# The role of slab geometry in the exhumation of cordilleran-type orogens and their forelands: Insights from northern Patagonia

Marie Genge, Massimiliano Zattin, Elisa Savignano, Marta Franchini, Cécile Gautheron, Victor Ramos, Stefano Mazzoli

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## ► To cite this version:

Marie Genge, Massimiliano Zattin, Elisa Savignano, Marta Franchini, Cécile Gautheron, et al. The role of slab geometry in the exhumation of cordilleran-type orogens and their forelands: Insights from northern Patagonia. Geological Society of America Bulletin, 2021, 133 (11-12), pp.2535-2548. 10.1130/B35767.1. hal-04505231

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# The role of slab geometry in the exhumation of cordilleran-type orogens and their forelands: Insights from northern Patagonia

Marie C. Genge<fnref n="1" />,<fnref n="2" />, Massimiliano Zattin<fnref n="1" />, Elisa Savignano<fnref n="1" />, Marta Franchini<fnref n="3" />,<fnref n="4" />, Cécile Gautheron<fnref n="5" />, Victor A. Ramos<fnref n="6" />, and Stefano Mazzoli<fnref n="7" />,†

## ABSTRACT

In cordilleran-type orogens, subduction geometry exerts a fundamental control on the tectonic behavior of the overriding plate. An integrated low-temperature, large thermochronological data set is used in this study to investigate the burial and exhumation history of the overriding plate in northern Patagonia (40°–45°S). Thermal inverse modeling allowed us to establish that a ~2.5–4-km-thick section originally overlaid the Jurassic–Lower Cretaceous successions deposited in half-graben systems that are presently exposed in the foreland. Removal of the sedimentary cover started in the late Early Cretaceous. This was coeval with an increase of the convergence rate and a switch to a westward absolute motion of the South American Plate that was accompanied by shallowing of the subducting slab. Unroofing was probably further enhanced by Late Cretaceous to early Paleogene opening of a slab window beneath the overriding plate. Following a tectonically quiescent period, renewed exhumation occurred in the orogen during relatively fast Neogene plate convergence. However, even the highly sensitive apatite (U-Th)/He thermochronometer does not record any coeval cooling in the foreland. The comparison between Late Cretaceous and Neogene exhumation patterns provides clear evidence of the fundamental role played by inter-plate coupling associated with shallow slab configurations in controlling plate-scale

deformation. Our results, besides highlighting for the first time how the whole northern Patagonia foreland was affected by an exhumation of several kilometers since the Late Cretaceous, provide unrivalled evidence of the link between deep geodynamic processes affecting the slab and the modes and timing of unroofing of different sectors of the overriding plate.

## INTRODUCTION

Mechanical plate coupling is governed by subduction geometry and particularly by slab dip. This latter is controlled by various parameters including: (1) upper plate absolute motion, (2) subducting slab buoyancy, (3) overriding plate temperature, and (4) faulting of the oceanic lithosphere (Gutscher, 2002; Gutscher and Peacock, 2003; Lallemand et al., 2005; Rodríguez-González et al., 2012; Cerpa et al., 2018). Many authors also pointed out correlations in time and space between shallow slab segments and the subduction of buoyant oceanic features such as aseismic ridges or oceanic plateau (Pilger, 1981; McGeary et al., 1985; Gutscher et al., 2000). Upper plate compression is generally associated with strong interplate coupling during shallow to flat-slab subduction, while extension is correlated with weak interplate coupling during slab rollback. Therefore, plate-coupling variations are believed to result in alternating phases of upper plate shortening and extension (Lallemand et al., 2005; Horton, 2018), thereby regulating orogenic growth and foreland evolution (Martinod et al., 2010). Although a high degree of interplate coupling can also characterize steep subduction (depending on convergence rate), this is gener-

ally less effective with respect to that associated with shallow to flat-slab segments (Gutscher, 2002). At present, slab dip varies significantly along the Andean margin from the dominant dip angle of ~30° to flat-slab segments (Ramos and Folguera, 2009; Horton and Fuentes, 2016; Maksymowicz and Tassara, 2018). This results in along-strike segmentation and coupling variations that influence magmatism and the style of recent to active deformation of the overriding plate (Coira et al., 1993; Yáñez and Cembrano, 2004; Ramos and Folguera, 2009).

The geological record of pulsed, upper plate shortening episodes in Patagonia has been related to variable plate coupling associated with changes of plate convergence settings and particularly slab dip (Echaurren et al., 2016; Gianni et al., 2015; Horton, 2018; Orts et al., 2012). Two orogenic stages, a late Early Cretaceous to early Paleogene stage and a middle Mio-cene to Pliocene stage, have been recognized in the Patagonian Cordillera, the Patagonian Precordillera (Fig. 1A; also named the Precordilleran system, or the Eastern Precordillera, or the North Patagonian fold and thrust belt; e.g., Bilmes et al., 2013; Folguera et al., 2018; Orts et al., 2015; Ramos et al., 2014), and the so-called broken foreland generated by the uplift of basement blocks related to the contractional reactivation of older crustal discontinuities and the deformation of former graben/half graben systems (Echaurren et al., 2016; López et al., 2019). These latter were formed in the context of Early Jurassic to Early Cretaceous extension associated with a protracted slab rollback, which is coherent with the westward shift of arc magmatism (Ramos, 1999, 2010). Similar processes have been documented for the intervening late

<footnote n="1">¹Department of Geosciences, University of Padua, Italy</footnote>

<footnote n="2">²Université de Lille, CNRS, Université du Littoral—Côte d’Opale, UMR 8187, LOG, Laboratoire d’Océanologie et de Géosciences, F 59000 Lille, France</footnote>

<footnote n="3">³Instituto de Investigación en Paleobiología y Geología, Universidad Nacional de Río Negro, Argentina</footnote>

<footnote n="4">⁴Centro Patagónico de Estudios Metalogenéticos, Universidad Nacional del Comahue, Neuquén, Argentina</footnote>

<footnote n="5">⁵GEOPS, Université Paris Sud, CNRS, Université Paris-Saclay, Orsay, France</footnote>

<footnote n="6">⁶Instituto de Estudios Andinos, UBA-CONICET, Buenos Aires, Argentina</footnote>

<footnote n="7">⁷School of Sciences and Technology, Geology Division, University of Camerino, Italy</footnote>

<footnote>†Corresponding author: stefano.mazzoli@unicam.it.</footnote>
<figure><img src="image_3.png" /></figure>

# The role of slab geometry in the exhumation of cordilleran-type orogens and their forelands: Insights from northern Patagonia

Marie C. Genge<fnref n="1" />,<fnref n="2" />, Massimiliano Zattin<fnref n="1" />, Elisa Savignano<fnref n="1" />, Marta Franchini<fnref n="3" />,<fnref n="4" />, Cécile Gautheron<fnref n="5" />, Victor A. Ramos<fnref n="6" />, and Stefano Mazzoli<fnref n="7" />,†

## ABSTRACT

In cordilleran-type orogens, subduction geometry exerts a fundamental control on the tectonic behavior of the overriding plate. An integrated low-temperature, large thermochronological data set is used in this study to investigate the burial and exhumation history of the overriding plate in northern Patagonia (40°–45°S). Thermal inverse modeling allowed us to establish that a ~2.5–4-km-thick section originally overlaid the Jurassic–Lower Cretaceous successions deposited in half-graben systems that are presently exposed in the foreland. Removal of the sedimentary cover started in the late Early Cretaceous. This was coeval with an increase of the convergence rate and a switch to a westward absolute motion of the South American Plate that was accompanied by shallowing of the subducting slab. Unroofing was probably further enhanced by Late Cretaceous to early Paleogene opening of a slab window beneath the overriding plate. Following a tectonically quiescent period, renewed exhumation occurred in the orogen during relatively fast Neogene plate convergence. However, even the highly sensitive apatite (U-Th)/He thermochronometer does not record any coeval cooling in the foreland. The comparison between Late Cretaceous and Neogene exhumation patterns provides clear evidence of the fundamental role played by inter-plate coupling associated with shallow slab configurations in controlling plate-scale

deformation. Our results, besides highlighting for the first time how the whole northern Patagonia foreland was affected by an exhumation of several kilometers since the Late Cretaceous, provide unrivalled evidence of the link between deep geodynamic processes affecting the slab and the modes and timing of unroofing of different sectors of the overriding plate.

## INTRODUCTION

Mechanical plate coupling is governed by subduction geometry and particularly by slab dip. This latter is controlled by various parameters including: (1) upper plate absolute motion, (2) subducting slab buoyancy, (3) overriding plate temperature, and (4) faulting of the oceanic lithosphere (Gutscher, 2002; Gutscher and Peacock, 2003; Lallemand et al., 2005; Rodríguez-González et al., 2012; Cerpa et al., 2018). Many authors also pointed out correlations in time and space between shallow slab segments and the subduction of buoyant oceanic features such as aseismic ridges or oceanic plateau (Pilger, 1981; McGeary et al., 1985; Gutscher et al., 2000). Upper plate compression is generally associated with strong interplate coupling during shallow to flat-slab subduction, while extension is correlated with weak interplate coupling during slab rollback. Therefore, plate-coupling variations are believed to result in alternating phases of upper plate shortening and extension (Lallemand et al., 2005; Horton, 2018), thereby regulating orogenic growth and foreland evolution (Martinod et al., 2010). Although a high degree of interplate coupling can also characterize steep subduction (depending on convergence rate), this is gener-

ally less effective with respect to that associated with shallow to flat-slab segments (Gutscher, 2002). At present, slab dip varies significantly along the Andean margin from the dominant dip angle of ~30° to flat-slab segments (Ramos and Folguera, 2009; Horton and Fuentes, 2016; Maksymowicz and Tassara, 2018). This results in along-strike segmentation and coupling variations that influence magmatism and the style of recent to active deformation of the overriding plate (Coira et al., 1993; Yáñez and Cembrano, 2004; Ramos and Folguera, 2009).

