Tectonic controls on the building of the North Patagonian fold-thrust belt (~ 43°S)

CHAPTER

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Tectonic controls on the building of the North Patagonian fold-thrust belt (~43°S) ⁎

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A. Echaurren , Guido M. Gianni†, Lucía Fernández Paz , C. Navarrete‡, Verónica Oliveros§, A. Encinas§, Mario Giménez†, F. Lince-Klinger†, Andrés Folguera‖ Department of Geological Sciences, IDEAN—Institute of Andean Studies “Don Pablo Groeber”, UBA—CONICET, FCEN, University of Buenos Aires, Buenos Aires, Argentina* Seismological Geophysical Institute Ing. Volponi (IGSV), FCEFyN, National University of San Juan, San Juan, Argentina† Department of Geology, Faculty of Natural Sciences, Patagonia National University San Juan Bosco, Comodoro Rivadavia, Argentina‡ Department of Earth Sciences, University of Concepción, Concepción, Chile§ Faculty of Exact and Natural Sciences, National Scientific and Technical Research Council (CONICET), IDEAN—Institute of Andean Studies “Don Pablo Groeber”, University of Buenos Aires, Buenos Aires, Argentina¶ Department of Geological Sciences, National Scientific and Technical Research Council (CONICET), IDEAN— Institute of Andean Studies “Don Pablo Groeber”, FCEN, University of Buenos Aires, Buenos Aires, Argentina‖

1 ­Introduction In active continental margins, mountain building processes have been classically referred to as “Andeantype” orogenesis (e.g., Uyeda and Kanamori, 1979) wherever the coupling between an oceanic plate subducted beneath a continent is associated with stress transmission to the upper plate, leading to fold-thrust belt development and arc magmatism. In these settings, orogenesis is controlled by parameters related to properties of the upper (continental) and lower (oceanic) plates, such as the upper plate velocity (e.g., Silver et al., 1998; Heuret and Lallemand, 2005; Sobolev and Babeyko, 2005), convergence rates (e.g., PardoCasas and Molnar, 1987; Somoza, 1998; Maloney et  al., 2013), climate-tectonic feedback (e.g., Lamb and Davis, 2003), geometry and subducted bathymetric elements of the oceanic slab (e.g., Gutscher et al., 2000; Folguera and Ramos, 2011), and a variety of processes involving mantle convection dynamics (e.g., Doglioni et al., 2009; Schellart, 2017). Even though interaction of these processes ultimately defines effective interplate coupling that characterizes the growth and development of a particular orogen, discerning the active relationship between these factors remains as a challenging task. The Andes are the largest of these cordilleran systems, where variation in crustal shortening and thickening expressed through topography and amplitude indicates that orogenesis has not been, either spatially or temporarily, in a steady state since its early stages in Jurassic time. On the contrary, even though Andean segmentation is defined by first-order differences in these features (e.g., Gansser, 1973; Mpodozis and Ramos, 1989; Kley et al., 1999; Tassara and Yáñez, 2003), the tectonic parameters behind the alternation of extensional and contractional phases remain unclear. Furthermore, even though this has led to proposals of a cyclic behavior governing Andean building (e.g., Ramos, 2009; DeCelles et al., 2009, 2015), these approaches describe more adequately the extreme orogenic end members, such as the Altiplano-Puna Andean Tectonics. https://doi.org/10.1016/B978-0-12-816009-1.00022-8 © 2019 Elsevier Inc. All rights reserved.

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segments in the Central Andes, rather than providing a holistic characterization of large and segmented mountain system. The recent proposal of Horton (2018) considers the alternation of tectonic regimes in the Central and Southern Andes as the result of differential plate coupling, product of distinctive plate convergence rates and slab shallowing episodes, leading to a more integrated perspective of Andean orogenesis. Patagonia, the southern tip of the South American continent from approximately ~39° to ~56°S, shows a southward trend in decreasing topography, width and crustal thickness in the Andes, along with a narrowing continental platform to the east (Fig. 1). In the northern segment, ~39–47°S, the main cordillera reaches maximum altitudes of ~2300 m in the active volcanic centers emplaced on its western slope, supported by a ~32-km-thick crust (Tassara and Echaurren, 2012), while in the westerncentral foreland several ~N-S oriented ranges are distributed over an eastward thickening crust that reaches ~40 km beneath the cratonic block. The abundant geological record of various ­sedimentary,

FIG. 1 Tectonic setting North Patagonia. The Chile Triple Junction between the Nazca, Antarctic, and South American plates is used to indicate the adopted division between northern and southern Patagonia (NP and SP, respectively, in inset). The Liquiñe Ofqui Fault Zone (LOFZ) dissects the western Andean slope, while a broad fold-thrust belt deforms the retroarc zone. The basins discussed in this chapter are shown (Mesozoic = Chubut, Austral and Cañadón Asfalto; Cenozoic = Ñirihuau).

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igneous and metamorphic rocks exposed in this area and deformed by an east-vergent fold-thrust belt allows recognition of distinctive tectonic patterns in the Mesozoic and Cenozoic evolution. This chapter is a review of Andean tectonic processes in northern Patagonia, analyzing the main structural, petrological, geochronological, and basin-formation events that gave rise to the formation of a wide and long-lived fold-thrust belt. These parameters will be set in a tectonic framework in order to identify the principal factors that have governed the Mesozoic and Cenozoic evolution of this area, in an attempt to recognize the roles of the continental upper plate and the different oceanic plates throughout Andean history. For detailed descriptions of the data presented in this review, the reader is referred to Echaurren et al. (2016, 2017), Fernández Paz et al. (2018), and Folguera et al. (2018).

2 ­Tectonic setting of North Patagonia The Patagonian platform is constituted by two main blocks of Paleozoic crystalline basement, the North Patagonian and Deseado massifs (Fig. 2A). Even though there are different interpretations concerning the timing and mechanisms of their amalgamation with the rest of the continent during the Paleozoic, there is more agreement in the allochthonous character of the Deseado block, whereas the North Patagonian Massif shows evidence of an authochthonous/parauthochthonous origin (see Pankhurst et al., 2006; Ramos, 2008; Schilling et al., 2017; Castillo et al., 2017). On the other hand, the Andean tectonic cycle (e.g., Charrier et al., 2007; Ramos, 2009) started in Early Mesozoic time with the construction of a continuous trench-parallel mountain system along western South America. In Patagonia, this was propagated over basement exhibiting heterogeneities associated with previous (Paleozoic) sutures of lithospheric blocks accreted to southwestern Gondwana (e.g., Pankhurst et al., 2014; Hervé et al., 2016). One of the major current tectonic features of Patagonia is the Chile Triple Junction, defined by the subduction of the seismic Chile Rise that separates the Nazca, Antarctic and the South American plates (Fig.  1). The Nazca plate is subducted obliquely with respect to the Chilean trench, with a N78°E orientation and a velocity of ~7 cm/y, while the Antarctic plate confronts the subduction zone almost orthogonally at ~2 cm/a (DeMets et al., 1990; Angermann et al., 1999). This is a relatively young tectonic configuration, considering that the active ridge began colliding against the southernmost continent approximately ~15 Ma ago, after first interacting with the Antarctic Peninsula and then migrating northwards to its present position. Bearing in mind formal proposals of Andean margin segmentation (see Folguera et al., 2016), for the purposes of this chapter we will informally place the division of northern and southern Patagonia at ~47°S, where the triple junction is located. Following this nomenclature, the North Patagonian margin can be divided into several morphotectonic units, comprising from west to east (i) the trench, (ii) a low elevation mountain system known as the Coastal Cordillera, which extends parallel to the trench almost continuously along the Chilean margin and disappears in the vicinity of the triple junction, (iii) the Central Valley, a discontinuous depression separating the coastal and Andean mountain systems, (iv) the North Patagonian Andes, representing the main cordillera, (v) the Precordilleran ranges, and (vi) the Patagonian foreland. Considering a cross section at ~43°S (Fig. 2), the main characteristics of these units can be summarized as follows. First, the trench is associated with a thick sedimentary infilling (>1 km) and a reduced frontal accretionary prism (~5 km wide) (Contreras-Reyes et  al., 2010), with a northward bathymetric gradient that acts as a sediment transport channel (Tebbens and Cande, 1997; Melnick and Echtler, 2006). The Coastal Cordillera is a subdued-relief range, represented by Chiloé island and

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FIG. 2 Geological and stratigraphical data for the main units of northern Patagonia. (A) Simplified geological map (based on Lizuain et al., 1995; Sernageomin, 2003; Echaurren et al., 2016), showing the wide distribution of the Mesozoic-Cenozoic igneous units exposed in Patagonia, indicating the trace of the simplified cross section shown in (B). Main ranges are indicated: CP, Cordón Pirámides; CG, Cordón Galeses; CR, Cordón Situación; CR, Cordón Rivadavia; CE, Cordón Esquel; STT, Sierra Tecka-Tepuel; SL, Sierra Languiñeo; SA, Sierra de Agnia; ST, Sierra Taquetrén; SSB, Sierra San Bernardo; SLT, Sierra Lonco Trapial. RCMV indicates the Río Chubut Middle Valley. (B) Cross section A-A′. Two main decollements are proposed as controlling the fold-thrust belt; a deeper one at ~22 km exhuming a series of basement blocks throughout the section, and a shallower one beneath the Río Chubut Middle Valley, associated with lower amplitude folds affecting the Cenozoic cover (modified from Echaurren et al., 2016).

