event (Fig. 11D). By this time, the continental deposits of the Rayoso Formation were filling the Neuquén Basin, and basement-fault inversion had started in the Agrio fold-and-thrust belt, uplifting the Cerro Mocho anticline in the inner (western) sector (Fig. 7). Shortening between the inner (western) and outer (eastern) sector was accommodated through a basal detachment in the Auquilco evaporites (Figs. 6 and 7) that transferred the horizontal displacement to the east. The outer sector was deformed at this time by thin-skinned tectonics, generating detachment folds seen as the Cerro La Mula, Pichi Mula, etc.

Kay (2001) and Kay et al. (this volume, chapter 2) present an 40Ar/39Ar biotite cooling age of 69.09 ± 0.13 Ma from a granodiorite pluton near Varvarcó in the western Cordillera del Viento, which they argue marks a time of uplift. Linares and Gonzalez (1990) and Franchini and Schalamuk (1999) presented ages from 65 to 75 Ma for the Paleogene andesitic series that unconformably overlie the deformed western flank of the Cordillera del Viento. Similar values have been presented here for the igneous rocks of the Collipilli area, which overlie deformed sediments. This event would have produced the uplifting of the Cordillera del Viento as a reactivation of the pre-existing normal faults (Zapata et al., 1999a, 1999b).

Late Cretaceous to Early Miocene

The structural relief of the Agrio fold-and-thrust belt generated during the first deformational stage could be responsible for the intra–Rayoso Formation unconformity (Ponce et al. 2002), as well as the unconformity between the Rayoso Formation and Neuquén Group. In this sector of the Neuquén Andes, the continental sediments of the Neuquén Group were deposited over synclines, such as the Collipilli, Pampa de Naunauco, and those in the external part of the Agrio fold-and-thrust belt that were produced in the first deformation stage. Miocene deformation would have eroded these deposits, leaving only those now found in the outer sector of the Agrio fold-and-thrust belt.

At this time, an important magmatic activity started at this latitude with the eruption of the Eocene Cayanta Formation on the back limb of the Cordillera del Viento and intrusion of the laccoliths of Cerro Nevazón, Varvarcó, etc. In the study area, the Collipilli and Naunauco andesitic rocks cut through the Collipilli syncline.

The Collipilli and Naunauco magmas have the chemistry expected of magmas erupted in or near the frontal volcanic arc over the subducting Nazca plate. If they are compared with the andesitic series and rocks from Cerro Nevazón (Franchini et al., 2003), their composition, volume, and regional distribution are in accord with eruption through a thin crust through which mantle-generated melts easily passed to the surface.

Middle Miocene to Pliocene (Second Compressional Stage)

A new change in regional stress produced a new deformational event. In the Agrio fold-and-thrust belt, this event is reflected in the reactivation of basement faults that inverted previous Jurassic half-grabens. It produced the tightening of all previous Cretaceous structures (Fig. 8), such as the Cordon del Salado, Pichi Mula, and Cerro Rayoso anticlines. This uplift is associated with the deposition of synorogenic deposits like those in the Puesto Burgos Formation and the Tralalhué conglomerates (Figs. 2 and 9). This deformation event is also recognized in the Chos Malal area by Kozlowski et al. (1996) where deposits of latest Miocene age and Pliocene are not folded.

CONCLUDING REMARKS

The new data presented here allow us to create a tectonic model for the history of the Agrio fold-and-thrust belt. The inversion of Jurassic half-grabens structured the inner sector in the latest Early Cretaceous through thick-skinned tectonics. Shortening was transferred to the foreland, producing thin-skinned structures in the outer sector. Later, the Collipilli igneous rocks intruded these structures, and a second deformational event took place in the Paleocene to middle Miocene. This event reactivated the previous structures and produced basement-fault inversion in the outer sector. The Tralalhué and Puesto Burgos synorogenic deposits record this event.

The Naunauco andesites and Collipilli igneous rocks have geochemical characteristics like those of other Andean-margin arc magmatic rocks erupted through a normal-thickness crust. They are assigned to a redefined latest Cretaceous, Maastrichtian-Paleocene Collipilli Group based on Ar/Ar ages ranging from 73 Ma to 65 Ma. The 102 Ma Cerro Mocho volcanic rocks, which were emplaced as E-W–trending dikes, reflect an earlier Cretaceous magmatic event that was previously unknown in the region. The Collipilli and Cerro Mocho Groups both have trace-element patterns associated with convergent continental margins, but differences between them require that they came from distinct source regions.

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, La faja plegada y corrida del Agrio: Análisis estructural y su relación con los estratos Terciarios de la Cuenca Neuquina, Argentina: in V Congreso de exploración y desarrollo de Hidrocarburos (Mar del Plata).
Zapata
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Zamora, G., and Ansa, A.,
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143
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1
-91.

Figures & Tables

Contents

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