The geological record of pulsed, upper plate shortening episodes in Patagonia has been related to variable plate coupling associated with changes of plate convergence settings and particularly slab dip (Echaurren et al., 2016; Gianni et al., 2015; Horton, 2018; Orts et al., 2012). Two orogenic stages, a late Early Cretaceous to early Paleogene stage and a middle Mio-cene to Pliocene stage, have been recognized in the Patagonian Cordillera, the Patagonian Precordillera (Fig. 1A; also named the Precordilleran system, or the Eastern Precordillera, or the North Patagonian fold and thrust belt; e.g., Bilmes et al., 2013; Folguera et al., 2018; Orts et al., 2015; Ramos et al., 2014), and the so-called broken foreland generated by the uplift of basement blocks related to the contractional reactivation of older crustal discontinuities and the deformation of former graben/half graben systems (Echaurren et al., 2016; López et al., 2019). These latter were formed in the context of Early Jurassic to Early Cretaceous extension associated with a protracted slab rollback, which is coherent with the westward shift of arc magmatism (Ramos, 1999, 2010). Similar processes have been documented for the intervening late

<footnote n="1">¹Department of Geosciences, University of Padua, Italy</footnote>

<footnote n="2">²Université de Lille, CNRS, Université du Littoral—Côte d’Opale, UMR 8187, LOG, Laboratoire d’Océanologie et de Géosciences, F 59000 Lille, France</footnote>

<footnote n="3">³Instituto de Investigación en Paleobiología y Geología, Universidad Nacional de Río Negro, Argentina</footnote>

<footnote n="4">⁴Centro Patagónico de Estudios Metalogenéticos, Universidad Nacional del Comahue, Neuquén, Argentina</footnote>

<footnote n="5">⁵GEOPS, Université Paris Sud, CNRS, Université Paris-Saclay, Orsay, France</footnote>

<footnote n="6">⁶Instituto de Estudios Andinos, UBA-CONICET, Buenos Aires, Argentina</footnote>

<footnote n="7">⁷School of Sciences and Technology, Geology Division, University of Camerino, Italy</footnote>

<footnote>†Corresponding author: stefano.mazzoli@unicam.it.</footnote>
<figure><img src="image_2.png" /></figure>

Figure 1. (A) Map view of major tectonic features of the Southern Andes (modified after Navarrete et al., 2016) shows locations of diagrams B–C. Blue dashed lines outline foreland basins (AMB—Austral-Magallanes Basin, CAB—Cañadón Asfalto Basin, NB—Neuquén Basin, and SJGB—San Jorge Gulf Basin); purple dashed lines outline foreland reliefs (DM—Deseado Massif, NPM—North Patagonian Massif, NP—Neuquén Precordillera, and PBF—Patagonian broken foreland). White arrows indicate current motions of oceanic plates relative to South America according to the global model NUVEL-1. (B) Map view of main morphotectonic features between 40°S and 45°S with new and published (Thomson, 2002; Thomson et al., 2010) apatite fission track (AFT) ages. (C) New and published (Thomson et al., 2010; Savignano et al., 2016) apatite (U-Th)/He (AHe) ages.

Eocene to early Miocene period, characterized by abundant magmatism toward the Cordillera and modest foreland subsidence (Folguera and Ramos, 2011; Horton, 2018). The localization of deformation in former basins emphasizes the role of inherited upper plate weaknesses in controlling the propagation of the deformation to the broken foreland during regional shortening episodes (Gianni et al., 2017). However, little is known about the amount of regional unroofing in the foreland of the Patagonian Andes, as

thermochronological studies focused mainly on the growth of the Cordillera (Thomson et al., 2010). In this study we present the results obtained by thermal inverse modeling of low-temperature thermochronology data collected in the Patagonian Precordillera and the broken foreland between 40°S and 45°S (Figs. 1B and 1C) with the goal of evaluating the impact of slab geometry changes—and associated plate coupling variations—on the deformation of the overriding plate. Our results provide a new and

unexpected picture of the large unroofing (in excess of 3 km considering an average geothermal gradient of ~34 ± 11°C/km) experienced by the whole northern Patagonia foreland—and not just the Cordillera—since the late Early Cretaceous.

## GEOLOGICAL SETTING

The northern Patagonian margin is characterized by several morphotectonic units, including the Patagonian Precordillera and the
<figure><img src="image_4.png" /></figure>

Figure 1. (A) Map view of major tectonic features of the Southern Andes (modified after Navarrete et al., 2016) shows locations of diagrams B–C. Blue dashed lines outline foreland basins (AMB—Austral-Magallanes Basin, CAB—Cañadón Asfalto Basin, NB—Neuquén Basin, and SJGB—San Jorge Gulf Basin); purple dashed lines outline foreland reliefs (DM—Deseado Massif, NPM—North Patagonian Massif, NP—Neuquén Precordillera, and PBF—Patagonian broken foreland). White arrows indicate current motions of oceanic plates relative to South America according to the global model NUVEL-1. (B) Map view of main morphotectonic features between 40°S and 45°S with new and published (Thomson, 2002; Thomson et al., 2010) apatite fission track (AFT) ages. (C) New and published (Thomson et al., 2010; Savignano et al., 2016) apatite (U-Th)/He (AHe) ages.

Eocene to early Miocene period, characterized by abundant magmatism toward the Cordillera and modest foreland subsidence (Folguera and Ramos, 2011; Horton, 2018). The localization of deformation in former basins emphasizes the role of inherited upper plate weaknesses in controlling the propagation of the deformation to the broken foreland during regional shortening episodes (Gianni et al., 2017). However, little is known about the amount of regional unroofing in the foreland of the Patagonian Andes, as

thermochronological studies focused mainly on the growth of the Cordillera (Thomson et al., 2010). In this study we present the results obtained by thermal inverse modeling of low-temperature thermochronology data collected in the Patagonian Precordillera and the broken foreland between 40°S and 45°S (Figs. 1B and 1C) with the goal of evaluating the impact of slab geometry changes—and associated plate coupling variations—on the deformation of the overriding plate. Our results provide a new and

unexpected picture of the large unroofing (in excess of 3 km considering an average geothermal gradient of ~34 ± 11°C/km) experienced by the whole northern Patagonia foreland—and not just the Cordillera—since the late Early Cretaceous.

## GEOLOGICAL SETTING

The northern Patagonian margin is characterized by several morphotectonic units, including the Patagonian Precordillera and the
Unroofing Patagonia

<figure><img src="image_3.png" /></figure>

Figure 2. Chronological table for northern Patagonia at around 43°S shows from left to right columns: (1) oceanic plates subducting beneath South America at this latitude through time and slab age (Maloney et al., 2013); (2) plate convergence rate (note the change of the South American Plate motion to the west during the Early Cretaceous as well as periods of plate convergence and divergence; modified from Maloney et al. (2013); (3) distance between the magmatic arc and the trench, which is considered to be a fixed reference marker. Note trenchward migration due to slab rollback during the entire Jurassic and the Oligocene–early Miocene time span and migration toward the eastern foreland during the Late Cretaceous and the middle–late Miocene as well as period of arc shut-off; modified after Gianni et al. (2018); (4) time-space evolution of the magmatic arc and basin development with the different sedimentary and magmatic formations identified in the studied area (note magmatic arc migration toward either the trench or the eastern foreland during slab steepening or slab shallowing episodes, respectively; Echaurren et al., 2016; Horton, 2018; Butler et al., 2020); and (5) the thermochronological data (apatite fission track and apatite (U-Th)/He ages versus longitudinal distribution) used in this study with the time-space evolution of deformation according to Horton (2018) and Butler et al. (2020).

broken foreland to the east (Fig. 1A; Echaurren et al., 2016). The Precordillera is a retroarc thick-skinned fold-thrust belt (Giacosa and Heredia, 2004; Orts et al., 2012). On the other hand, the broken foreland got its name because it is structurally articulated and fragmented by a series of uplifted blocks (e.g., Bilmes et al., 2013). Regional deformation in northern Patagonia is characterized by the shortening of former sedimentary basins (Piltriquitrón Basin and Rio Mayo Basin along the Precordillera, Cañadón Asfalto Rift Basin to the foreland) that developed on top of a low-grade metamorphic basement. The latter is constituted of Paleozoic-

Triassic, volcano-sedimentary successions (e.g., Cushamen, Calcatapul Formations; Volkheimer, 1964; von Gosen and Loske, 2004) and coeval intrusions (e.g., Colohuincul Complex, Mamil Choique, and Lipetrén Formations; Rapela et al., 1992; Ravazzoli and Sesana, 1977; Turner, 1965; Varela et al., 2005). The Cañadón Asfalto Rift Basin is mainly the product of extension induced by Gondwana break-up during the Early Jurassic and of the following Early Cretaceous sag stage (Fig. 2; Figari et al., 2015; Mpodozis and Ramos, 2008). The sedimentary infill includes the continental Las Leoneras Formation, followed above by the Cañadón Asfalto and Cañadón

Calcáreo Formations, which are laterally interbedded with the volcanic Lonco Trapial Formation (Zaffarana and Somoza, 2012; Cúneo et al., 2013). Few discordances observed in the basin succession are correlated with minor shortening events associated with changes in plate motion and volcanic activity (Maloney et al., 2013; Figari et al., 2015; Navarrete et al., 2016, 2018). Along the Patagonian Precordillera, roughly coeval back-arc extension was triggered by a protracted slab rollback generated by low convergence rates and eastward motion of the South American Plate (Giacosa and Heredia, 2004; Suarez and Marquez, 2007; Seton et al., 2012;
Unroofing Patagonia

<figure><img src="image_5.png" /></figure>

Figure 2. Chronological table for northern Patagonia at around 43°S shows from left to right columns: (1) oceanic plates subducting beneath South America at this latitude through time and slab age (Maloney et al., 2013); (2) plate convergence rate (note the change of the South American Plate motion to the west during the Early Cretaceous as well as periods of plate convergence and divergence; modified from Maloney et al. (2013); (3) distance between the magmatic arc and the trench, which is considered to be a fixed reference marker. Note trenchward migration due to slab rollback during the entire Jurassic and the Oligocene–early Miocene time span and migration toward the eastern foreland during the Late Cretaceous and the middle–late Miocene as well as period of arc shut-off; modified after Gianni et al. (2018); (4) time-space evolution of the magmatic arc and basin development with the different sedimentary and magmatic formations identified in the studied area (note magmatic arc migration toward either the trench or the eastern foreland during slab steepening or slab shallowing episodes, respectively; Echaurren et al., 2016; Horton, 2018; Butler et al., 2020); and (5) the thermochronological data (apatite fission track and apatite (U-Th)/He ages versus longitudinal distribution) used in this study with the time-space evolution of deformation according to Horton (2018) and Butler et al. (2020).

broken foreland to the east (Fig. 1A; Echaurren et al., 2016). The Precordillera is a retroarc thick-skinned fold-thrust belt (Giacosa and Heredia, 2004; Orts et al., 2012). On the other hand, the broken foreland got its name because it is structurally articulated and fragmented by a series of uplifted blocks (e.g., Bilmes et al., 2013). Regional deformation in northern Patagonia is characterized by the shortening of former sedimentary basins (Piltriquitrón Basin and Rio Mayo Basin along the Precordillera, Cañadón Asfalto Rift Basin to the foreland) that developed on top of a low-grade metamorphic basement. The latter is constituted of Paleozoic-