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bounded by two forearc basins—the Chiloé Basin on the oceanic platform to the west and the Ancud Basin in the submerged Central Valley to the east. The cordilleran region reaches an average height of ~2000 m, the highest altitudes being defined by active volcanoes such as Michinmahuida, Chaitén, Melimoyu, Hornopirén, and Yanteles (e.g., López-Escobar et al., 1993). Kinematic and petrological analysis has revealed a close connection between the activity of these centers and the structural control exerted by the Liquiñe-Ofqui Fault Zone (LOFZ), a NNE-trending dextral strike-slip to transpressional system that runs for more than 1200 km and interacts with NE-trending extensional faults and NW-trending sinistral transpressive faults (Cembrano et al., 1996; Arancibia et al., 1999; Pérez-Flores et al., 2016). This major structure originates at the Chile Triple Junction by partitioning of deformation due to oblique subduction and cuts through the western Andean slope at the latitude of interest. It was responsible for enhanced denudation during Plio-Quaternary time (Cembrano et al., 1996; Thomson, 2002; Thomson et al., 2010). The eastern Andean slope constitutes a retroarc fold-thrust belt characterized by thick-skinned deformation that uplifts several N-S striking ranges classically known as Precordilleran (Ramos and Cortés, 1984; Giacosa and Heredia, 2004; Giacosa et al., 2005; Orts et al., 2015; Echaurren et al., 2016). This fold-thrust belt had previously been unrecognized in the literature, due to low shortening values that were interpreted as deformation being primarily accommodated by the LOFZ (e.g., Cembrano et al., 2002). However, Giacosa and Heredia (2004) and Giacosa et al. (2005) quantitatively determined the Tertiary activity of east-vergent thrusts, recognizing the amplitude and distribution of the deformation involved, whereas the role of Cretaceous deformation regarding Andean orogenesis and initial foldthrust belt growth has been recently addressed by Echaurren et al. (2016, 2017). Toward the east, the foreland is disrupted by a series of uplifted blocks, especially in the southwestern border of the North Patagonian Massif (Fig. 1), which led Bilmes et al. (2013) to integrate the Precordilleran ranges and the Patagonian foreland into a single genetic unit known as the Patagonian broken foreland (see Fig. 1). For the period after the continental fragmentation of Gondwana and the establishment of an eastdipping subduction zone in Jurassic time, different plate kinematic models have been proposed for reconstruction of the various oceanic plates subducted beneath South America. Initially, rates of convergence were determined for the Farallon/Nazca-South America system for post-Cretaceous time (PardoCasas and Molnar, 1987; Somoza, 1998), a record that has been extended to ~170 Ma by Maloney et al. (2013) based on the global plate reconstruction model of Seton et al. (2012). More recently, the global model presented by Müller et al. (2016) proposes a different arrangement of the subducted oceanic (Pacific) plates that have interacted with the Patagonian margin, which can be summarized as the Chasca-Catequil (Cretaceous), Phoenix-Aluk (Paleogene), Farallon (Paleogene-Miocene) and, since ~23 My, the Nazca-Antarctica plates.

3 ­Mesozoic evolution: From continental breakup to initial Andean orogeny Two main tectonic processes characterize Patagonia at ~43°S at the inception of the Andean cycle in Early Mesozoic time: arc magmatism linked to the eastward subduction of oceanic (proto-Pacific) plates beneath the continent and, at a relatively short distance inboard (~500 km), the later stages of plume-related volcanism related to the breakup of Gondwana in the proto-Atlantic margin (Gust et al., 1985; Uliana and Biddle, 1987; Rapela and Kay, 1988; Pankhurst et al., 2000; Suárez and Márquez,

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2007; Navarrete et al., 2016). These magmatic associations led to the formation of a trench-parallel magmatic belt by the time of opening of the South Atlantic Ocean in the Early Cretaceous. They are characterized by distinctive compositional features that as follows: (i) “Pacific” or subduction-derived for the Patagonian Batholith, the Subcordilleran Batholith and the Ibáñez-Lago la Plata Formation and Divisadero Group, (ii) “Continental” or transitional with mixed parental magmatic sources for the Batholith of Central Patagonian and the Lonco Trapial Formation, and (iii) “Atlantic” or crust-related for the Marifil, Chon Aike, and Tobífera formations (Echaurren et al., 2017 and references therein).

3.1 ­Geology of the Mesozoic igneous rocks 3.1.1 ­The Andean domain ­Patagonian Batholith Forming the core of the cordillera, the Patagonian Batholith extends as a margin-parallel magmatic belt over ~1500 km from ~40° to 56°S and constitutes one of the largest igneous bodies on the planet. It can be divided into northern (~40°–47°S), southern (~47°–53°S), and Fueguian (~53°–56°S) segments (e.g., Weaver et al., 1990; Bruce et al., 1991; Pankhurst et al., 1999; Suárez and De la Cruz, 2001; Thomson et al., 2001; Hervé et al., 2007a; Castro et al., 2011) roughly correlating with different times of magma generation and emplacement. The North Patagonian Batholith (NPB), the focus of this review, is limited to the south by the Chile Triple Junction at the latitude of Lago General Carrera/ Buenos Aires (Figs. 2 and 3). Individualization, space-time distribution, and a regional compositional zonation of different plutons in the NPB has proven difficult, since mapping and sampling have been hampered by restricted access and dense vegetation conditions of the main cordilleran ranges. Even so, magmatic rates defined by the available radiometric data indicate two major episodic but protracted pulses in Late Jurassic-Late Cretaceous and Neogene time, separated by magmatic waning/ gap through the Paleogene (e.g., Pankhurst et al., 1999; Suárez and De la Cruz, 2001; Suárez et al., 2010a,b). In the northeastern NPB at ~41°S, monzogranite, leucogranite, tonalite, and granodiorite with restricted gabbro intrude Paleozoic plutonic rocks, geochronologically constrained to ~170–150 Ma by U-Pb and, according to plagioclase-amphibole thermobarometry, crystallized under low pressures (0.5–1.5 kbar) at shallow crustal depths (Castro et al., 2011). Jurassic plutonic suites are also found in the southern NPB, where Suárez and De la Cruz (2001) dated granitoid rocks at ~150 Ma (K-Ar) in the Lago General Carrera/Buenos Aires area. Data from the western-Chilean slope indicate that the plutonic assemblages intrude different metamorphic complexes of Paleozoic age (Hervé et al., 2007a,b) and are composed primarily of granodiorite and tonalite deformed by shear zones of the LiquiñeOfqui Fault Zone (Cembrano et al., 1996), bracketed to the ~120–100 Ma interval (K-Ar, SernageominBRGM, 1995) and emplaced at ~14–8 km depth (Siefert et al., 2003). Most of the geochronological data from widely spaced areas east of the LOFZ indicate Cretaceous ages of crystallization. Rb-Sr isochrons for continental Chiloé (~43°S) carried out by Pankhurst et al. (1992) constrain the axial zone of the batholith to ~120–100 Ma, where it is composed of monzogranite and granodiorite emplaced in the Paleozoic basement and the volcano-sedimentary Jurassic rocks, as recognized farther south at ~43° and 44°S by Barbieri et al. (1994) with ages of ~138 Ma and ~118 Ma using the same method. In the southern Aysén sector between ~44° and 47°S, Pankhurst et al. (1999) describe mid-Cretaceous monzogranite, granite, and subordinate diorite and gabbro spanning the axial Andean zone, with Early Cretaceous units farther west in the Chonos Archipelago (See Fig. 2).

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FIG. 3 Schematic geological chart of the main units discussed in the text. See Echaurren et al. (2016) for detailed references.

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Late Cretaceous plutonic suites are scarce to absent in the main Andes, although restricted granodiorites and tonalites intrude previous phases of the NPB in the central-eastern border of the batholith in the northern and southern segments (Toubes and Spikermann, 1973; Sepúlveda and Viera, 1980; Pankhurst et al., 1999). More widespread are satellite plutons to the east in the extra-Andean zone, intruding Paleozoic metasedimentary rocks and Jurassic volcanic and sedimentary sequences, and ranging compositionally from tonalite to granite. They have yielded K-Ar ages of 90–75 Ma (Turner, 1982; Benito and Chernicoff, 1986; López de Luchi et al., 1992).

­The Subcordilleran Batholith This plutonic association, also known as the Subcordilleran Plutonic Belt, was originally defined as a series of granodioritic, tonalitic, and granitic rocks exposed in the pre-cordilleran ranges at ~42°S that intrude metamorphic and plutonic Paleozoic rocks (Gordon and Ort, 1993). To the south, between ~42°30′ and 44°S, and continuing the morphological trend of these ranges, outcrops of granodiorite, monzonite, and diorite, along with alkaline gabbro and norite (Page, 1984; Page and Page, 1999), have been identified by Rapela et al. (2005) as a single, N/NNE-trending belt. U-Pb zircon dating by the latter authors constrains the plutonic activity to ~185–181 Ma. This suite is contemporaneous with shallow marine to deltaic sedimentary sequences of Early Jurassic age, part of the “Liassic Basin” (e.g., Lizuaín, 1980; Suárez and Márquez, 2007), roughly in agreement with initial Rb/Sr isochrons of 200 ± 24 and 183 ± 13 Ma (Gordon and Ort, 1993). Closer to the cordilleran axis, exposures of gabbroic rocks near Lago Fontana at ~45°S have been dated at ~190 Ma (Rolando et al., 2002), even though this age is at variance with contact relations with Lower Cretaceous sedimentary rocks cropping out in the area (e.g., Depine and Ramos, 2004). The Subcordilleran Batholith has been considered the first clear subduction-related magmatic unit in north Patagonia (e.g., Page and Page, 1999; Rapela et al., 2005).

­Mesozoic volcanic arc Early studies grouped the Andean volcanic sequences as “porphyritic complexes” due to their essentially rhyolitic composition (e.g., Quensel, 1913; Heim, 1940). Much of the volcanic cover of the central foreland and the Atlantic coast (e.g., Rapela and Kay, 1988; Rapela and Pankhurst, 1993; Pankhurst and Rapela, 1995; Feraud et al., 1999; Suárez et al., 1999; Zubia et al., 1999) has been assigned to the “Chon Aike Magmatic Province” (Kay et  al., 1989; Pankhurst et  al., 1998). This large igneous province includes volcanic units exposed in the Antarctic Peninsula, being divided by Pankhurst et al. (2000) into separate volcanic episodes: V1 (187–178 Ma), V2 (172–162 Ma), and V3 (157–153 Ma). Genetic classification and temporal extent of these events have advanced since the pioneer study of the latter authors, led by the refinement of the geochronological constraints and petrological classification that allowed them to be assigned to a subduction or crustal melting origin. In this section, we will examine the distribution, age constraints, deformational features, and geochemical signatures of these Patagonian Mesozoic magmatic units. Volcanic sequences of Mesozoic age are distributed as continuous outcropping sections in the high cordillera of the North Patagonian Andes, and more scattered to the east in the precordilleran ranges (Haller and Lapido, 1982, Fig. 2). Jurassic volcanic rocks, the Ibáñez Formation in Chile and Lago la Plata Formation in Argentina, constitute thick associations (800–1500 m; Ramos, 1981; Bruce, 2001) of dacitic-rhyolitic ignimbrites, acidic domes, and breccias, with a less differentiated component of basalt and andesite lavas. They represent a continental volcanic episode, though scarce fine-grained lacustrine sedimentary rocks have been identified as intercalations at the top of the formation (Suárez

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et al., 2009). This nomenclature (Ibáñez/Lago la Plata) includes local denominations on both Andean slopes: the Huemul, Arroyo Cajón, and Elizalde formations, among others (Thiele et al., 1978; Pesce, 1978; Skármeta and Charrier, 1976; Fuenzalida, 1968). The silicic ignimbrites are locally associated with thick packages (500–1000 m) of mafic rocks. For example, in the northern Futaleufú segment (Fig. 3), basalt and basaltic andesite with tholeiitic affinity have been described (Haller and Lapido, 1982; Massaferro, 1999; Blesa, 2004; Echaurren et al., 2017), resting unconformably over the Paleozoic metamorphic complexes that crop out in the Chilean slope. Even though no clear stratigraphic relation of these rocks to the silicic units has been described, radiometric dating between ~180 Ma and 170 Ma (K-Ar, Haller and Lapido, 1982; Rb-Sr, Ghiara et al., 1999; U-Pb, Blesa, 2004) suggests that these lava flows correspond to an older magmatic pulse. In the southern Aysén segment (Fig.  3), thicker sections of Jurassic volcanic rocks are exposed in the locality of Coyhaique and in the Lago General Carrera/Buenos Aires area. These sequences have been described as dominantly silicic pyroclastic rocks, ignimbrites and lithic tuffs, surge deposits, local sedimentary intercalations, and restricted andesitic-basaltic lava flows. A basal conglomerate succession has been identified in this region, with metamorphic clasts probably derived from the Eastern Andean Metamorphic Complex (De La Cruz et al., 2004). Geochronological determinations by different methods (K-Ar, U-Pb, Ar-Ar) have consistently bracketed this volcanic activity to a ~157–150 Ma interval (see Fig.  3), even though younger ages up to ~138 Ma (U-Pb zircon, Pankhurst et  al., 2003; K-Ar, Suárez et al., 2009) have been determined. Cretaceous volcanic activity is represented by the Divisadero Group, composed of mainly silicic volcanoclastic sequence of dacitic-rhyolitic tuffs and domes, andesitic lava flows, and local sedimentary intercalations. Eruption in the type area is constrained to 116–118 Ma (U-Pb zircon, Pankhurst et al., 2003), even though volcanic activity probably extends from ~120 to ~100 Ma (Haller and Lapido, 1982; Aragón et al., 2011a; Suárez et al., 2015). Outcropping sections in the main cordillera define continuous exposure between ~41° and 47°S; unlike the Jurassic units, they do not have correlative units to the south but they have been associated with volcanoclastic, tuff-rich and distal epiclastic deposits of the Kachaike Formation and the Baqueró Group in the Deseado Massif area (Césari et al., 2011; PérezLoinaze et al., 2013; Passalia et al., 2015). The Divisadero Group includes local denominations such as Cordón de las Tobas, Ventisquero Member of the Tamango Formation, Carrenleufú, Chile Chico, Payaniyeu, Ñirehuau, El Gato and Pico Solo formations (Fuenzalida, 1968; Skármeta and Charrier, 1976; Thiele et al., 1978; Pesce, 1978; Ploszkiewicz and Ramos, 1977; Haller and Lapido, 1982); those in the Aysén segment consist of subvolcanic stocks and hypabyssal bodies. The two volcanic associations (Ibáñez/Lago la Plata Formation and Divisadero Group) are stratigraphically separated by a sedimentary succession consisting of marine and deltaic deposits: stratigraphically from base to top, the Toqui or Tres Lagunas, Katterfeld and Apeleg formations (Ramos, 1981). This sedimentary package consists of fossiliferous sandstones that turn to finer-grained lithologies like shale and siltstone locally known as the Coyhaique Group filling the Río Mayo Basin (part of the northern Austral Basin, Suárez et al., 2009).

3.1.2 ­Extra-Andean domain

The Mesozoic evolution of the extra-Andean domain in northern Patagonia is marked by formation of the Cañadón Asfalto Basin in a series of subparallel depocenters along the southwestern margin of the North Patagonian Massif containing more than ~3 km of fill (e.g., Figari, 2005; Figari et al., 2015). The basin has been described as reflecting different states of continental rift maturity, related to the

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main extensional phase that affected the Patagonian foreland during Gondwana breakup and spanning a sedimentation interval from Early Jurassic to early Paleocene. The western margin of the basin is bounded by a main NW-trending and east-dipping half-graben fault. Contractional inversion along with conjugate NW-faulting exhumed Late Paleozoic to Early Mesozoic plutonic igneous basement (Fig. 4). It also raised the Sierra de Taquetrén, a ~300-km-long feature discordant to the N-S striking configuration of the western precordilleran ranges that delimits the Patagonian broken foreland at these latitudes. Here, following the same structural trend, the Batholith of Central Patagonia is a major NW-striking plutonic belt, composed of calc-alkaline suites of mainly granodioritic composition of Late Triassic age (~220–200 Ma; Rapela et  al., 1991; Zaffarana et  al., 2014a) emplaced in other Late Paleozoic intrusive units (the Mamil Choique Complex of Martínez Dopico et al., 2011). Based on field relations and compositional and radiometric characteristics, the batholith has been divided into two main superunits, Gastre and Lipetrén: the former composed of granodiorite, monzogranite, and diorite with an age of ~222–213 Ma and the latter by syenogranite of ~213–204 Ma (Rb-Sr, Rapela et al., 1991; Ar-Ar, Zaffarana et al., 2014a). Pervasive deformation of these plutonic units follows NE-trending as ductile shear zones, developing mylonitic foliation at meso- and microscopic scale. These observations led Rapela and Pankhurst (1992) to interpret this syntectonic deformation as provoked by continent-scale dextral strike-slip movement along a subvertical major structure (the Gastre Fault Zone; Rapela et al., 1991). Zaffarana and Somoza (2012) and Zaffarana et al. (2014a,b, 2017) instead attributed this deformation to a syntectonic sinistral transpressional regime producing NW-SE directed magmatic foliation in both superunits. The early rifting stage and sedimentary infill of the Cañadón Asfalto Basin was characterized by continental fluvial and deltaic facies of the Las Leoneras Formation, correlative with the Piltriquitrón Formation of the Liassic Basin to the west (e.g., Suárez and Márquez, 2007). Unconformably overlying the plutonic units of the Batholith of Central Patagonia, the Lonco Trapial Formation is a volcanic and sedimentary association that consists mainly of calc-alkaline volcanic facies of predominantly andesitic composition, along with dacitic lava flows, domes, breccias, silicic ignimbrites, and a sedimentary-volcanoclastic component represented by conglomerates and silicic tuffaceous levels (Lesta and Ferello, 1972; Zaffarana et  al., 2014b). The most continuous sections are exposed in the Sierra de Lonco Trapial, Sierra de Taquetrén and Sierra de Agnia with thicknesses over ~500 m (Fig. 4). Recent studies date this magmatic activity to the Early Jurassic (~185 Ma, Ar-Ar, Zaffarana and Somoza, 2012; ~188–179 Ma, U-Pb, Cúneo et al., 2013; ~172 Ma, U-Pb, Hauser et al., 2017; ~182 Ma, U-Pb, Marquez et al., 2016), temporally closer to the Marifil Formation rhyolites in the Atlantic coast than the Middle Jurassic period previously proposed (~160 Ma, Aragón et al., 2000). The thickest sedimentary filling of the basin is represented by the Cañadón Asfalto Formation, a continental succession of lacustrine, fluvial, and deltaic deposits of Middle Jurassic to lowermost Cretaceous age. This unit is divided into two members. The lower one (Cañadón Asfalto Formation or Las Chacritas member) is characterized by fossil-rich calcareous and pelitic lacustrine deposits intercalated with lava and pyroclastic flows at its base (Volkheimer et al., 2009; Cabaleri et al., 2010). The upper member, known as the Cañadón Calcáreo Formation or the Puesto Almada member (Silva Nieto et  al., 2003; Cabaleri et al., 2005; Cabaleri and Benavente, 2013), is formed by limestones with sandy and tuffaceous intercalations. The volcanic units in the Cañadón Asfalto Formation have been recently studied in the Navidad sub-basin corresponding to the northern depocenters by Bouhier et al. (2017). They are characterized as basaltic to andesitic lava flows, which are highly altered and host polymetallic mineralization in basement-related faults. Geochronological dating by these authors (U-Pb in zircons) constrains these

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FIG. 4 Figure caption continued page on 12

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units to ~170–173 Ma, while the tuffaceous intercalations and sandstone horizons in both members of the Cañadón Asfalto Formation were recently dated by Hauser et al. (2017) as ~176–158 Ma. The Cañadon Asfalto Basin also hosts part of the Chubut Group that extends all over central Patagonia (Lesta, 1968). The Chubut Group is a thick continental succession mainly distributed between ~45° and 47°S in drill holes through the Golfo de San Jorge Basin and in the uplifted and deformed San Bernardo fold-thrust belt, where it presents a wide-ranging stratigraphy and fossil content (from base to top, Matasiete/Pozo D-129, Castillo, Bajo Barreal, and Laguna Palacios formations; Fitzgerald et al., 1990; Suárez et al., 2009; Paredes et al., 2013; Gianni et al., 2015). The rocks are mainly fluvial and lacustrine with variable input of pyroclastic materials: whereas the base of the group has been dated at ~120 Ma (Paredes et al., 2007), the top members possess maximum depositional ages of ~97–99 Ma (U-Pb; Suárez et al., 2014). In the Cañadón Asfalto area, the Chubut Group is present as thick packages of continental sedimentary rocks divided into the Los Adobes and Cerro Barcino formations (Codignotto et al., 1978). Final infilling of the basin is represented by the Paso del Sapo and Lefipán formations of fluvial to shallow marine facies, representing an Atlantic transgression that covered the southwestern edge of the North Patagonian Massif (Lesta and Ferello, 1972; Scasso et al., 2012). While the sandstones with tuffaceous horizons of the Paso del Sapo Formation have a maximum depositional age of ~83 Ma (UPb, Echaurren et al., 2016), the Lefipán Formation contains abundant marine fossils that allow correlation with the southern Salamanca Formation, dated by Clyde et al. (2014) as ~67 Ma (Ar-Ar).

3.2 ­Basin evolution and deformational processes Several Jurassic-Cretaceous basins were generated in northern Patagonia; their infilling and geometrical arrangement reflects the dominant extensional stress field that affected vast portions of the arc and retro-arc zone. In Early Jurassic time, simultaneously with the main activity of the Subcordilleran Batholith, a Pacific transgression filled a series of NNW-trending intra-arc to retroarc depocenters with fine-grained sedimentary sequences of both continental and marine origin. This has been referred to as the “Chubut Liassic Basin” (Suárez and Márquez, 2007), the Chubut Basin (Navarrete et al., 2016), or the Pampa de Agnia Basin (Zaffarana et al., 2017), which Vicente (2005) correlated with northern sections of the Neuquén Basin at ~37°S. At the studied latitudes, the westernmost exposures are of ~191– 194 Ma continental sandstones (Spalletti et al., 2010; Orts et al., 2012) that are uplifted to ~1700 m in the Cerro Plataforma area of the main cordillera, preserving syn-extensional features (Orts et al., 2012). These sedimentary units are deformed in most of the western precordilleran ranges at ~43°S, such as the sierras of Esquel and Tecka (see Fig. 2C). FIG. 4, CONT'D Geochronological data for the Mesozoic igneous units in the cordilleran zone of North Patagonia. Note a nonuniform pattern of temporal zonation within the batholith, and the marked inferred contact between the mid-Cretaceous and Neogene rocks, separated by the main trace of the Liquiñe Ofqui fault Zone. Radiometric ages were taken from Haller and Lapido (1982), González Díaz (1982), Benito and Chernicoff (1986), Rapela et al. (1988), Pankhurst et al. (1992, 1999, 2000, 2003), Gordon and Ort (1993), Carrasco (1995), Sernageomin-BRGM (1995), Ghiara et al. (1999), Muñoz et al. (2000), Bruce (2001), Parada et al. (2001), Suárez and De la Cruz (2001), Rolando et al. (2002), Arenas and Duhart (2003), Blesa (2004), Vattuone and Latorre (2004), Rapela et al. (2005), Suárez et al. (2009), Castro et al. (2011), Aragón et al. (2011a), Poblete et al. (2014), and Hervé et al. (2017).