Triassic, volcano-sedimentary successions (e.g., Cushamen, Calcatapul Formations; Volkheimer, 1964; von Gosen and Loske, 2004) and coeval intrusions (e.g., Colohuincul Complex, Mamil Choique, and Lipetrén Formations; Rapela et al., 1992; Ravazzoli and Sesana, 1977; Turner, 1965; Varela et al., 2005). The Cañadón Asfalto Rift Basin is mainly the product of extension induced by Gondwana break-up during the Early Jurassic and of the following Early Cretaceous sag stage (Fig. 2; Figari et al., 2015; Mpodozis and Ramos, 2008). The sedimentary infill includes the continental Las Leoneras Formation, followed above by the Cañadón Asfalto and Cañadón

Calcáreo Formations, which are laterally interbedded with the volcanic Lonco Trapial Formation (Zaffarana and Somoza, 2012; Cúneo et al., 2013). Few discordances observed in the basin succession are correlated with minor shortening events associated with changes in plate motion and volcanic activity (Maloney et al., 2013; Figari et al., 2015; Navarrete et al., 2016, 2018). Along the Patagonian Precordillera, roughly coeval back-arc extension was triggered by a protracted slab rollback generated by low convergence rates and eastward motion of the South American Plate (Giacosa and Heredia, 2004; Suarez and Marquez, 2007; Seton et al., 2012;
Maloney et al., 2013; Horton, 2018). This result-
ed in steepening of the slab, as evidenced by the
westward migration of the magmatic arc (Fig. 2)
marked by the Lower Jurassic Subcordillera
Plutonic Belt (Gordon and Ort, 1993; Page and
Page, 1999) and by the Upper Jurassic–Lower
Cretaceous North Patagonian Batholith (Suárez
and De la Cruz, 2001; Pankhurst et al., 2003).
Indeed, lateral shifts of the magmatic arc either
toward the trench or toward the eastern foreland
are promoted by variations of the subduction
zone configuration involving slab steepening or
shallowing episodes, respectively (e.g., Coira
et al., 1993; England et al., 2004; Kay et al.,
2005; Ramos and Folguera, 2005; Syracuse and
Abers, 2006; Folguera and Ramos, 2011; Spag-
nuolo et al., 2012; Schellart, 2017; Fernández
Paz et al., 2019).

Coeval volcanism is recorded by the Middle-
Upper Jurassic Lago La Plata Formation (cor-
related with the Ibáñez Group in Chile; Folguera
and Iannizzotto, 2004; Olivero, 1982) and the
Lower Cretaceous Divisadero Group (Ramos,
1981; Suárez et al., 2009). Respectively, these
volcanic deposits overlie the N-NE-trending,
graben/half-graben systems of the Piltriquitrón
Basin (filled by the Lower-Middle Jurassic Osta
Arena Formation, Piltriquitrón Formation, and
equivalent units intercalated with marine suc-
cessions; Gabaldón, 1982; Giacosa and Here-
dia, 2004; Suarez and Marquez, 2007), and the
Upper Jurassic–Lower Cretaceous sedimentary
successions of the Coyhaique Group hosted in
the Rio Mayo Basin (Skarmeta, 1976; Olivero,
1982; Echaurren et al., 2017). As revealed by
the regional unconformity observed in the up-
per volcanic succession of the Divisadero Group
and on top of the Cañadón Asfalto Rift Basin
deposits, extension was suddenly interrupted
at ca. 120 Ma by a regional shortening episode
(Folguera and Iannizzotto, 2004; Suárez et al.,
2009; Horton, 2018). This latter, acting on a con-
tinental crust mechanically weakened by faults,
produced both the earliest stage of Andean up-
lift and deformation of the Patagonian broken
foreland. Indeed, the deformation, reaching ar-
eas ~500 km away from the trench (Navarrete
et al., 2016), was dominated by the reactivation
of existing faults associated with the shortening
of former depocenters (Gianni et al., 2015). The
initiation of regional shortening coincided with
a change of absolute motion of the South Ameri-
can Plate, which became west directed (Fig. 2;
Maloney et al., 2013; Müller et al., 2016; Se-
ton et al., 2012; Silver et al., 1998). Trenchward
absolute motion of the overriding plate and an
increase of convergence rates (Eagles, 2007;
Maloney et al., 2013; Müller et al., 2016) were
accompanied by slab shallowing manifested in
the upper crust by the eastward expansion of the

magmatic arc during the Late Cretaceous (Haller
et al., 2010; Aragón et al., 2013). This change in
slab dip favored a strong increase in coupling
of the subducting plates and the overriding plate
(Horton and Fuentes, 2016). Growth strata in
foreland basins have been detected in the con-
tinental Upper Cretaceous Chubut Group, Paso
del Sapo Formations, and in the marine Paleo-
cene Lefipán Formations (Gianni et al., 2015;
Echaurren et al., 2016; Navarrete et al., 2016),
which indicates that crustal shortening persisted
throughout Late Cretaceous to Paleocene times
(Echaurren et al., 2016; Horton, 2018). Subduc-
tion of mid-ocean ridges may have sustained
this long-term and significant deformation, as
the positive buoyancy of younger lithosphere
tends to resist subduction (Cloos, 1993), and al-
lowed the opening of a slab window during latest
Cretaceous (Echaurren et al., 2016). A slab win-
dow model, supported by seismic tomography
(Aragón et al., 2011), is generally accompanied
by an isostatic uplift of the overriding plate and a
dynamic topography related to limited deforma-
tion of the retroarc (Aragón et al., 2011, 2013;
Ávila and Dávila, 2020).

Growth strata in the lower Eocene deposits
are observed north and south of the area studied
(Cobbold and Rossello, 2003; Charrier et al.,
2007; Navarrete et al., 2016; Gianni et al., 2017),
where only syn-extensional strata have been rec-
ognized in the upper Paleocene–Eocene Huitrera
Formation associated with bimodal within-plate
volcanism (Pilcaniyeu Belt; Fig. 3; Echaurren
et al., 2016; Iannelli et al., 2018; Mazzoni et al.,
1991). Cessation of the contraction between
40°S and 45°S during the Paleocene may be re-
lated to the restepening of the subducting slab
induced by a sharp reduction in average trench-
ward velocity, an increase of slab pull forces
after asthenospheric window development, and/
or the decreasing buoyancy of oceanic segments
(Suárez and De la Cruz, 2001; Aragón et al.,
2011; Maloney et al., 2013; Echaurren et al.,
2016; Horton, 2018). Pronounced slab rollback
during the late Paleogene generated decoupling
of the subducting plate and intake of hot atmo-
sphere (De Ignacio et al., 2001; Encinas et al.,
2016). The steepening of the slab is revealed by
pervasive magmatism close to the Patagonian
Precordillera (El Maitén Belt; Fig. 3; Iannelli
et al., 2018; Paz et al., 2018; Rapela et al., 1988)
and a modest extensional setting (Ventana For-
mation) reported in the whole Patagonia foreland
(from Neuquén to Magallanes—Austral Basins;
George et al., 2020; Horton et al., 2016; Orts
et al., 2012). The Eocene–Miocene Somuncura
Plateau and associated formations (e.g., El Bu-
itre Formation) represented magmatism in the
distal retroarc zone. Several origins have been
proposed (e.g., slab detachment, plume-like

mantle upwelling) due to the complex geo-
chemical signatures of these volcanic plateaus
(De Ignacio et al., 2001; Kay et al., 2007; Aragón
et al., 2013).

Trench-normal absolute velocity of the South American Plate remained low through ca. 23 Ma, when the break-up of the oceanic Farallon Plate into Nazca and Cocos Plate reorganized the plate convergence (Somoza, 1998; Lonsdale, 2005). This new configuration allowed an increase of interplate coupling that was not necessarily linked to a change of slab dip (Cerpa et al., 2018; Schellart, 2020). Resumption of the upper plate shortening is recorded along the Patagonian Precordillera and associated with the Miocene rejuvenation of the North Patagonian Batholith (Echaurren et al., 2016). Mild contractional deformation of the foreland has been proposed based on the growth strata documented in the early-middle Miocene foreland basins known as the Nñirhuau and Collón Curá Basins (Fig. 2; Echaurren et al., 2016; Orts et al., 2012; Ramos et al., 2011). Substantial Miocene deformation controlled by former heterogeneities is also suggested in the foreland (Bilmes et al., 2013; Gianni et al., 2015).

MATERIALS AND METHODS

The low closure temperatures of apatite
(U-Th)/He (AHe, ~65 °C; e.g., Flowers et al.,
2009; Gautheron et al., 2009) and apatite fis-
sion track (AFT, ~110 °C; e.g., Ketcham et al.,
2007) systems provide information on cooling
histories and exhumation within the upper crust.
In this study, we present 33 AHe and 31 new
AFT ages from samples collected from the Pa-
tagonian Precordillera and its broken foreland
between 40°S and 45°S. Fifteen of the new AFT
ages (with track measurements) were obtained
from the same samples that further yielded 34
single-grain AHe ages published by Savignano
et al. (2016), and thus an extensive data set
was achieved (Fig. 2). Geographic coordinates,
elevation, formation (mainly metamorphic
basement, Jurassic sedimentary rocks, and Cre-
taceous granitoids), stratigraphic age, and rock
lithology of all samples are included in Table 1.

Apatite Fission Tracks (AFT)

We performed AFT analysis at the University
of Padua, Italy. Apatite grains were mounted in
epoxy resin, polished, and etched at 5.5 M HNO
3 for 20 sec at 20 °C to reveal spontaneous tracks.
The samples were analyzed by applying the
external detector method (Gleadow, 1981) us-
ing low-uranium muscovite foils as an external
detector to cover apatite mounts and then irra-
diated at the Radiation Center of Oregon State
Maloney et al., 2013; Horton, 2018). This result-
ed in steepening of the slab, as evidenced by the
westward migration of the magmatic arc (Fig. 2)
marked by the Lower Jurassic Subcordillera
Plutonic Belt (Gordon and Ort, 1993; Page and
Page, 1999) and by the Upper Jurassic–Lower
Cretaceous North Patagonian Batholith (Suárez
and De la Cruz, 2001; Pankhurst et al., 2003).
Indeed, lateral shifts of the magmatic arc either
toward the trench or toward the eastern foreland
are promoted by variations of the subduction
zone configuration involving slab steepening or
shallowing episodes, respectively (e.g., Coira
et al., 1993; England et al., 2004; Kay et al.,
2005; Ramos and Folguera, 2005; Syracuse and
Abers, 2006; Folguera and Ramos, 2011; Spag-
nuolo et al., 2012; Schellart, 2017; Fernández
Paz et al., 2019).

Coeval volcanism is recorded by the Middle-
Upper Jurassic Lago La Plata Formation (cor-
related with the Ibáñez Group in Chile; Folguera
and Iannizzotto, 2004; Olivero, 1982) and the
Lower Cretaceous Divisadero Group (Ramos,
1981; Suárez et al., 2009). Respectively, these
volcanic deposits overlie the N-NE-trending,
graben/half-graben systems of the Piltriquitrón
Basin (filled by the Lower-Middle Jurassic Osta
Arena Formation, Piltriquitrón Formation, and
equivalent units intercalated with marine suc-
cessions; Gabaldón, 1982; Giacosa and Here-
dia, 2004; Suarez and Marquez, 2007), and the
Upper Jurassic–Lower Cretaceous sedimentary
successions of the Coyhaique Group hosted in
the Rio Mayo Basin (Skarmeta, 1976; Olivero,
1982; Echaurren et al., 2017). As revealed by
the regional unconformity observed in the up-
per volcanic succession of the Divisadero Group
and on top of the Cañadón Asfalto Rift Basin
deposits, extension was suddenly interrupted
at ca. 120 Ma by a regional shortening episode
(Folguera and Iannizzotto, 2004; Suárez et al.,
2009; Horton, 2018). This latter, acting on a con-
tinental crust mechanically weakened by faults,
produced both the earliest stage of Andean up-
lift and deformation of the Patagonian broken
foreland. Indeed, the deformation, reaching ar-
eas ~500 km away from the trench (Navarrete
et al., 2016), was dominated by the reactivation
of existing faults associated with the shortening
of former depocenters (Gianni et al., 2015). The
initiation of regional shortening coincided with
a change of absolute motion of the South Ameri-
can Plate, which became west directed (Fig. 2;
Maloney et al., 2013; Müller et al., 2016; Se-
ton et al., 2012; Silver et al., 1998). Trenchward
absolute motion of the overriding plate and an
increase of convergence rates (Eagles, 2007;
Maloney et al., 2013; Müller et al., 2016) were
accompanied by slab shallowing manifested in
the upper crust by the eastward expansion of the

magmatic arc during the Late Cretaceous (Haller
et al., 2010; Aragón et al., 2013). This change in
slab dip favored a strong increase in coupling
of the subducting plates and the overriding plate
(Horton and Fuentes, 2016). Growth strata in
foreland basins have been detected in the con-
tinental Upper Cretaceous Chubut Group, Paso
del Sapo Formations, and in the marine Paleo-
cene Lefipán Formations (Gianni et al., 2015;
Echaurren et al., 2016; Navarrete et al., 2016),
which indicates that crustal shortening persisted
throughout Late Cretaceous to Paleocene times
(Echaurren et al., 2016; Horton, 2018). Subduc-
tion of mid-ocean ridges may have sustained
this long-term and significant deformation, as
the positive buoyancy of younger lithosphere
tends to resist subduction (Cloos, 1993), and al-
lowed the opening of a slab window during latest
Cretaceous (Echaurren et al., 2016). A slab win-
dow model, supported by seismic tomography
(Aragón et al., 2011), is generally accompanied
by an isostatic uplift of the overriding plate and a
dynamic topography related to limited deforma-
tion of the retroarc (Aragón et al., 2011, 2013;
Ávila and Dávila, 2020).

Growth strata in the lower Eocene deposits
are observed north and south of the area studied
(Cobbold and Rossello, 2003; Charrier et al.,
2007; Navarrete et al., 2016; Gianni et al., 2017),
where only syn-extensional strata have been rec-
ognized in the upper Paleocene–Eocene Huitrera
Formation associated with bimodal within-plate
volcanism (Pilcaniyeu Belt; Fig. 3; Echaurren
et al., 2016; Iannelli et al., 2018; Mazzoni et al.,
1991). Cessation of the contraction between
40°S and 45°S during the Paleocene may be re-
lated to the restepening of the subducting slab
induced by a sharp reduction in average trench-
ward velocity, an increase of slab pull forces
after asthenospheric window development, and/
or the decreasing buoyancy of oceanic segments
(Suárez and De la Cruz, 2001; Aragón et al.,
2011; Maloney et al., 2013; Echaurren et al.,
2016; Horton, 2018). Pronounced slab rollback
during the late Paleogene generated decoupling
of the subducting plate and intake of hot atmo-
sphere (De Ignacio et al., 2001; Encinas et al.,
2016). The steepening of the slab is revealed by
pervasive magmatism close to the Patagonian
Precordillera (El Maitén Belt; Fig. 3; Iannelli
et al., 2018; Paz et al., 2018; Rapela et al., 1988)
and a modest extensional setting (Ventana For-
mation) reported in the whole Patagonia foreland
(from Neuquén to Magallanes—Austral Basins;
George et al., 2020; Horton et al., 2016; Orts
et al., 2012). The Eocene–Miocene Somuncura
Plateau and associated formations (e.g., El Bu-
itre Formation) represented magmatism in the
distal retroarc zone. Several origins have been
proposed (e.g., slab detachment, plume-like

mantle upwelling) due to the complex geo-
chemical signatures of these volcanic plateaus
(De Ignacio et al., 2001; Kay et al., 2007; Aragón
et al., 2013).

Trench-normal absolute velocity of the South American Plate remained low through ca. 23 Ma, when the break-up of the oceanic Farallon Plate into Nazca and Cocos Plate reorganized the plate convergence (Somoza, 1998; Lonsdale, 2005). This new configuration allowed an increase of interplate coupling that was not necessarily linked to a change of slab dip (Cerpa et al., 2018; Schellart, 2020). Resumption of the upper plate shortening is recorded along the Patagonian Precordillera and associated with the Miocene rejuvenation of the North Patagonian Batholith (Echaurren et al., 2016). Mild contractional deformation of the foreland has been proposed based on the growth strata documented in the early-middle Miocene foreland basins known as the Nñirhuau and Collón Curá Basins (Fig. 2; Echaurren et al., 2016; Orts et al., 2012; Ramos et al., 2011). Substantial Miocene deformation controlled by former heterogeneities is also suggested in the foreland (Bilmes et al., 2013; Gianni et al., 2015).

MATERIALS AND METHODS

The low closure temperatures of apatite
(U-Th)/He (AHe, ~65 °C; e.g., Flowers et al.,
2009; Gautheron et al., 2009) and apatite fis-
sion track (AFT, ~110 °C; e.g., Ketcham et al.,
2007) systems provide information on cooling
histories and exhumation within the upper crust.
In this study, we present 33 AHe and 31 new
AFT ages from samples collected from the Pa-
tagonian Precordillera and its broken foreland
between 40°S and 45°S. Fifteen of the new AFT
ages (with track measurements) were obtained
from the same samples that further yielded 34
single-grain AHe ages published by Savignano
et al. (2016), and thus an extensive data set
was achieved (Fig. 2). Geographic coordinates,
elevation, formation (mainly metamorphic
basement, Jurassic sedimentary rocks, and Cre-
taceous granitoids), stratigraphic age, and rock
lithology of all samples are included in Table 1.

Apatite Fission Tracks (AFT)

We performed AFT analysis at the University
of Padua, Italy. Apatite grains were mounted in
epoxy resin, polished, and etched at 5.5 M HNO
for 20 sec at 20 °C to reveal spontaneous tracks.
The samples were analyzed by applying the
external detector method (Gleadow, 1981) us-
ing low-uranium muscovite foils as an external
detector to cover apatite mounts and then irra-
diated at the Radiation Center of Oregon State
<figure><img src="image_4.png" /></figure>

Figure 3. Geological map of the study area shows the locations of samples used in this study (modified after Anselmi et al., 2004; Ardolino et al., 2011; Cucchi et al., 2001, 1998; Escosteguy et al., 2013; Giacosa et al., 2001; González et al., 2000; Haller et al., 2010; Lizuaín et al., 2010; Lizuaín and Nieto, 2011; Orts et al., 2015; Remesal et al., 2001; Savignano et al., 2016; Silva Nieto et al., 2005).
<figure><img src="image_6.png" /></figure>