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FIG. 5 (A) Geological map of the northwestern edge of the Cañadòn Asfalto Basin, where the Sierra de Taquetrén delineates the main inverted front of the basin. Radiometric ages for the volcanic units are from Zaffarana and Somoza (2012), Hauser et al. (2017), and fission track data from Savignano et al. (2016). (B) Sesimic line 7613 showing the deformation at the Taquetrén thrust front, with syn-contractional deposition of the Los Adobes Formation. (C) Seismic line 7602, where the syn-extensional control on the basal units of the basin is exerted by NE-trending normal faults with no signs of tectonic inversion.

Even though an extensional regime controlling these depocenters has been invoked as a dominant influence throughout northern Patagonia during the ~200–180 Ma interval, in accordance with the inferred stretched crust hosting the emplacement of the Subcordilleran Batholith (Page and Page, 1999), subsurface sections in the extra-Andean domain reveal contractional phases expressed through angular unconformities between Lower Jurassic and Middle Jurassic strata (Navarrete et  al., 2016).

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Here, clockwise block rotations and intrabasin unconformities (Geuna et al., 2000), possibly linked to the same contractional event proposed by Suárez and Márquez (2007) in the precordilleran ranges at ~45°S, were interpreted by Navarrete et al. (2016) as caused by a major change in plate motion vectors (Muller et al., 2016) aided by magmatically induced lithospheric weakening by the “Chon Aike LIP.” Closer to the cordilleran zone, Late Jurassic-Early Cretaceous subsidence occurred in a new position within the retroarc where the Coyhaique Group was deposited. This extensional regime has been associated with a late synrift-to-sag stage, the basal members of the basin being intercalated with the youngest volcanic rocks of the Ibáñez/Lago la Plata Formation (~140–138 Ma; Pankhurst et al., 2003; Suárez et al., 2009). This transgression-regression episode reached its northernmost extent at the considered latitudes (~43°S), represented by isolated outcrops that widen and thicken in the Aysén or Río Mayo depocenters (e.g., Aguirre-Urreta and Ramos, 1981). In contrast to the protracted extensional regime of Jurassic rifting and thermal subsidence deduced by the stratigraphic and structural relations of Mesozoic units in the Aysén segment (e.g., Suárez et al., 2009), the northern Futaleufú segment exhibits evidence of contraction and tectonic inversion. In the Cordón de Las Pirámides (Fig. 2), steeply deformed strata of the Coyhaique Group are unconformably overlain by gently folded rocks of the Divisadero Group; an angular unconformity also recognized in the Cordón Situación immediately to the east (Echaurren et al., 2016, 2017). The change in the dominant stress field is well constrained by syn-extensional geometry of the Lago la Plata Formation in the Sierra de Galeses, where a half-graben controls the thickness of the Jurassic volcanic sequences through NE-trending normal faults (Echaurren et al., 2017). These structures are similar to synrift wedges in the Cordón Situación, indicating active NE-directed normal faults contemporaneous with the Lago la Plata volcanism. This pre-Aptian/Albian deformational episode is compatible with the observations of Suárez et al. (2010b) who identify volcanic activity of ~122–120 Ma immediately before the closure of the northern Austral Basin. Early Jurassic detrital zircon (~180–170 Ma) in late Early Cretaceous sedimentary sequences at ~45–46°S (Pankhurst et al., 2003; Ghiglione et al., 2015) implies exhumation of the Lago la Plata Formation and Subcordilleran Batholith as terrigenous source of the basin. In the Cañadón Asfalto Basin, the major NW-directed half-graben structure that accommodated thick sedimentary sequences almost uninterruptedly throughout Jurassic time, suffered a kinematic change during mid-to-Late Cretaceous time that can be recognized along the Sierra de Taquetrén (Fig. 6). A contractional regime can be first identified in the structural arrangement of the basal Chubut Group here (Los Adobes Formation). In the back limb of the Sierra de Taquetrén, sandstones horizons of this sequence exhibit progressive east-vergent unconformities that are seen in subsurface seismic sections as tightly folded strata with onlapping reflectors and a rearrangement of the depocenter, thickening eastwards away from the fault trace (Echaurren et al., 2016; Fig. 6B). As noted by Navarrete et  al. (2016), selective inversion of the pre-extensional faults occurred according to the preferential orientation of the structures against the dominant stress field, for both the Jurassic and Cretaceous contractional episodes (see Fig. 6B and C). During the Late Cretaceous-early Paleocene, the Paso del Sapo and Lefipán formations marked the final infilling of the basin during an Atlantic marine transgression that encroached far to the west at these latitudes. In the Sierra de Taquetrén, the Paso del Sapo Formation displays a fan-like arrangement of progressive unconformities toward the west, limited by a reverse fault that thrusts rocks of the Los Adobes Formation over this unit, with growth strata and onlapping relations. The contractional character of this structure is further corroborated by the marked supply of ~116 Ma detrital zircon in the Los Adobes Formation (Echaurren et al., 2016). The deformational episode continued up to Danian time, as indicated

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FIG. 6 (A) Multielement spider diagrams for the Jurassic and Cretaceous cordilleran volcanic rocks normalized against primitive mantle values (Sun and McDonough, 1989) (modified from Echaurren et al., 2017). (B) Ba/La vs Nb/La ratios distinguishing subduction-related and within-plate associations. (C) Rare earth diagrams for the Jurassic and Cretaceous cordilleran volcanic rocks normalized against chondrite values (Nakamura, 1974) (modified from Echaurren et al., 2017). (D) Initial isotopic values of 87Sr/86Sr vs εNd for igneous rocks in both the cordilleran and the Cañadón Asfalto area. For data sources, see Echaurren et al. (2017). For (A) and (C), plotted samples correspond to Echaurren et al. (2017) and compositional fields (blue = Jurassic, green = Cretaceous) from several references specified in the same study.

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by progressive unconformities in the Lefipán Formation related to SW-directed thrusting of the Lonco Trapial Formation (Echaurren et al., 2016) with a foredeep geometrical arrangement (Ruiz et al., 2005): it was concluded by emplacement of the undeformed volcanic neck and related lavas of the Eocene Gorro Frigio alkaline volcanic center. This Cretaceous-early Paleocene setting differs from previous ideas of dominant extension and transpression-transtension linked to block rotations (e.g., Aragón et al., 2013; Figari et al., 2015) but is supported by recent thermochronological approaches that indicate Late Cretaceous-Paleogene exhumation of the broken foreland at these latitudes (Savignano et al., 2016).

3.3 ­Geochemical features and magma sources The available geochemical data of igneous rocks (major, minor, and trace elements and isotopic compositions) from both the western (cordilleran) and eastern (Cañadón Asfalto Basin) domains are summarized in order to characterize their compositional features and discuss the different tectonomagmatic setting interpreted for different igneous rocks across Patagonia. Datasets are specified in Fig. 6. In the Andean realm, the compositional spectrum of total alkalis vs silica for the considered plutonic (North Patagonian and Subcordilleran batholiths) and volcanic (Ibáñez/Lago la Plata Formation and Divisadero Group) rocks follows a partly continuous trend from basalt (gabbro) to rhyolite (granite), showing a direct correlation of SiO2 with Al2O3 and an inverse correlation with MgO, Fe2O3 , and TiO2 (Echaurren et al., 2017). The volcanic association corresponds to a medium-K calc-alkaline domain (Echaurren et al., 2017). For the North Patagonian Batholith, the Jurassic plutons described by Castro et al. (2011) define a less differentiated component compared to other Mesozoic suites (Pankhurst et al., 1992, 1999), while the slight bimodal composition of the Subcordilleran Batholith was interpreted by Rapela et al. (2005) as due to fractional crystallization from gabbro to granite. For the volcanic rocks, multielement diagrams show enrichment of the large ion lithophile elements (LILE) compared to the high-field strength elements (HFSE), an enrichment in Pb, and depletion in Nb, Ta, Ti, and P for both Jurassic and Cretaceous units (Fig. 6A), the latter with higher contents of all immobile elements. The rare earth elements (REE) also show similar patterns for both groups, with a progressive depletion of the heavy REE (HREE) with respect to the light REE (LREE), less pronounced in the Cretaceous rocks, along with a marked negative Eu anomaly and higher overall contents in all elements for the latter (Fig. 6C). These compositional features indicate that the Ibáñez/Lago la Plata Formation and the North Patagonian Batholith are a cogenetic continental arc association with a slab-derived origin (Pankhurst et al., 2003; Echaurren et al., 2017). Nevertheless, Ba/La vs Nb/La diagrams (Fig. 6B) suggest that some of the Jurassic volcanic rocks fall in the within-plate compositional field. For the northern Futaleufú segment, ~42–44°S (see Fig. 4), Pankhurst et al. (1992) proposed a juvenile magmatic source without significant crustal contributions for late Early Cretaceous monzogranites and monzonites, according to (87Sr/86Sr)i and εNd(i) values (~0.7042 and +0.5 to +1.5, respectively). The volcanic rocks of the Divisadero Group, constrained to ~120–100 Ma (see Section 3.1.1 and Fig. 4), show mostly similar 87Sr/86Sr values of 0.7035–0.7051 and εNd between +0.9 and +3.1 (Echaurren et al., 2017). The Jurassic and Cretaceous volcanic units analyzed by Echaurren et al. (2017) show a covariation of initial 87Sr/86Sr with initial εNd from the depleted mantle field toward radiogenic (crustal) compositions (Fig.  6D). In the Aysén segment between ~44° and 46°S (see Fig.  4), the Cretaceous granitoids show a notable decrease in initial 87Sr/86Sr, from ~0.7050–0.7054 (Early Cretaceous) to ~0.7036–0.7043 (mid-Cretaceous), indicating a change from a more radiogenic to a juvenile source (Pankhurst et al., 1999).