Figure 3. Geological map of the study area shows the locations of samples used in this study (modified after Anselmi et al., 2004; Ardolino et al., 2011; Cucchi et al., 2001, 1998; Escosteguy et al., 2013; Giacosa et al., 2001; González et al., 2000; Haller et al., 2010; Lizuaín et al., 2010; Lizuaín and Nieto, 2011; Orts et al., 2015; Remesal et al., 2001; Savignano et al., 2016; Silva Nieto et al., 2005).
<table><caption>TABLE 1. SUMMARY OF SAMPLE INFORMATION AND APATITE FISSION TRACK (AFT) DATA</caption><thead><tr><th rowspan="2">Samples<sup>†</sup></th><th colspan="2">Field*</th><th rowspan="2">Elev.<br>(m)</th><th colspan="2">Geology</th><th rowspan="2">Stratigraphic age</th><th colspan="2">AFT<br>(Ma)</th><th colspan="2">AFT ages</th><th rowspan="2">N<sub>i</sub></th><th rowspan="2">Nd</th><th rowspan="2">µD</th><th rowspan="2">P(χ<sup>2</sup>)</th><th colspan="2">AFT length</th></tr><tr><th>Latitude<br>(°S)</th><th>Longitude<br>(°W)</th><th>σ</th><th>Ns</th><th>σs</th><th>MTL<br>(µm)</th><th>σ</th><th>n°</th></tr></thead><tbody><tr><td>G01</td><td>-42.1928</td><td>-69.1656</td><td>1134</td><td>granite</td><td>Lipatren</td><td>Triassic</td><td>96.3</td><td>6.4</td><td>400</td><td>5.90</td><td>822</td><td>12.12</td><td>4798</td><td>11.56</td><td>93.29</td><td>12.86</td><td>0.18</td><td>80</td><td>1.88</td></tr><tr><td>G02</td><td>-42.2858</td><td>-69.4086</td><td>929</td><td>granite</td><td>Mamil Choique</td><td>Permian</td><td>68</td><td>5.2</td><td>268</td><td>5.15</td><td>775</td><td>14.89</td><td>4798</td><td>11.46</td><td>98.68</td><td>11.95</td><td>0.23</td><td>66</td><td>1.81</td></tr><tr><td>G04</td><td>-42.3878</td><td>-69.5219</td><td>997</td><td>granite</td><td>Mamil Choique</td><td>Permian</td><td>87.3</td><td>4.3</td><td>867</td><td>10.86</td><td>1938</td><td>24.27</td><td>4798</td><td>11.39</td><td>98.3</td><td>11.95</td><td>0.23</td><td>66</td><td>1.88</td></tr><tr><td>G05</td><td>-42.3694</td><td>-69.6292</td><td>963</td><td>conglomerate</td><td>Canadon Asfalto</td><td>Upper Jurassic</td><td>108.2</td><td>11.2</td><td>155</td><td>4.15</td><td>282</td><td>7.55</td><td>8875</td><td>11.51</td><td>96.7</td><td>100</td><td>100</td><td>100</td><td>100</td></tr><tr><td>G06</td><td>-42.4444</td><td>-68.7803</td><td>1150</td><td>arkose</td><td>Canadon Asfalto</td><td>Upper Jurassic</td><td>109.2</td><td>7.8</td><td>352</td><td>3.23</td><td>625</td><td>5.73</td><td>8875</td><td>11.34</td><td>100</td><td>12.47</td><td>0.13</td><td>100</td><td>1.88</td></tr><tr><td>G07</td><td>-42.5597</td><td>-69.0039</td><td>1070</td><td>granite</td><td>Mamil Choique</td><td>Lower Permian</td><td>154.4</td><td>6</td><td>2211</td><td>39.86</td><td>2832</td><td>51.05</td><td>4798</td><td>11.6</td><td>99.9</td><td>12.11</td><td>0.23</td><td>92</td><td>2.33</td></tr><tr><td>G08</td><td>-42.5639</td><td>-69.0042</td><td>1043</td><td>granite</td><td>Mamil Choique</td><td>Lower Permian</td><td>74.1</td><td>4</td><td>643</td><td>12.01</td><td>1670</td><td>31.20</td><td>4798</td><td>11.22</td><td>73.5</td><td>12.02</td><td>0.27</td><td>56</td><td>2.39</td></tr><tr><td>G09</td><td>-42.4131</td><td>-69.7822</td><td>995</td><td>arkose</td><td>Canadon Asfalto</td><td>Upper Jurassic</td><td>99.9</td><td>5.6</td><td>602</td><td>5.81</td><td>1152</td><td>11.11</td><td>8875</td><td>11.17</td><td>91</td><td>12.02</td><td>0.27</td><td>56</td><td>2.39</td></tr><tr><td>G10</td><td>-42.0953</td><td>69.7749</td><td>1060</td><td>granite</td><td>Mamil Choique</td><td>Lower Permian</td><td>99.9</td><td>5.6</td><td>602</td><td>5.81</td><td>1152</td><td>11.11</td><td>8875</td><td>11.17</td><td>91</td><td>12.02</td><td>0.27</td><td>56</td><td>2.39</td></tr><tr><td>G11</td><td>-42.3114</td><td>-70.0367</td><td>999</td><td>dyke (granite)</td><td>Mamil Choique</td><td>Lower Permian</td><td>112</td><td>6</td><td>710</td><td>11.24</td><td>1192</td><td>18.87</td><td>8875</td><td>11</td><td>88.66</td><td>11.73</td><td>0.19</td><td>61</td><td>1.90</td></tr><tr><td>G12</td><td>-42.0353</td><td>-70.2992</td><td>976</td><td>granite</td><td>Mamil Choique</td><td>Lower Permian</td><td>95</td><td>4.8</td><td>1475</td><td>15.71</td><td>2904</td><td>30.93</td><td>8875</td><td>10.94</td><td>63</td><td>11.73</td><td>0.19</td><td>61</td><td>1.90</td></tr><tr><td>G13</td><td>-42.3736</td><td>-69.6039</td><td>973</td><td>sandstone</td><td>Canadon Asfalto</td><td>Upper Jurassic</td><td>112.6</td><td>8.6</td><td>1184</td><td>13.84</td><td>2001</td><td>23.40</td><td>8875</td><td>10.89</td><td>92.78</td><td>100</td><td>100</td><td>100</td><td>100</td></tr><tr><td>G14</td><td>-42.4253</td><td>-68.8347</td><td>1177</td><td>sandstone</td><td>Canadon Asfalto</td><td>Upper Jurassic</td><td>871</td><td>9</td><td>147</td><td>3.19</td><td>313</td><td>6.80</td><td>4798</td><td>10.83</td><td>99</td><td>12.11</td><td>0.23</td><td>92</td><td>2.33</td></tr><tr><td>G15</td><td>-42.5861</td><td>-68.8128</td><td>1103</td><td>granite</td><td>Mamil Choique</td><td>Lower Permian</td><td>73.6</td><td>3.7</td><td>718</td><td>13.20</td><td>1794</td><td>32.99</td><td>8875</td><td>10.72</td><td>99.55</td><td>12.70</td><td>0.16</td><td>94</td><td>1.67</td></tr><tr><td>G17</td><td>-41.7119</td><td>-69.6192</td><td>1275</td><td>granite</td><td>Mamil Choique</td><td>Lower Permian</td><td>73.6</td><td>3.7</td><td>718</td><td>13.20</td><td>1794</td><td>32.99</td><td>8875</td><td>10.72</td><td>99.55</td><td>12.70</td><td>0.16</td><td>94</td><td>1.67</td></tr><tr><td>G18</td><td>-42.9306</td><td>-71.2606</td><td>617</td><td>granite</td><td>Subcordilleran</td><td>Lower Jurassic</td><td>73.6</td><td>3.7</td><td>718</td><td>13.20</td><td>1794</td><td>32.99</td><td>8875</td><td>10.72</td><td>99.55</td><td>12.70</td><td>0.16</td><td>94</td><td>1.67</td></tr><tr><td>G19</td><td>-42.8064</td><td>-71.6608</td><td>576</td><td>granite</td><td>Plutonic Belt</td><td>Upper Cretaceous</td><td>73.6</td><td>3.7</td><td>718</td><td>13.20</td><td>1794</td><td>32.99</td><td>8875</td><td>10.72</td><td>99.55</td><td>12.70</td><td>0.16</td><td>94</td><td>1.67</td></tr><tr><td>G20</td><td>-42.3847</td><td>-71.2453</td><td>663</td><td>granite</td><td>Batholith</td><td>Upper Jurassic</td><td>11.5</td><td>1.3</td><td>92</td><td>2.08</td><td>1464</td><td>33.06</td><td>4798</td><td>10.6</td><td>100</td><td>100</td><td>100</td><td>100</td><td>100</td></tr><tr><td>G21</td><td>-42.1672</td><td>-71.3572</td><td>616</td><td>granodiorite</td><td>Leleque</td><td>Lower Jurassic</td><td>11.5</td><td>1.3</td><td>162</td><td>3.14</td><td>1995</td><td>38.47</td><td>8875</td><td>10.55</td><td>36.74</td><td>100</td><td>100</td><td>100</td><td>100</td></tr><tr><td>G22</td><td>-42.0303</td><td>-71.5761</td><td>275</td><td>granite</td><td>Subcordilleran</td><td>Upper Cretaceous</td><td>14.8</td><td>1.3</td><td>162</td><td>3.14</td><td>1995</td><td>38.47</td><td>8875</td><td>10.55</td><td>36.74</td><td>100</td><td>100</td><td>100</td><td>100</td></tr><tr><td>G30</td><td>-43.7439</td><td>-69.8186</td><td>670</td><td>sandstone</td><td>Osta Arena</td><td>Lower Jurassic</td><td>89.8</td><td>8.3</td><td>211</td><td>2.11</td><td>416</td><td>4.15</td><td>5000</td><td>10.3</td><td>100</td><td>12.61</td><td>0.15</td><td>65</td><td>1.39</td></tr><tr><td>G31</td><td>-43.6650</td><td>-70.0839</td><td>825</td><td>sandstone</td><td>Tapuel Group</td><td>Carboniferous to</td><td>67.4</td><td>4.6</td><td>418</td><td>9.98</td><td>1078</td><td>25.74</td><td>5000</td><td>10.1</td><td>98.9</td><td>12.14</td><td>0.2</td><td>25</td><td>1.33</td></tr><tr><td>G35</td><td>-43.3244</td><td>-70.8961</td><td>875</td><td>granitoid</td><td>Rio Hielo</td><td>Permian</td><td>73.5</td><td>10.4</td><td>76</td><td>1.98</td><td>178</td><td>4.64</td><td>5000</td><td>10</td><td>99.98</td><td>12.14</td><td>0.2</td><td>25</td><td>1.33</td></tr><tr><td>G37</td><td>-43.5250</td><td>-71.5161</td><td>473</td><td>granodiorite</td><td>Rio Hielo</td><td>Cretaceous</td><td>8.6</td><td>1</td><td>88</td><td>1.34</td><td>1743</td><td>26.57</td><td>5000</td><td>9.85</td><td>100</td><td>100</td><td>100</td><td>100</td><td>100</td></tr><tr><td>G38</td><td>-41.8508</td><td>-71.4219</td><td>627</td><td>granodiorite</td><td>Colohuincul</td><td>Lower Permian</td><td>8.6</td><td>1</td><td>88</td><td>1.34</td><td>1743</td><td>26.57</td><td>5000</td><td>9.85</td><td>100</td><td>100</td><td>100</td><td>100</td><td>100</td></tr><tr><td>G39</td><td>-41.5564</td><td>-71.4806</td><td>731</td><td>granite</td><td>Patagonian</td><td>Lower Cretaceous</td><td>83.5</td><td>5.6</td><td>489</td><td>4.84</td><td>968</td><td>9.58</td><td>5000</td><td>9.61</td><td>94.66</td><td>12.09</td><td>0.1</td><td>61</td><td>1.32</td></tr><tr><td>G40</td><td>-40.3447</td><td>-71.5064</td><td>898</td><td>granodiorite</td><td>Batholith</td><td>Lower Permian</td><td>14.2</td><td>1.7</td><td>86</td><td>1.14</td><td>1026</td><td>13.61</td><td>5000</td><td>9.77</td><td>99.95</td><td>13.87</td><td>0.09</td><td>60</td><td>1.34</td></tr><tr><td>G41</td><td>-41.1061</td><td>-71.4831</td><td>849</td><td>granite</td><td>Complejo</td><td>Lower Cretaceous</td><td>9</td><td>1.1</td><td>73</td><td>2.03</td><td>1364</td><td>37.95</td><td>5000</td><td>9.69</td><td>99.37</td><td>13.87</td><td>0.09</td><td>60</td><td>1.34</td></tr><tr><td>G44</td><td>-41.2089</td><td>-70.1667</td><td>999</td><td>granite</td><td>Patagonian</td><td>Lower Cretaceous</td><td>83.5</td><td>5.6</td><td>489</td><td>4.84</td><td>968</td><td>9.58</td><td>5000</td><td>9.61</td><td>94.66</td><td>12.09</td><td>0.1</td><td>61</td><td>1.32</td></tr><tr><td>G45</td><td>-40.9181</td><td>-70.2247</td><td>810</td><td>granodiorite</td><td>Plicanicyeu</td><td>Lower Permian</td><td>77.9</td><td>5.7</td><td>368</td><td>6.74</td><td>775</td><td>14.20</td><td>5000</td><td>9.54</td><td>9769</td><td>13.87</td><td>0.09</td><td>60</td><td>1.34</td></tr><tr><td>G46</td><td>-40.9017</td><td>-70.0461</td><td>1189</td><td>granodiorite</td><td>Mamil Choique</td><td>Lower Permian</td><td>71.6</td><td>5.1</td><td>382</td><td>5.76</td><td>869</td><td>13.10</td><td>5000</td><td>9.46</td><td>81.3</td><td>13.87</td><td>0.09</td><td>60</td><td>1.34</td></tr><tr><td>G47</td><td>-40.6692</td><td>-69.8833</td><td>1298</td><td>granodiorite</td><td>Mamil Choique</td><td>Lower Permian</td><td>81.8</td><td>4.9</td><td>683</td><td>10.54</td><td>1349</td><td>20.81</td><td>5000</td><td>9.39</td><td>88.24</td><td>11.90</td><td>0.15</td><td>33</td><td>1.33</td></tr><tr><td>G48</td><td>-40.8936</td><td>-71.4950</td><td>805</td><td>granodiorite</td><td>Los Machis</td><td>Up. Jurassic to</td><td>14.5</td><td>1.2</td><td>188</td><td>2.79</td><td>2093</td><td>31.03</td><td>5000</td><td>9.31</td><td>99.85</td><td>13.87</td><td>0.09</td><td>60</td><td>1.34</td></tr><tr><td>G49</td><td>-40.4853</td><td>-71.5894</td><td>903</td><td>granodiorite</td><td>Los Machis</td><td>Cretaceous to</td><td>6.9</td><td>0.9</td><td>60</td><td>1.07</td><td>1387</td><td>24.84</td><td>5000</td><td>9.23</td><td>100</td><td>13.87</td><td>0.09</td><td>60</td><td>1.34</td></tr><tr><td>G50</td><td>-40.1956</td><td>-71.3614</td><td>976</td><td>granodiorite</td><td>Complejo</td><td>Up. Jurassic to</td><td>4.9</td><td>0.6</td><td>66</td><td>1.12</td><td>2131</td><td>36.27</td><td>5000</td><td>9.15</td><td>100</td><td>13.87</td><td>0.09</td><td>60</td><td>1.34</td></tr><tr><td>G51</td><td>-40.1289</td><td>-71.6456</td><td>791</td><td>tonalite</td><td>Colohuincul</td><td>Cretaceous</td><td>8.1</td><td>1.1</td><td>65</td><td>0.92</td><td>1242</td><td>17.56</td><td>5000</td><td>9</td><td>99.34</td><td>13.87</td><td>0.09</td><td>60</td><td>1.34</td></tr><tr><td>G53</td><td>-40.3683</td><td>-70.6619</td><td>645</td><td>granite</td><td>Los Machis</td><td>Devonian to</td><td>59.3</td><td>4.2</td><td>392</td><td>6.66</td><td>1015</td><td>17.24</td><td>5000</td><td>8.92</td><td>99.98</td><td>12.66</td><td>0.11</td><td>86</td><td>1.29</td></tr><tr><td>G54</td><td>-40.2822</td><td>-70.3803</td><td>986</td><td>granodiorite</td><td>Cushamen</td><td>Carboniferous</td><td>69.6</td><td>4.6</td><td>486</td><td>8.69</td><td>1063</td><td>19.00</td><td>5000</td><td>8.85</td><td>100</td><td>12.83</td><td>0.13</td><td>100</td><td>1.28</td></tr></tbody></table>

Notes: Central ages reported with a confidence interval of ±1σ. N—number of apatite crystal counted; and i—track density (×10⁵ tracks/cm²); subscripts s, l, and d denote spontaneous, induced, and dosimeter, respectively. P(χ²)—probability of obtaining a Chi-square value for n degrees of freedom; Dpar—mean diameter of fission-track etch pit parallel to the c-axis. MTL—Mean Track Length.

*Longitude and latitude coordinates are given in WGS84.

†Zeta = 345 ± 8

‡Zeta = 346 ± 12.
<table><caption>TABLE 1. SUMMARY OF SAMPLE INFORMATION AND APATITE FISSION TRACK (AFT) DATA</caption><thead><tr><th rowspan="2">Samples<sup>†</sup></th><th colspan="2">Field*</th><th rowspan="2">Elev.<br>(m)</th><th colspan="2">Geology</th><th colspan="2">AFT ages</th><th colspan="2">AFT ages</th><th colspan="2">AFT length</th></tr><tr><th>Latitude<br>(°S)</th><th>Longitude<br>(°W)</th><th>Lithology</th><th>Formation</th><th>Stratigraphic age</th><th>AFT<br>(Ma)</th><th>σ</th><th>Ns</th><th>βs</th><th>Ni</th><th>βi</th><th>Nd</th><th>βD</th><th>P(χ<sup>2</sup>)</th><th>MTL<br>(μm)</th><th>σ</th><th>n°</th><th>Dpar</th></tr></thead><tbody><tr><td>G01</td><td>-42.1928</td><td>-69.1656</td><td>1134</td><td>granite</td><td>Lipetren</td><td>Triassic</td><td>96.3</td><td>6.4</td><td>400</td><td>5.90</td><td>822</td><td>12.12</td><td>4798</td><td>11.56</td><td>93.29</td><td>12.86</td><td>0.18</td><td>80</td><td>1.88</td></tr><tr><td>G02</td><td>-42.2858</td><td>-69.4086</td><td>929</td><td>granite</td><td>Mamil Choique</td><td>Permian</td><td>68</td><td>5.2</td><td>268</td><td>5.15</td><td>775</td><td>14.89</td><td>4798</td><td>11.46</td><td>98.68</td><td>11.95</td><td>0.23</td><td>66</td><td>1.81</td></tr><tr><td>G04</td><td>-42.3878</td><td>-69.5219</td><td>997</td><td>granite</td><td>Mamil Choique</td><td>Permian</td><td>87.3</td><td>4.3</td><td>867</td><td>10.86</td><td>1938</td><td>24.27</td><td>4798</td><td>11.39</td><td>98.3</td><td>11.95</td><td>0.23</td><td>66</td><td>1.88</td></tr><tr><td>G05</td><td>-42.3694</td><td>-69.6292</td><td>963</td><td>conglomerate</td><td>Canadon Asfalto</td><td>Upper Jurassic</td><td>108.2</td><td>11.2</td><td>155</td><td>4.15</td><td>282</td><td>7.55</td><td>8875</td><td>11.51</td><td>96.7</td><td>100</td><td>100</td><td>100</td><td>100</td></tr><tr><td>G06</td><td>-42.4444</td><td>-68.7803</td><td>1150</td><td>arkose</td><td>Canadon Asfalto</td><td>Upper Jurassic</td><td>109.2</td><td>7.8</td><td>352</td><td>3.23</td><td>625</td><td>5.73</td><td>8875</td><td>11.34</td><td>100</td><td>12.47</td><td>0.13</td><td>100</td><td>1.88</td></tr><tr><td>G07</td><td>-42.5597</td><td>-69.0039</td><td>1070</td><td>granite</td><td>Mamil Choique</td><td>Lower Permian</td><td>154.4</td><td>6</td><td>2211</td><td>39.86</td><td>2832</td><td>51.05</td><td>4798</td><td>11.6</td><td>99.9</td><td>12.11</td><td>0.23</td><td>92</td><td>2.33</td></tr><tr><td>G08</td><td>-42.5639</td><td>-69.0042</td><td>1043</td><td>granite</td><td>Mamil Choique</td><td>Lower Permian</td><td>74.1</td><td>4</td><td>643</td><td>12.01</td><td>1670</td><td>31.20</td><td>4798</td><td>11.22</td><td>73.5</td><td>12.02</td><td>0.27</td><td>56</td><td>2.39</td></tr><tr><td>G09</td><td>-42.4131</td><td>-69.7822</td><td>995</td><td>arkose</td><td>Canadon Asfalto</td><td>Upper Jurassic</td><td>99.9</td><td>5.6</td><td>602</td><td>5.81</td><td>1152</td><td>11.11</td><td>8875</td><td>11.17</td><td>91</td><td>12.02</td><td>0.27</td><td>56</td><td>2.39</td></tr><tr><td>G10</td><td>-42.0953</td><td>69.7749</td><td>1060</td><td>granite</td><td>Mamil Choique</td><td>Lower Permian</td><td>99.9</td><td>5.6</td><td>602</td><td>5.81</td><td>1152</td><td>11.11</td><td>8875</td><td>11.17</td><td>91</td><td>12.02</td><td>0.27</td><td>56</td><td>2.39</td></tr><tr><td>G11</td><td>-42.3114</td><td>-70.0367</td><td>999</td><td>dyke (granite)</td><td>Mamil Choique</td><td>Lower Permian</td><td>112</td><td>6</td><td>710</td><td>11.24</td><td>1192</td><td>18.87</td><td>8875</td><td>11</td><td>88.66</td><td>11.73</td><td>0.19</td><td>61</td><td>1.90</td></tr><tr><td>G12</td><td>-42.0353</td><td>-70.2992</td><td>976</td><td>granite</td><td>Mamil Choique</td><td>Lower Permian</td><td>95</td><td>4.8</td><td>1475</td><td>15.71</td><td>2904</td><td>30.93</td><td>8875</td><td>10.94</td><td>63</td><td>11.73</td><td>0.19</td><td>61</td><td>1.90</td></tr><tr><td>G13</td><td>-42.3736</td><td>-69.6039</td><td>973</td><td>sandstone</td><td>Canadon Asfalto</td><td>Upper Jurassic</td><td>112.6</td><td>8.6</td><td>1184</td><td>13.84</td><td>2001</td><td>23.40</td><td>8875</td><td>10.89</td><td>92.78</td><td>100</td><td>100</td><td>100</td><td>100</td></tr><tr><td>G14</td><td>-42.4253</td><td>-68.8347</td><td>1177</td><td>sandstone</td><td>Canadon Asfalto</td><td>Upper Jurassic</td><td>871</td><td>9</td><td>147</td><td>3.19</td><td>313</td><td>6.80</td><td>4798</td><td>10.83</td><td>99</td><td>12.11</td><td>0.23</td><td>92</td><td>2.33</td></tr><tr><td>G15</td><td>-42.5861</td><td>-68.8128</td><td>1103</td><td>granite</td><td>Mamil Choique</td><td>Lower Permian</td><td>73.6</td><td>3.7</td><td>718</td><td>13.20</td><td>1794</td><td>32.99</td><td>8875</td><td>10.72</td><td>99.55</td><td>12.70</td><td>0.16</td><td>94</td><td>1.67</td></tr><tr><td>G17</td><td>-41.7119</td><td>-69.6192</td><td>1275</td><td>granite</td><td>Mamil Choique</td><td>Lower Permian</td><td>73.6</td><td>3.7</td><td>718</td><td>13.20</td><td>1794</td><td>32.99</td><td>8875</td><td>10.72</td><td>99.55</td><td>12.70</td><td>0.16</td><td>94</td><td>1.67</td></tr><tr><td>G18</td><td>-42.9306</td><td>-71.2606</td><td>617</td><td>granite</td><td>Subcordilleran</td><td>Lower Jurassic</td><td>73.6</td><td>3.7</td><td>718</td><td>13.20</td><td>1794</td><td>32.99</td><td>8875</td><td>10.72</td><td>99.55</td><td>12.70</td><td>0.16</td><td>94</td><td>1.67</td></tr><tr><td>G19</td><td>-42.8064</td><td>-71.6608</td><td>576</td><td>granite</td><td>Plutonic Belt</td><td>Upper Cretaceous</td><td>73.6</td><td>3.7</td><td>718</td><td>13.20</td><td>1794</td><td>32.99</td><td>8875</td><td>10.72</td><td>99.55</td><td>12.70</td><td>0.16</td><td>94</td><td>1.67</td></tr><tr><td>G20</td><td>-42.3847</td><td>-71.2453</td><td>663</td><td>granite</td><td>Batholith</td><td>Upper Jurassic</td><td>11.5</td><td>1.3</td><td>92</td><td>2.08</td><td>1464</td><td>33.06</td><td>4798</td><td>10.6</td><td>100</td><td>100</td><td>100</td><td>100</td><td>100</td></tr><tr><td>G21</td><td>-42.1672</td><td>-71.3572</td><td>616</td><td>granodiorite</td><td>Leleque</td><td>Lower Jurassic</td><td>11.5</td><td>1.3</td><td>162</td><td>3.14</td><td>1995</td><td>38.47</td><td>8875</td><td>10.55</td><td>36.74</td><td>100</td><td>100</td><td>100</td><td>100</td></tr><tr><td>G22</td><td>-42.0303</td><td>-71.5761</td><td>275</td><td>granite</td><td>Subcordilleran</td><td>Upper