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In the cordilleran igneous series, there is a trend in Sr and Nd isotopic signatures from the mantle array toward more crustal values, which suggests intermediate or mixed sources of asthenospheric mantle and crustal origin, with the Paleozoic metamorphic complexes as a probable contaminant (Fig. 6G). The cordilleran volcanic series (Ibáñez/Lago la Plata, Quemado, and Tobifera, see Fig.  2) have extremely diverse isotopic signatures, even within the Lago la Plata Formation (Fig. 6G). Particularly, the data of Echaurren et al. (2017) indicate a relatively juvenile source for the northern Lago la Plata Fm. that contrasts with the highly crustal signature seen in the “V3” episode of Patagonia and the Antarctic Peninsula (Pankhurst et al., 2000; Riley et al., 2001; Rapela et al., 2005 and references therein). This suggests that the primitive volcanic rocks of the Lago la Plata Formation could be part of the initial Andean subduction magmatism along with the Subcordilleran Batholith and thus would not share a genetic relationship with the highly crust-contaminated V3 rocks that include the younger (~150 Ma) and silicic members of this unit. The Lonco Trapial Formation and the volcanic units intercalated in the Cañadón Asfalto and Cañadón Calcáreo formations show generally consistent geochemical characteristics: a calc-alkaline affinity, general enrichment of LILE vs HFSE elements, negative Nb-Ta anomalies, and high La/Ta ratios (>25) (Zaffarana et  al., 2014a,b; Bouhier et  al., 2017). The Lonco Trapial Formation, however, shows mixed characteristics between arc-related and rift-related settings (Dejonghe et al., 2002; Zaffarana et al., 2014b), reinforced by intermediate Hf initial isotopic values (εHf between −2.2 and +4.0; Hauser et al., 2017). Furthermore, the Nb-Ta anomaly in these rocks was interpreted by Bouhier et al. (2017) as not derived from slab-related fluids, but due to magmatic assimilation of crustal materials depleted in these elements. Basaltic units of the Cañadón Asfalto/Calcáreo formations show similar geochemical signatures, with εHf between −5.6 and +2.1 interpreted by Hauser et al. (2017) as crustal reworking of Late Paleozoic basement units, namely the Mamil Choique complex. Pb isotopic compositions of the Lago la Plata Formation also exhibit a mixed character, between asthenospheric mantle and continental crust signatures, although they are more radiogenic with respect to 206Pb and 207Pb than the Lonco Trapial Formation and the active volcanic arc emplaced at the same latitudes (~41–44° S; Echaurren et al., 2017). A possible explanation for this difference is thicker crust toward the west in Jurassic time and hence more interaction between the magmas and the continental crust. On the other hand, the less-enriched radiogenic character of the Lonco Trapial Formation could be derived from different crustal domains, which seems a plausible explanation according to Zaffarana et al. (2014b) who separate two domains within this unit: a northern one distributed over the North Patagonian Massif, more similar to the Marifil Formation with evidence of higher degrees of crustal contamination (La, Yb, Sr, Y contents suggesting a garnet-bearing source) and a southern domain over the Cañadón Asfalto Basin with less evolved rocks suggesting an extension-related genesis.

4 ­Cenozoic evolution: Paleogene extensional regime and Neogene Andean rise During the Early Cenozoic, there was a drastic change in North Patagonian magmatism, accompanied by Paleogene-early Miocene retro-arc extension and widespread volcanism in the central foreland. Contraction was renewed in middle Miocene time, leading to final consolidation and uplift of the main Andes and the broken foreland system. In the early extensional stages, several changes in the tectonic configuration resulted from subduction of the Farallón-Aluk active ridge at ~50 Ma (Cande and Leslie,

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1986) and the following breakup of the Farallón plate into the Nazca and Antarctica plates at ~23 Ma (Lonsdale, 2005); this had been preceded by high convergence rates at ~28–25 Ma (Cande and Leslie, 1986; Somoza, 1998). During this period at ~40–44°S, abundant volcanism spread from the forearc to the distal foreland, interrupting Andean development (e.g., the Coastal magmatic belt emplaced in Chiloé island at 24–18 Ma), temporally preceded by the ~29–25 Ma plateau lavas of the Somun Cura Magmatic Province (Muñoz et al., 2000; Kay et al., 2006). We will focus on the volcanic units of the central foreland, known as the Pilcaniyeu and El Maitén belts.

4.1 ­Geology of Cenozoic magmatic rocks After the Late Cretaceous eastward expansion of arc magmatism, there was a Paleogene lull in activity between ~75 and ~50 Ma (e.g., Suárez and De la Cruz, 2001; Suárez et al., 2010a,b; Aragón et al., 2011b). Isolated plutons were emplaced in the central foreland, west of the Cañadón Asfalto Basin at ~57 Ma (U-Pb, Rodríguez et al., 2017), followed by scattered Late Eocene granites and granodiorites in the forearc—in the eastern Chonos Archipelago, 44–39 Ma (Rb-Sr, Pankhurst et al., 1999) and Chiloé island (~39 Ma, U-Pb, Arenas and Duhart, 2003; ~37 Ma, K-Ar, Muñoz et al., 2000) As the vigorous plutonic activity of the North Patagonian Batholith waned, voluminous volcanism occurred in the central foreland, between the Andean axis and the Cañadón Asfalto Basin (Fig. 2). Following the early recognition of volcanic rocks with subordinate sedimentary sequences at ~40–44°S (“Andesitic Series”; Groeber, 1918; Feruglio, 1927), a distinction was made between an eastern association of PaleoceneEocene age (Huitrera Formation) and a western volcanic assemblage of Oligocene-early Miocene age (Ventana Formation) on the basis of radiometric dating (González-Bonorino, 1979; González Díaz, 1979; Rapela et al., 1983, 1988). These units were later integrated into the Pilcaniyeu and El Maitén belts (Rapela et al., 1988), whose distinctive distribution in age and composition has been related to different petrogenetic models (e.g., Rapela and Kay, 1988; Aragón et al., 2011a,b, 2013; Iannelli et al., 2017; Fernández Paz et al., 2018). The Pilcaniyeu Belt is located in the central broken foreland, to the west of the Cañadón Asfalto Basin; while the younger, El Maitén Belt, is located to the east of the Andes (Figs. 2 and 7). The Pilcaniyeu Belt strikes N-S, being strongly deflected toward the cordilleran axis at ~42°S. It comprises bimodal volcanic sequences included in the Huitrera Formation, geochronologically constrained in the northern sector between ~55 and ~43 Ma (K-Ar and Ar-Ar; Wilf et al., 2010; Iannelli et al., 2017), while southern exposures in the Río Chubut Middle Valley are bracketed within the ~58– 47 Ma interval (K-Ar; Mazzoni et al., 1991). Volcanic rocks of the Pilcaniyeu Belt are characterized by a wide compositional range, from rhyolite to basalt. The northern sector (north of 42°S) is characterized by typical lavic-pyroclastic facies in which the lava flow facies alternates from basalt to rhyolite (Rapela et al., 1988; Iannelli et al., 2017). South of ~42°S, larger amounts of ignimbrites, tuffs, and breccias are present, with subordinate andesites and basalts. These lithological variations over time, from large volumes of silicic calc-alkalic magmatism (ignimbrite flare-up) to final stages characterized by alkali basalts and tholeiitic basalts-trachytes, have been interpreted as a transition from arc to within-plate associations (Rapela et al., 1988; Aragón et al., 2011a,b). Late Eocene-early Miocene volcanic rocks crop out in the N-S trending El Maitén Belt, which comprises the Ventana Formation (González Bonorino and González Bonorino, 1978) between 41° and 43°S, and the Auca Pan Formation (Turner, 1973) cropping out further north at 39°30′S. The Ventana Formation constitutes the basal unit of the Nahuel Huapi Group and forms part of the initial infill of

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FIG. 7 (A) Distribution and radiometric ages of the Pilcaniyeu and El Maitén belts and the Ñirihuau Formation, taken from Orts et al. (2012), Fernández Paz et al. (2018), Bilmes et al., 2013, Ramos et al. (2014), Iannelli et al. (2017), and Bechis et al. (2014), see inset for location of different figures. (B) Residual Bouguer anomaly map, filtered up to 40 km, where the basement segmentation is evident. (C) Aeromagnetic data showing how the NW-oriented Sierra de Taquetrén truncates the N-S striking structural fabric corresponding to the precordilleran ranges. Note how the eastward shift of the Cretaceous intrusive units has a spatial association with the Jurassic-Early Cretaceous basins and Oligocene inverted structures. Thick black line in (A) and (B) indicates a NW-directed lineament that obliterates the strike of main structures (“Nahuel Huapi lineament” of Bechis et al. (2014)).

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the Ñirihuau basin (Cazau et al., 2005; Bechis et al., 2014). The belt encompasses an extended time span of 37–20 Ma with a wide lithological variation all over the belt and through time (Rapela et al., 1988; Bechis et al., 2014; Fernández Paz et al., 2018). This sequence reaches a thickness of 3500 m and is composed of andesitic and dacitic lava flows with lesser amounts of rhyolite and basalt. Although the most voluminous pulses are Oligocene in age, older late Eocene and younger early Miocene ages have also been registered. The older pulses are volumetrically restricted: U-Pb ages of 37 ± 0.7 Ma and 35.8 ± 1.7 Ma and paleontological content confirm a Late Eocene age (Sepúlveda, 1980; Benedini et  al., 2017; Fernández Paz et  al., 2018). They are mainly composed of basaltic to andesitic lavas and volcanic breccias, with interbedded pyroclastic flows and continental deposits (Sepúlveda, 1980; Fernández Paz et al., 2018). The main Oligocene pulse has K-Ar ages that range from 33 to 23 Ma and is characterized by andesitic to dacitic lava flows that prevail over ignimbrite and plinian facies (Rapela et al., 1984, 1988). Younger (23–19 Ma) exposures at ~41°S are of andesite, andesitic breccia, and pyroclastic deposits replaced by basaltic lavas interbedded with marine and pyroclastic deposits toward the top of the section (Bechis et al., 2014). Arc plutonism was renewed in the Miocene, primarily concentrated in the western Andean slope and spatially related to the western side of the Liquiñe-Ofqui Fault Zone (see Fig.  2). Here, in the inner forearc zone from ~41°30′ to 47°S, the batholith is mainly composed of mafic-intermediate assemblages of tonalite, diorite, gabbro, and subordinate granitic suites with early Miocene (~20–18 Ma) and late Miocene-Pliocene (10–4 Ma) crystallization ages (Rb-Sr, Pankhurst et al., 1999), emplaced at shallow crustal levels (<0.3 GPa; Adriasola et al., 2006). In the northern NPB at ~41°S, granitoids with subordinate gabbroic suites dated at ~18–12 Ma occur farther east (González Díaz, 1982; Aragón et al., 2011a,b).