Cretaceous</td><td>14.8</td><td>1.3</td><td>162</td><td>3.14</td><td>1995</td><td>38.47</td><td>8875</td><td>10.55</td><td>36.74</td><td>100</td><td>100</td><td>100</td><td>100</td></tr><tr><td>G30</td><td>-43.7439</td><td>-69.8186</td><td>670</td><td>sandstone</td><td>Osta Arena</td><td>Lower Jurassic</td><td>89.8</td><td>8.3</td><td>211</td><td>2.11</td><td>416</td><td>4.15</td><td>5000</td><td>10.3</td><td>100</td><td>12.61</td><td>0.15</td><td>65</td><td>1.39</td></tr><tr><td>G31</td><td>-43.6650</td><td>-70.0839</td><td>825</td><td>sandstone</td><td>Tapuel Group</td><td>Carboniferous to</td><td>67.4</td><td>4.6</td><td>418</td><td>9.98</td><td>1078</td><td>25.74</td><td>5000</td><td>10.1</td><td>98.9</td><td>12.14</td><td>0.2</td><td>25</td><td>1.33</td></tr><tr><td>G35</td><td>-43.3244</td><td>-70.8961</td><td>875</td><td>granitoid</td><td>Rio Hielo</td><td>Permian</td><td>73.5</td><td>10.4</td><td>76</td><td>1.98</td><td>178</td><td>4.64</td><td>5000</td><td>10</td><td>99.98</td><td>12.14</td><td>0.2</td><td>25</td><td>1.33</td></tr><tr><td>G37</td><td>-43.5250</td><td>-71.5161</td><td>473</td><td>granodiorite</td><td>Rio Hielo</td><td>Cretaceous</td><td>8.6</td><td>1</td><td>88</td><td>1.34</td><td>1743</td><td>26.57</td><td>5000</td><td>9.85</td><td>100</td><td>100</td><td>100</td><td>100</td><td>100</td></tr><tr><td>G38</td><td>-41.8508</td><td>-71.4219</td><td>627</td><td>granodiorite</td><td>Colohuincul</td><td>Lower Permian</td><td>8.6</td><td>1</td><td>88</td><td>1.34</td><td>1743</td><td>26.57</td><td>5000</td><td>9.85</td><td>100</td><td>100</td><td>100</td><td>100</td><td>100</td></tr><tr><td>G39</td><td>-41.5564</td><td>-71.4806</td><td>731</td><td>granite</td><td>Patagonian</td><td>Lower Cretaceous</td><td>83.5</td><td>5.6</td><td>489</td><td>4.84</td><td>968</td><td>9.58</td><td>5000</td><td>9.61</td><td>94.66</td><td>12.09</td><td>0.1</td><td>61</td><td>1.32</td></tr><tr><td>G40</td><td>-40.3447</td><td>-71.5064</td><td>898</td><td>granodiorite</td><td>Batholith</td><td>Lower Permian</td><td>14.2</td><td>1.7</td><td>86</td><td>1.14</td><td>1026</td><td>13.61</td><td>5000</td><td>9.77</td><td>99.95</td><td>13.87</td><td>0.09</td><td>60</td><td>1.34</td></tr><tr><td>G41</td><td>-41.1061</td><td>-71.4831</td><td>849</td><td>granite</td><td>Plutonitas</td><td>Lower Cretaceous</td><td>9</td><td>1.1</td><td>73</td><td>2.03</td><td>1364</td><td>37.95</td><td>5000</td><td>9.69</td><td>99.37</td><td>13.87</td><td>0.09</td><td>60</td><td>1.34</td></tr><tr><td>G44</td><td>-41.2089</td><td>-70.1667</td><td>999</td><td>granite</td><td>Plutonitas</td><td>Lower Jurassic</td><td>83.5</td><td>5.6</td><td>489</td><td>4.84</td><td>968</td><td>9.58</td><td>5000</td><td>9.61</td><td>94.66</td><td>12.09</td><td>0.1</td><td>61</td><td>1.32</td></tr><tr><td>G45</td><td>-40.9181</td><td>-70.2247</td><td>810</td><td>granodiorite</td><td>Plicanhyeu</td><td>Lower Permian</td><td>77.9</td><td>5.7</td><td>368</td><td>6.74</td><td>775</td><td>14.20</td><td>5000</td><td>9.54</td><td>9769</td><td>13.87</td><td>0.09</td><td>60</td><td>1.34</td></tr><tr><td>G46</td><td>-40.9017</td><td>-70.0461</td><td>1189</td><td>granodiorite</td><td>Mamil Choique</td><td>Lower Permian</td><td>71.6</td><td>5.1</td><td>382</td><td>5.76</td><td>869</td><td>13.10</td><td>5000</td><td>9.46</td><td>81.3</td><td>13.87</td><td>0.09</td><td>60</td><td>1.34</td></tr><tr><td>G47</td><td>-40.6692</td><td>-69.8833</td><td>1298</td><td>granodiorite</td><td>Mamil Choique</td><td>Lower Permian</td><td>81.8</td><td>4.9</td><td>683</td><td>10.54</td><td>1349</td><td>20.81</td><td>5000</td><td>9.39</td><td>88.24</td><td>11.90</td><td>0.15</td><td>33</td><td>1.33</td></tr><tr><td>G48</td><td>-40.8936</td><td>-71.4950</td><td>805</td><td>granodiorite</td><td>Los Machis</td><td>Up. Jurassic to</td><td>14.5</td><td>1.2</td><td>188</td><td>2.79</td><td>2093</td><td>31.03</td><td>5000</td><td>9.31</td><td>99.85</td><td>13.87</td><td>0.09</td><td>60</td><td>1.34</td></tr><tr><td>G49</td><td>-40.4853</td><td>-71.5894</td><td>903</td><td>granodiorite</td><td>Los Machis</td><td>Cretaceous to</td><td>6.9</td><td>0.9</td><td>60</td><td>1.07</td><td>1387</td><td>24.84</td><td>5000</td><td>9.23</td><td>100</td><td>13.87</td><td>0.09</td><td>60</td><td>1.34</td></tr><tr><td>G50</td><td>-40.1956</td><td>-71.3614</td><td>976</td><td>granodiorite</td><td>Plutonic Belt</td><td>Up. Jurassic to</td><td>4.9</td><td>0.6</td><td>66</td><td>1.12</td><td>2131</td><td>36.27</td><td>5000</td><td>9.15</td><td>100</td><td>13.87</td><td>0.09</td><td>60</td><td>1.34</td></tr><tr><td>G51</td><td>-40.1289</td><td>-71.6456</td><td>791</td><td>tonalite</td><td>Batholith</td><td>Cretaceous</td><td>8.1</td><td>1.1</td><td>65</td><td>0.92</td><td>1242</td><td>17.56</td><td>5000</td><td>9</td><td>99.34</td><td>13.87</td><td>0.09</td><td>60</td><td>1.34</td></tr><tr><td>G53</td><td>-40.3683</td><td>-70.6619</td><td>645</td><td>granite</td><td>Colohuincul</td><td>Devonian to</td><td>59.3</td><td>4.2</td><td>392</td><td>6.66</td><td>1015</td><td>17.24</td><td>5000</td><td>8.92</td><td>99.98</td><td>12.66</td><td>0.11</td><td>86</td><td>1.29</td></tr><tr><td>G54</td><td>-40.2822</td><td>-70.3803</td><td>986</td><td>granodiorite</td><td>Cushamen</td><td>Carboniferous</td><td>69.6</td><td>4.6</td><td>486</td><td>8.69</td><td>1063</td><td>19.00</td><td>5000</td><td>8.85</td><td>100</td><td>12.83</td><td>0.13</td><td>100</td><td>1.28</td></tr></tbody><tfoot><tr><td colspan="17">Notes: Central ages reported with a confidence interval of ±1σ. N—number of apatite crystal counted; and β—track density (×10<sup>5</sup> tracks/cm<sup>2</sup>); subscripts s, l, and d denote spontaneous, induced, and dosimeter, respectively. P(χ<sup>2</sup>)—probability of obtaining a Chi-square value for n degrees of freedom; Dpar—mean diameter of fission-track etch pit parallel to the c-axis. MTL—Mean Track Length.</td></tr><tr><td colspan="17">*Longitude and latitude coordinates are given in WGS84.</td></tr><tr><td colspan="17">†Zeta = 345 ± 8</td></tr><tr><td colspan="17">‡Zeta = 346 ± 12.</td></tr></tfoot></table>

1Zeta = 345 ± 8

2Zeta = 346 ± 12
University with a nominal fluence of $9 \times 10^{15}$ neutrons/cm$^2$. After irradiation, we etched mica detectors for 40 min in 40% HF at 20 °C to reveal induced tracks. We counted tracks and measured track length distribution using an Olympus optical microscope at a magnification of $\times$1250. We carried out age calculations and statistics with Trackkey software (Dunkl, 2002). We report AFT ages as central age with $1\sigma$ errors (Gla breath and Laslett, 1993) using a zeta calibration approach (Hurford and Green, 1983) with a zeta value of $345 \pm 8$ (samples G1-G22) and a zeta value of $346 \pm 12$ (samples G30-G55) for the CN5 dosimeter glass. Dpar measurements were used to characterize the chemical kinetic properties of the apatite crystals (Burtner et al., 1994). Both track density ratio and average track etch pit diameter (Dpar) were recorded for 20 grains per sample. The results are presented in Table 1.