4.2 ­Basin evolution and deformational processes Major Cenozoic basin formation was related to extensional and contractional periods in the North Patagonian margin: first, protracted Paleogene extension with a late Oligocene-early Miocene climax and, second, foreland basins with an eastward-growing fold-thrust belt from middle to late Miocene times. The late Oligocene-early Miocene extension-related retro- to intra-arc basins are represented by isolated outcrops in the forearc with maximum sedimentation ages of ~26–17 Ma (U-Pb, see Encinas et al., 2018). These are partly contemporaneous with accentuated extension in the southern Traiguén Basin (~45°S), where primitive mafic volcanic rocks and marine sequences were emplaced under thinning crustal conditions at ~26–23 Ma (e.g., Hervé et al., 1995; Encinas et al., 2016). In the retroarc, the Ñirihuau Basin extends along and to the east of the Andean axis (Fig. 1) and contains ~3000 m of continental and marine strata in a main depocenter at ~41°S (Ñirihuau depocenter, Fig. 7A and B), while minor depocenters are located toward the main Andes (El Bolsón depocenter) and dispersed as thinner sections scattered south to ~42°S (e.g., Bechis et al., 2014; Martínez et al., 2016, see Fig. 2). These sedimentary rocks are fossil-rich continental, mainly fluvial and lacustrine, and shallow marine facies with abundant pyroclastic supply, included in the Nahuel Huapi Group as the Ventana Formation (forming part of the El Maitén belt) and the Ñirihuau Formation in the Ñirihuau depocenter, both being temporal equivalents of the Río Foyel Group of the El Bolsón depocenter (Cazau et al., 1989; Paredes et al., 2009; Bechis et al., 2014). Age constraints of these sequences are mainly provided by fossil content and U-Pb maximum detrital zircon sedimentation ages of ~23–16 Ma, although older Paleogene depositional ages have been proposed for the El Bolsón depocenter (Asensio et al., 2005;

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Martínez et al., 2016). Zircon provenance patterns indicate terrigenous supply from Paleozoic igneous sources and the Mesozoic magmatic arc (Bechis et al., 2014). These sections are intensely deformed in tight east-vergent folds in the thick-skinned Ñirihuau fold-thrust belt (Giacosa et al., 2005; Bechis and Cristallini, 2005; see Fig.  2). Paleozoic and Jurassic plutonic rocks are affected together with the Ventana/Nirihuau formations, focusing maximum topography, exhumation, and shortening in the North Patagonian fold-thrust belt (Tobal et al., 2015; Orts et al., 2015). Deformed sequences are also found to the west disconnected from the main fold-thrust belt in the inner cordilleran zone at ~42°S. Here, Orts et al. (2012) describe the Cerro Plataforma strata as a clastic continental sequence of ~18 Ma (maximum depositional age, U-Pb) affected by syn-sedimentary thrusts. To the south, at ~43°S, tuffaceous sandstones of the Ñirihuau Formation are gently deformed, with angular unconformities in the eastern slope of the Cordón Rivadavia, exhibiting milder deformation than in the northern exposures (Echaurren et al., 2016). The distribution of depocenters of the Ñirihuau Basin is controlled by basement segmentation in the eastern Andean slope and the uplifted blocks that characterize the broken foreland. Their geometry is observed through gravity and magnetic data, which delimit the eastern extent of the Cretaceous suites of the North Patagonian Batholith (Fig. 7B and C). Tectonic inversion of the structures that controlled accumulation of the Jurassic-Early Cretaceous units and the Ventana Formation is thought to be the main mechanism of deformation: it resulted in the establishment of a foreland basin and closure of the Pacific connection between the two Andean slopes. However, the retroarc extension of the early Ñirihuau Basin could also have been caused by a late Oligocene transtensional regime, as suggested by Ramos et al. (2014) at ~40°S in a zone of structural interference of NW- and NE-directed lineaments that reactivated the Mesozoic extensional fabric. Thick middle Miocene sequences of reworked tuffaceous material of the Collón Cura Formation are distributed from the southern Neuquén Basin at ~39°S, near the main Andes, to the surroundings of Sierra de Taquetrén in the Cañadón Asfalto Basin at ~43°S. A contractional regime has been proposed as the dominant setting during deposition of these strata (~16–10 Ma), where progressive unconformities have been identified in the eastern Andean slope and toward the foreland (Ramos et al., 2011; Huyghe et al., 2015; Echaurren et al., 2016). Similar structures are found in the eastern Cañadón Asfalto Basin (Bilmes et al., 2013; Bucher et al., 2017), where Bilmes et al. (2013) dated deformation at ~14 Ma (Ar-Ar, see Fig. 7A). The geometrical arrangement of depocenters around basement blocks characterizes this system as a broken foreland basin that evolved to intermontane depocenters in late Miocene-Pliocene time (Huyghe et al., 2015; Bucher et al., 2017).

4.3 ­Geochemical data The Pilcaniyeu Belt comprises tholeiitic basalt, calc-alkaline rhyolite, and dacite with similar major element geochemistry to the present SVZ, although with a slightly higher alkali content (Aragón et al., 2011a,b). Trace element patterns display relative enrichment in LILE (Cs, Ba, Rb), but differ from typical arc signatures in the absence of characteristic troughs in Nb and Ta contents. REE patterns are relatively flat with higher overall La/Yb ratios than the SVZ, which could imply residual garnet and thus a deeper mantle source for these magmas (Aragón et al., 2011a,b). Incompatible elements show low Ba/La and high Nb/La ratios, along with high Ta/Hf, allowing assignment of these rocks to withinplate rather than subduction settings (Aragón et al., 2011a,b, Fig. 8E and F).

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FIG. 8 Geochemical composition of the Cenozoic volcanic rocks of northern Patagonia (Pilcaniyeu and El Maitén belts). (A) Silica content vs total alkalis (TAS; Le Bas et al., 1986). (B) Tectonic discrimination diagrams based on Hf-Ta-Th and Zr-Th-Nb (after Wood, 1980). (C) Multielement spider diagrams normalized against primitive mantle values (normalizing values after Sun and McDonough, 1989). (D) Rare earth diagrams normalized to chondrite values (normalizing values after Nakamura, 1974). Ba/La vs Nb/La (E) and Ta/Hf vs Th/Hf (F) ratios distinguishing subduction-related and within-plate associations.

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Tectonic discrimination diagrams suggest a within-plate signature with mostly EMORB-like sources, with some samples even plotting in OIB fields (Fig. 8). Geochemical analyses of the northernmost part of this belt, near the main Andean cordillera, exhibit more arc-like trace element patterns, with small negative Nb-Ta anomalies. Although these rocks still show enriched sources and an alkaline tendency, some trace elements ratios show more arc-like values and hence allow distinction from previously described rocks of the Pilcaniyeu Belt (Iannelli et al., 2017). Overall, the syn-extensional and HFSE-enriched nature of the Pilcaniyeu Belt suggests a within-plate setting with high degrees of decompression melting of a lithosphere enriched by remnant subduction components (Aragón et al., 2011a,b). Genesis of this belt was related to asthenospheric upwelling, with the weak influence of slab-derived fluids, at a time when the Farallon-Aluk active ridge reached the South American margin (Aragón et al., 2011a,b; Iannelli et al., 2017). Magmatic arc activity resumed in the upper Eocene forming part of the El Maitén Belt, initially composed of tholeiitic basalt and andesite (Fernández Paz et al., 2018). Multielement diagrams display arc-like patterns with LILE enrichment relative to the negative Nb-Ta, Sr-P, and Ti anomalies (Fig. 8). REE diagrams show flat patterns with low La/Yb and Dy/Yb that reflect low-pressure equilibrium with residual mineral assemblages. The Ba/Nb ratios of these volcanic rocks typically exceed 30, indicative of an arc setting (Fernández Paz et  al., 2018) (Fig.  8). The genesis of these initial low-volume magmas was controlled by extensional structures and characterized by a tholeiitic composition with limited slab contributions to the mantle wedge, decompression melting being the main process involved (Fernández Paz et al., 2018). The El Maitén Belt evolved progressively to a more mature calc-alkaline composition (Fernández Paz et  al., 2017; Iannelli et  al., 2017). As shown by Wood (1980), discrimination diagrams (Fig. 8), together with high Th/Ta and low Ta/Hf, indicate that most of the El Maitén Belt samples fall within the field of tholeiitic to calc-alkaline volcanic rocks. The contrasting signatures of these two Paleogene volcanic episodes imply evolution from Paleocenemiddle Eocene within-plate magmatism in the retroarc zone to late Eocene arc-like magmatism to the west in the North Patagonian Andes.

5 ­Discussion: Tectonic evolution and controls on orogenesis The main petrological, structural, and stratigraphic characteristics of the North Patagonian margin indicate that both the Andean and the Cañadón Asfalto domains were affected by similar deformational and magmatic processes that resulted in the construction of a wide fold-thrust system. The recognized stages of contraction and extension seem to have affected the whole of Patagonia nearly simultaneously. In North Patagonia, the transition toward the “typical” Andean subduction setting, i.e., trenchparallel magmatic belt and mountain building processes, took place simultaneously with JurassicEarly Cretaceous destabilization of the Gondwana supercontinent, where the proto-Pacific and proto-Atlantic margins were characterized by subduction-related mobile belts and abundant withinplate volcanism, respectively. Plate reconstructions for this time interval include the Antarctic Peninsula as part of the developing Andean margin south of ~48°S, prior to its detachment and southward drift in middle-late? Jurassic time (e.g., Jokat, 2003; Martin, 2007; Ghidella et al., 2007; Poblete et al., 2016), while the westward-advancing plume-related thermal anomaly of the KarooFerrar LIP developed in Patagonia, South Africa, and west Antarctica, prior to effective opening of the South Atlantic Ocean at ~130 Ma (e.g., Dalziel et al., 2013; Franke, 2013).

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FIG. 9 (A) Isotopic variations (initial 87Sr/86Sr and εNd values) of the Mesozoic igneous rocks. Crosses and triangles indicate plutonic and volcanic rocks, respectively, and are color-coded according to 87Sr/86Sr (i) (blue) and εNd (i) (red) values. Note the correspondence of units with temporal scale at bottom (data from Pankhurst et al., 1999; Rapela et al., 2005; Hervé et al., 2007a,b; Echaurren et al., 2017). (B) Histogram of the radiometric age data distribution of magmatic events between ~41 and 47°S. See text and Figs. 3–5 and 7 for references. Abbreviations correspond to: BCP, Batholith of Central Patagonia; LP, Lago la Plata Formation (~180–170 Ma, older pulse); SCB, Subcordilleran Batholith; Ib-LP, Ibáñez-Lago la Plata Formation (~150–135 Ma, younger pulse); DV, Divisadero Group; PC, Pilcaniyeu belt; EM, El Maitén belt; NPB, North Patagonian Batholith. (C) Crustal thickness estimations based on methodology of Chapman et al. (2015) and Profeta et al. (2015). See Echaurren et al. (2017) and Fernández Paz et al. (2018) for detailed dataset sources and compositional considerations for plotted samples. (D) Absolute velocity of the South American Plate and relative convergence with Pacific oceanic plates (taken from Maloney et al., 2013).

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During this transition, the continent experienced clockwise rotations in the Early Jurassic (Iglesia Llanos, 2018). After a rather stationary period during the middle Jurassic, there was a southward drift of ~8 cm/yr in Late Jurassic-Early Cretaceous time (Müller et al., 2016), resulting in overall low rates of trench-normal interplate convergence (< 4 cm/yr, Maloney et al., 2013; Fig. 9). Continental rifting led to a Pacific transgression that flooded the protocordilleran zone through ~NNW-directed extensional fault systems, and spilled into the Piltriquitrón-Osta Arena marine/ continental depocenters in the retroarc. The Cañadón Asfalto Basin registered NW-directed rifting with deposition of shallow marine facies (Las Leoneras depocenter) and the short-lived (~15 My) primitive volcanism of the Lonco Trapial and Cañadón Asfalto formations, which may have shared a parental melting source(s) with the older plume-related volcanism (Marifil Formation, V1 event, 187–178 Ma). The tectonomagmatic events and basin evolution described for northern Patagonia are consistent with an extensional regime ruled by a slab-rollback scenario. Intraplate deformation in the San Jorge and Cañadón Asfalto basins was related to a changing upper plate motion aided by thermal weakening in the Chon Aike Magmatic Province (Navarrete et al., 2016), hence producing no signs of orogenic contraction in the western margin. This constitutes a plausible mechanism to explain the observed magmatic migration pattern of ~180–130 Ma subduction-related units, such as the magmatic arc focus shifting from the NW-trending Subcordilleran Batholith to the ~NNE-trending suites of the North Patagonian Batholith in the eastern Chonos Archipelago (Fig. 10). Westward migration of the magmatic focus was at least ~50 km at the northern apex (~41°S) and ~300 km in the south (~44°S). This was accompanied by a trend toward more juvenile magmas, with increasing values of initial εNd and decreasing initial 87Sr/86Sr (Fig. 9). Even though this “scissor-like” slab rollback (Rapela et al., 2005; Echaurren et al., 2017) is preferred over the hypothesis that invokes accretion of a lithospheric block against the margin (the Fitz Roy terrane of Hervé and Mpodozis, 2005), the inception of rollback is not defined and the previous Late Triassic scenario needs to be considered. A slowed, uninterrupted, subduction during Late Triassic time in the Central and Southern Andes (e.g., Vásquez et al., 2011; del Rey et al., 2016; González et al., 2017) seems a consistent scenario for northern Patagonia, where high P/T metamorphic rocks of the Chonos Metamorphic Complex indicate accretionary processes in a subduction setting (Hervé et al., 2007a,b). This is also consistent with involvement of slab-derived fluids in the petrogenesis of the Late Triassic Batholith of Central Patagonia, which was exhumed by contractional tectonics prior to Lonco Trapial volcanism (Zaffarana and Somoza, 2012). In this case, kinematic reconstruction of Early Mesozoic mobile belts should also take into account the eastern Late Triassic and the western Early Jurassic units, with their southward correlatives in southern Patagonia (La Leona granite, ~200 Ma, Rapela and Pankhurst, 1996) and the Antarctic Peninsula (~188–181 Ma granitoids, Riley et al., 2017). After this Jurassic-Early Cretaceous extensional stage, formation of the North Patagonian foldthrust belt began in late Early Cretaceous time. Contraction in the upper plate, as evidenced by strong angular unconformities at the base of mid-Cretaceous strata in both the cordilleran and foreland realms (Divisadero Group and Los Adobes Formation, respectively), caused inversion of the northern Austral (Río Mayo-Aysén depocenter) and Cañadón Asfalto basins. This marked the beginning of a series of contractional pulses on the western border of the Cañadón Asfalto basin, represented by syntectonic deposition of the Late Cretaceous Paso del Sapo Formation and the early Paleocene Lefipán Formation. The exhumation event determined by Savignano et al. (2016) for the Cañadón Asfalto Basin is in accordance with field and seismic evidence of Navarrete et al. (2015) and Echaurren et al. (2016).

FIG. 10 Migration patterns of magmatic focus for (A) Late Triassic-Early Cretaceous and (B) Paleocene-Miocene periods. Both stages precede the contractional episodes that affected the whole north Patagonian margin, representing extensional, trench retraction periods from the main lithospheric heterogeneity located in the southwestern border of the North Patagonian Massif. This limit acted as an eastern boundary for the propagation of magmatism and fold-thrust belt development during mid Cretaceous-Paleocene and mid-late Miocene contractional periods.

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The first pulse of contraction is well registered in the North Patagonian Andes, where the outcropping volcanic units show evidence of syn-extensional (Lago la Plata Formation) and syn-contractional emplacement (Divisadero Group). Geochemical parameters (Ce/Y, La/Yb, Sr/Y) in these units show no sign of Aptian-Albian arc crustal thickening but, on the contrary, crustal thinning from Jurassic to midCretaceous time (Echaurren et al., 2017) simultaneous with high magmatic rates during emplacement of the North Patagonian Batholith (Fig. 9). This is shown in the trends of crustal thickness estimations in Fig. 9. A similar scenario is described for the Antarctic Peninsula, where the Antarctic Peninsula Batholith experienced a flare-up event (Lassister Coast batholith) related to crustal thinning during the late Early Cretaceous (Riley et al., 2016, 2017), followed by contractional deformation during the midCretaceous (Palmer Land event; Vaughan et al., 2012). Even though Riley et al. (2016, 2017) suggested delamination as the trigger for enhanced magmatism and the inferred crustal thinning, this does not seem to be the situation for northern Patagonia, as crustal thicknesses probably did not surpass ~45 km, and hence did not reach the garnet stability zone where density-driven crustal-lithospheric foundering could occur. Early Cretaceous crustal thinning was attributed by Echaurren et al. (2017) to extensional stretching due to a steeper slab angle, consistent with intra-retroarc extension during deposition of the Coyhaique Group. A notable eastward expansion of magmatism in the North Patagonian Batholith and correlative units of the Divisadero Group (Don Juan Formation, Franchi and Page, 1980) took place at the considered latitudes between ~118 Ma and ~75 Ma, followed by a lull in arc activity for the ~75–50 Ma period (e.g., Suárez et al., 2010a,b, see Fig. 9). The deformational pattern together with this eastward magmatic migration points to a flat-slab configuration (Gianni et al., 2015; Echaurren et al., 2016). The detailed timing of these events has been extensively discussed by Gianni et al. (2018), who proposed a large-scale flat-slab model that links the Masstrichtian-Danian Atlantic transgression of the Paso del Sapo/Lefipán formations to dynamic subsidence produced by the flattening. Lithospheric weakening in the upper plate would have enhanced this rapid spread of deformation, favored by the shallow subduction angle. Potential triggers of the Cretaceous flat slab are related to the collision of active midocean ridges (Chasca-Catequil), whose young and buoyant oceanic lithosphere would encourage the slab flattening (Gianni et al., 2015; Echaurren et al., 2016). Alternatively, westward acceleration of the South American plate driven by the opening of the South Atlantic Ocean may have led to a flat slab controlled by upper-plate parameters (e.g., Guillaume et al., 2018). Destabilization of this configuration may have been related to subduction of a highly oblique active oceanic ridge at ~52–50 Ma beneath a nearly stationary South American plate, separating the Farallon and Aluk plates (Cande and Leslie, 1986; Somoza and Ghidella, 2005). This could have caused effective opening of a slab window, provoking asthenospheric upwelling as the magmatic source of the within-plate members of the Pilcaniyeu Belt at the considered latitudes (Iannelli et al., 2017). As proposed by Fernández Paz et al. (2018), this mechanism could explain magmatic and structural patterns in a simpler way than pure slab rollback of the Farallon plate (Echaurren et al., 2016) or the complete detachment of the Aluk plate at depth with following formation of a transform margin (Aragón et al., 2011a,b, 2013). Field evidence shows the syn-extensional character of these volcanic units (Fig. 7), where caldera-type volcanism took place with almost no crustal assimilation (Aragón et al., 2011a,b). The emplacement of Late Eocene volcanic units closer to the Andean axis, controlled by extensional structures (Fig. 8), followed by the main lava pulse of El Maitén Belt (~23–17 Ma) and the final emplacement of North Patagonian Batholith granitoids in the present western forearc (Fig.  10), define a new SW-clockwise magmatic migration pattern in the continental margin. This changing tectonic scenario was probably controlled by a new slab rollback episode.

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The breakup of the Farallon plate into the Nazca and Antarctica plates at ~23 Ma was associated with a significant increase in the rate of relative interplate convergence (Fig. 10). Prior to this, the late Oligocene-early Miocene period saw accentuated extension in the margin, where Pacific and Atlantic transgressions inundated several parts of the Patagonian margin. These probably did not reach the cordilleran zone at ~43°S but were more accentuated to the south at ~45°S (Bechis et al., 2014). According to Encinas et al. (2018), the widespread distribution and simultaneity of transgressive sequences along the margin, and the presence of deep marine sedimentary rocks in the forearc, point to a more general extensional environment for these deposits rather than local effects. This could correspond to the enhanced thermal weakening and crustal thinning provoked by vigorous fore- to retroarc volcanism along the margin, where Fernández Paz et al. (2018) estimated thin crust during emplacement of the El Maitén Belt (Fig. 10). Nevertheless, the localization of deep depocenters of the Ñirihuau Basin (Fig. 8) suggests local transtension controlled by the interaction of ~ENE-directed Andean stresses and the NW-striking lineament where maximum deformation of the Ñirihuau and Collón Cura formations is focused. A change from extensional retroarc conditions to foreland basin development seems to have started at ~19–16 Ma with syn-contractional deposition of the Río Foyel Group and Ñirihuau Formation. Progressive uncomformities and growth strata (Ramos et al., 2011; Orts et al., 2012, 2015; Echaurren et al., 2016) represent incipient tectonic switching or a “transition phase” (Bechis et al., 2014). Later, syn-orogenic strata in the late Miocene Collón Cura Formation (~16–11 Ma) were related to this late contractional pulse, being spatially associated with the NW-trending Taquetrén thrust front (Ramos et al., 2015; Fig. 7). The contractional reactivation of the western border of the Cañadón Asfalto Basin (~14 Ma, Ar-Ar, Bilmes et  al., 2013), recognized in growth strata of the Collón Cura Formation in Sierra de Taquetrén, accounts for the notable spatial coincidence of the Miocene deformational front with the Cretaceous one. This observation reinforces the premise that the southwestern border of the North Patagonian Massif acted as a mechanical limit for growth of the fold-thrust belt (Fig. 10). Based on this analysis, the Cretaceous contractional phase appears as the dominant tectonic event defining the continental architecture of northern Patagonia after the pronounced extension associated with Gondwana breakup. However, the deformational front of the subsequent Neogene stage mimics the Cretaceous one, regardless of their different and contrasting mechanisms. Both the CretaceousPaleocene and Neogene contractional pulses and the Jurassic-Early Cretaceous and Paleoegene extensional stages were spatially controlled by the southwestern border of the North Patagonian Massif. This border was proposed as a terrane suture between the North Patagonian and Deseado massifs, with scattered Carboniferous S-type postcollisional granitoids exposed in the surroundings of the Sierra de Taquetrén (Pankhurst et al., 2006; Ramos, 2008; Schilling et al., 2017). Such a lithospheric anisotropy might control the emplacement path of magmas and act as a mechanical barrier for the propagation of deformation, thus exerting a first-order control in the tectonic evolution of the northern Patagonia margin (Fig. 10).

6 ­Conclusions Two main orogenic stages affected the North Patagonian margin: late Early Cretaceous-early Paleocene and middle Miocene to present. Associated deformation penetrated into the continent defining a complex fold-thrust belt structure from the cordilleran zone to the Cañadón Asfalto Basin, with a thickskinned style that absorbed reduced shortening values. Both contractional episodes were preceded by

­References

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protracted extension associated with enhanced within-plate magmatism and high rates of slab rollback, resulting in southwestward migrating patterns of magmatic focus and upper plate extension. These phases interrupted typical Andean development, not within a classic orogenic cycle setting, but related to major tectonic reorganization during early Mesozoic and early Cenozoic time, disrupting the trenchparallel configuration of arc magmatism and fold-thrust belt construction. In both contractional stages, similar primitive volcanism is recognized in the progression toward a more stable configuration, represented by the initial Lago la Plata Formation (~170 Ma) and the El Maitén Belt (~37 Ma), and showing preferential arrangement around a first-order NW-striking lithospheric weakness that may correspond to a suture between different Paleozoic blocks.

­Acknowledgments The authors declare no conflict of interest in the development of this investigation, supported by projects PIP 11220150100426, UBACYT 20020150100166BA, PICT-2016-2252 and Proyecto Fondecyt 1151146. We are truly grateful to R.J. Pankhurst and an anonymous reviewer for detailed corrections that greatly improved the quality of the present manuscript. We thank our partners at the Instituto de Estudios Andinos for fruitful discussions on the tectonics of Patagonia.

­References Adriasola, A.C., Thomson, S.N., Brix, M.R., Hervé, F., Stöckhert, B., 2006. Postmagmatic cooling and late Cenozoic denudation of the North Patagonian Batholith in the Los Lagos region of Chile, 41−42 15′ S. Int. J. Earth Sci. 95 (3), 504–528. Aguirre-Urreta, M.B., Ramos, V.A., 1981. Estratigrafía y paleontología de la alta cuenca del río Roble, Cordillera Patagónica, Provincia de Santa Cruz. In: VIII Congreso Geológico Argentino, pp. 101–138. Angermann, D., Klotz, J., Reigber, C., 1999. Space-geodetic estimation of the Nazca-South America Euler vector. Earth Planet. Sci. Lett. 171 (3), 329–334. Aragón, E., González, P.D., Aguilera, Y.E., Cavarozzi, C., 2000. Andesitas Alvar: volcanismo alcalino jurásico en el área de Paso del Sapo, provincia del Chubut. Rev. Asoc. Geol. Argent. 55, 44. Aragón, E., Castro, A., Díaz-Alvarado, J., Liu, D.-Y., 2011a. The North Patagonian batholith at Paso Puyehue (Argentina-Chile). SHRIMP ages and compositional features. J. S. Am. Earth Sci. 32, 547–554. https://doi. org/10.1016/j.jsames.2011.02.005. Aragón, E., D’Eramo, F., Castro, A., Pinotti, L., Brunelli, D., Rabbia, O., Rivalenti, G., Varela, R., Spakman, W., Demartis, M., Cavarozzi, C.E., Aguilera, Y.E., Mazzucchelli, M., Ribot, A., 2011b. Tectono-magmatic response to major convergence changes in the North Patagonian suprasubduction system: the Paleogene subduction–transcurrent plate margin transition. Tectonophysics 509, 218–237. https://doi.org/10.1016/j. tecto.2011.06.012. Aragón, E., Pinotti, L., D’eramo, F., Castro, A., Rabbia, O., Coniglio, J., Demartis, M., Hernando, I., Cavarozzi, C.E., Aguilera, Y.E., 2013. The Farallon-Aluk ridge collision with South America: implications for the geochemical changes of slab window magmas from fore- to back-arc. Geosci. Front. 4, 377–388. https://doi. org/10.1016/j.gsf.2012.12.004. Arancibia, G., Cembrano, J., Lavenu, A., 1999. Transpresión dextral y partición de la deformación en la Zona de Falla Liquiñe-Ofqui, Aisén, Chile (44-45oS). Rev. Geol. Chile 26, 3–22. Arenas, M., Duhart, P., 2003. Geología del Área Castro-Dalcahue. Servicio Nacional de Geología y Minería. Cart. Geológica Chile. Ser. Geol. Básica 79, 31.

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Vattuone, M.E., Latorre, C.O., 2004. 2004. Edades K/Ar al este del Cerro Nahuel Pan, Chubut. Implicancias en la correlación del Grupo Divisadero y del Choiyoi en el área. Rev. Asoc. Geol. Argentina. 59 (3), 510–513. Vaughan, A.P.M., Leat, P.T., Dean, A.A., Millar, I.L., 2012. Crustal thickening along the West Antarctic Gondwana margin during mid-Cretaceous deformation of the Triassic intra-oceanic Dyer Arc. Lithos 142–143, 130–147. Vicente, J.C., 2005. Dynamic paleogeography of the Jurassic Andean Basin: pattern of transgression and localisation of main straits through the magmatic arc. Rev. Asoc. Geol. Argent. 60, 221–250. Volkheimer, W., Gallego, O.F., Cabaleri, N.G., Armella, C., Narváez, P.L., Silva Nieto, D.G., Páez, M.a., 2009. Stratigraphy, palynology, and conchostracans of a Lower Cretaceous sequence at the Cañadón Calcáreo locality, Extra-Andean central Patagonia: age and palaeoenvironmental significance. Cretac. Res. 30, 270–282. https://doi.org/10.1016/j.cretres.2008.07.010. Weaver, S.G., Bruce, R., Nelson, E.P., Brueckner, H.K., LeHuray, A.P., 1990. The Patagonian batholith at 48°S latitude, Chile; Geochemical and isotopic variations. Geol. Soc. Am. 241, 33–50. https://doi.org/10.1130/ SPE241-p33. Wilf, P., Singer, B.S., del Carmen Zamaloa, M., Johnson, K.R., Cúneo, N.R., 2010. Early Eocene 40Ar/39Ar age for the Pampa de Jones plant, frog, and insect biota (Huitrera Formation, Neuquén Province, Patagonia, Argentina). Ameghiniana 47 (2), 207–217. Wood, D.A., 1980. The application of a Th-Hf-Ta diagram to problems of tectonomagmatic classification and to establishing the nature of crustal contamination of basaltic lavas of the British Tertiary Volcanic Province. Earth Planet. Sci. Lett. 50, 11–30. https://doi.org/10.1016/0012-821X(80)90116-8. Zaffarana, C.B., Somoza, R., 2012. Palaeomagnetism and 40Ar/39Ar dating from Lower Jurassic rocks in Gastre, central Patagonia: further data to explore tectonomagmatic events associated with the break-up of Gondwana. J. Geol. Soc. Lond. 169, 371–379. https://doi.org/10.1144/0016-76492011-089. Zaffarana, C.B., Somoza, R., López de Luchi, M., 2014a. The Late Triassic Central Patagonian Batholith: Magma hybridization, 40Ar/39Ar ages and thermobarometry. J. S. Am. Earth Sci. 55, 94–122. https://doi.org/10.1016/j. jsames.2014.06.006. Zaffarana, C.B., Poma, S., Lagorio, S.L., Gregori, D., Somoza, R., Busteros, A., Silva Nieto, D.G., Giacosa, R.E., 2014b. Petrogénesis de las volcanitas Lonco Trapial, magmatismo del Jurásico temprano de Patagonia Central. In: XIX Congreso Geológico Argentino, Córdoba. Zaffarana, C.B., Somoza, R., Orts, D.L., Mercader, R., Boltshauser, B., González, V.R., Puigdomenech, C., 2017. Internal structure of the Late Triassic Central Patagonian batholith at Gastre, southern Argentina: Implications for pluton emplacement and the Gastre fault system. Geosphere 13, 1973–1992. https://doi.org/10.1130/ GES01493.1. Zubia, M., Genini, A., Schalamuk, I.B., Zappettini, E.O., 1999. Yacimiento Cerro Vanguardia, Recursos Minerales de la República Argentina. Instituto de Geología y Recursos Minerales.

­Further reading Coira, B.L., 1979. Descripción geológica de la Hoja 40 d, Ingeniero Jacobacci. Servicio Geológico Nacional. Gill, J.B., 1981. Orogenic Andesites and Plate Tectonics, Minerals and Rocks. Springer Berlin Heidelberg, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-68012-0. González-Bonorino, F., 1973. Geología del área entre San Carlos de Bariloche y Llao-llao. Dep. Recur. Nat. y Energ. 16. Nullo, F.E., 1978. Descripción geológica de la hoja 41d, Lipetrén, Provincia de Río Negro: Carta geológicoeconómica de la República Argentina, escala 1: 200.000. Servicio Geológico Nacional. Pearce, J.A., Harris, N.B.W., Tindle, A.G., 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. J. Petrol. 25, 956–983.

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Ploszkiewicz, J.V., 1987. Descripcion geologica de la hoja 47 c, Apeleg. Dirección Nacional de Minería y Geología. Ramos, V.A., 1982. Las ingresiones pacíficas del terciario en el norte de la Patagonia (Argentina). In: III Congreso Geológico Chileno, Concepción, pp. 262–288. Ravazzoli, I.A., Sesana, F.L., 1977. Descripción geológica de la hoja 41c, Río Chico, Provincia de Río Negro: Carta geológico-económica de la República Argentina, escala 1: 200.000. República Argentina, Ministerio de Economía, Secretaría de Estado de Minería, Servicio Geológico Nacional.