## Apatite (U-Th-Sm)/He (AHe)

The first steps of AHe dating involved apatite picking at the University of Paris-Saclay, Orsay, France. Apatite grains were selected carefully according to their morphology, size (minimum width of 60 µm), and lack of visible inclusions or grain boundary phases (Murray et al., 2014) and then placed into a Niobium basket for He extraction. From one to three grains were dated per sample, depending on sample apatite quality. The Niobium baskets were heated twice using a diode laser at $1030 \pm 50$ °C for 5 min, allowing for total He degassing and to check the presence of He trapped in small inclusions (Fillon et al., 2013). The $^4$He content was determined by comparison with a known amount of $^3$He spike added during analysis. After He extraction, Nb baskets were placed into single-use polypropylene vials. Apatite grains were dissolved for 3 h at 70 °C in a $50 \mu L HNO_3 5N^-$ solution containing a known content of $^{235}$U, $^{230}$Th, and $^{149}$Sm, and additional $50 \mu L HNO_3 5N^-$ and then filled with 0.9 mL of ultrapure MQ water. The final solution was measured for U, Th, and Sm concentrations by quadrupole inductively coupled plasma (ICP-quadrupole) mass spectrometry (collision cell technology [CCT] Thermo-Electron at LSCE, Gif/Yvette, France). The analysis was calibrated using external age standards, including Limberg Tuff and Durango apatites. Mean AHe ages of $16.0 \pm 1.4$ Ma and $31.1 \pm 2.1$ Ma have been measured for the Limberg Tuff and yellow Durango apatite, respectively, which agree with published data (i.e., $16.8 \pm 1.1$ Ma and $31.0 \pm 1.0$ Ma; Kraml et al., 2006; McDowell et al., 2005). Single ages were corrected using the calculated ejection factor FT, determined using the Monte Carlo simulation technique of Ketcham et al. (2011); the equivalent-sphere

radius was calculated using the procedure of Gautheron and Tassan-Got (2010). Single-grain apatite He ages and supporting data are presented in Table 2. The $1\sigma$ error for single-grain AHe ages should be considered as 9%, reflecting the sum of errors in the ejection-factor correction and age dispersion of the standards.

## Time-Temperature Modeling

In this work we used the QTQt software (version PC 5.7.0) to model 30 samples, including those never before modeled, from Savignano et al. (2016). The program allows inversion of the AFT annealing and AHe diffusion parameters with the Markov chain Monte Carlo method (Gallagher et al., 2009; Gallagher, 2012). The inversion code incorporates kinetic models of He diffusion in apatite (Flowers et al., 2009; Gautheron et al., 2009) and AFT annealing in the multi-kinetic model (Ketcham et al., 2007). The modeling procedure is detailed in Gallagher (2012). The input parameters used to model each profile are: (1) central AFT ages, (2) track length distribution, (3) Dpar values, and (4) single-grain AHe ages with grain size and chemical characteristics. Chemical composition ranges of the apatites analyzed were taken into consideration during both AFT and AHe modeling by imposing the mean measured Dpar values for the sample following Gautheron et al. (2013). Thermal history constraints included: (1) age and emplacement depth of intrusive bodies such as the Permian Mamil Choique Formation ($t = 275 \pm 25$ Ma, $T = 350 \pm 50$ °C), the Lower Jurassic Subcordilleran Batholith and equivalents ($t = 190 \pm 20$ Ma, $T = 350 \pm 50$ °C), the Upper Jurassic to Lower Cretaceous North Patagonian Batholith ($t = 135 \pm 35$ Ma, $T = 350 \pm 50$ °C), and Upper Cretaceous shallow intrusions ($t = 80 \pm 20$ Ma and $T = 100 \pm 20$ °C); (2) depositional age of sedimentary rocks such as the Osta Arena Formation (Lower Jurassic) and Cañadon Asfalto Formation (Upper Jurassic); (3) age of the unconformity (Lower Jurassic) on top of the basement; and (4) present day temperature ($10 \pm 10$ °C for all of the rocks). Thermal history simulation results were obtained taking into account the influence of $\alpha$-recoil damage, grain size, and apatite kinetic properties.

## RESULTS

A homogeneous single grain AFT population ($\chi^2$ test passed) was detected in all of the samples, whereas significant intra-sample variability was observed in only two cases among the AHe data despite the large variety of rock types and U-Th-Sm composition (in both these latter samples, apatites derive from Paleozoic rocks from

the foreland). No particular relationships occur between age, eU (eU = [U] + 0.235[Th] + 0.047[Sm]), and grain radius. AFT ages obtained are in all instances older than AHe ages, as is expected given the closure temperatures. The regional picture is characterized by a marked thermochronological age difference between the Patagonian Precordillera and the broken foreland (Figs. 1, 2, and 4). In fact, the samples from the Precordillera show consistent middle Miocene to Pliocene cooling ages. In contrast, AFT and AHe ages in the broken foreland range from $154.4 \pm 6.0$ Ma to $59.3 \pm 4.2$ Ma, and from $109.2 \pm 9.8$ Ma to $26.9 \pm 2.4$ Ma, respectively. Both AFT and AHe ages are always younger than the depositional ages of the sedimentary rocks sampled. This testifies that all samples were affected by a significant heating that almost completely reset both the AFT and AHe systems. It is noteworthy that the samples from the broken foreland display short fission track lengths (Table 1), thus suggesting a long residence in the so-called Partial Annealing Zone (~60–120 °C, Green et al., 1989). Cretaceous granitoids collected in the Patagonian Precordillera record Late Neogene AFT and AHe cooling ages with the exception of three samples characterized by older AFT and/or AHe ages (Fig. 1).

Paleotemperatures were extracted from the best-fit models (Fig. 4; Figs. S1A and S1B<fnref n="1" />) at different times and then plotted on the map of Figure 5. During Early Jurassic times, most of the samples were at shallow depths (at temperatures between 40 °C and 80 °C), whereas some granitoids were emplaced along the so-called Subcordillera Plutonic Belt (red dots in Fig. 5A). Between the Early and early Late Cretaceous, maximum burial temperatures exceeding or very close to the total reset limit for fission tracks in apatite (i.e., ~120 °C; Green et al., 1989) were reached all across the foreland. Soon after, exhumation of the foreland samples started at very slow rates. From the Late Cretaceous, a clear differentiation occurred between the foreland and the orogen. Samples from the latter still lay at greater depth (except for a few intrusions away from the basins and therefore not affected by any burial, which were already close to the surface). The foreland area remained quite stable for the following 70 m.y., whereas the Patagonian Precordillera was affected by rapid cooling during late Miocene–Pliocene times.

<footnote n="1">¹Supplemental Material. Input files for inverse thermal modeling of the results presented in Figures S1a and S1b. Please visit https://doi.org/10.1130/GSAB.S.13863536 to access the supplemental material, and contact editing@geosociety.org with any questions.</footnote>
University with a nominal fluence of $9 \times 10^{15}$ neutrons/cm$^2$. After irradiation, we etched mica detectors for 40 min in 40% HF at 20 °C to reveal induced tracks. We counted tracks and measured track length distribution using an Olympus optical microscope at a magnification of $\times$1250. We carried out age calculations and statistics with Trackkey software (Dunkl, 2002). We report AFT ages as central age with $1\sigma$ errors (G labourth and Laslett, 1993) using a zeta calibration approach (Hurford and Green, 1983) with a zeta value of $345 \pm 8$ (samples G1-G22) and a zeta value of $346 \pm 12$ (samples G30-G55) for the CN5 dosimeter glass. Dpar measurements were used to characterize the chemical kinetic properties of the apatite crystals (Burtner et al., 1994). Both track density ratio and average track etch pit diameter (Dpar) were recorded for 20 grains per sample. The results are presented in Table 1.

## Apatite (U-Th-Sm)/He (AHe)

The first steps of AHe dating involved apatite picking at the University of Paris-Saclay, Orsay, France. Apatite grains were selected carefully according to their morphology, size (minimum width of 60 µm), and lack of visible inclusions or grain boundary phases (Murray et al., 2014) and then placed into a Niobium basket for He extraction. From one to three grains were dated per sample, depending on sample apatite quality. The Niobium baskets were heated twice using a diode laser at $1030 \pm 50$ °C for 5 min, allowing for total He degassing and to check the presence of He trapped in small inclusions (Fillon et al., 2013). The $^4$He content was determined by comparison with a known amount of $^3$He spike added during analysis. After He extraction, Nb baskets were placed into single-use polypropylene vials. Apatite grains were dissolved for 3 h at 70 °C in a $50 \mu L HNO_3 5N^-$ solution containing a known content of $^{235}$U, $^{230}$Th, and $^{149}$Sm, and additional $50 \mu L HNO_3 5N^-$ and then filled with 0.9 mL of ultrapure MQ water. The final solution was measured for U, Th, and Sm concentrations by quadrupole inductively coupled plasma (ICP-quadrupole) mass spectrometry (collision cell technology [CCT] Thermo-Electron at LSCE, Gif/Yvette, France). The analysis was calibrated using external age standards, including Limberg Tuff and Durango apatites. Mean AHe ages of $16.0 \pm 1.4$ Ma and $31.1 \pm 2.1$ Ma have been measured for the Limberg Tuff and yellow Durango apatite, respectively, which agree with published data (i.e., $16.8 \pm 1.1$ Ma and $31.0 \pm 1.0$ Ma; Kraml et al., 2006; McDowell et al., 2005). Single ages were corrected using the calculated ejection factor FT, determined using the Monte Carlo simulation technique of Ketcham et al. (2011); the equivalent-sphere

radius was calculated using the procedure of Gautheron and Tassan-Got (2010). Single-grain apatite He ages and supporting data are presented in Table 2. The $1\sigma$ error for single-grain AHe ages should be considered as 9%, reflecting the sum of errors in the ejection-factor correction and age dispersion of the standards.

## Time-Temperature Modeling

In this work we used the QTQt software (version PC 5.7.0) to model 30 samples, including those never before modeled, from Savignano et al. (2016). The program allows inversion of the AFT annealing and AHe diffusion parameters with the Markov chain Monte Carlo method (Gallagher et al., 2009; Gallagher, 2012). The inversion code incorporates kinetic models of He diffusion in apatite (Flowers et al., 2009; Gautheron et al., 2009) and AFT annealing in the multi-kinetic model (Ketcham et al., 2007). The modeling procedure is detailed in Gallagher (2012). The input parameters used to model each profile are: (1) central AFT ages, (2) track length distribution, (3) Dpar values, and (4) single-grain AHe ages with grain size and chemical characteristics. Chemical composition ranges of the apatites analyzed were taken into consideration during both AFT and AHe modeling by imposing the mean measured Dpar values for the sample following Gautheron et al. (2013). Thermal history constraints included: (1) age and emplacement depth of intrusive bodies such as the Permian Mamil Choique Formation ($t = 275 \pm 25$ Ma, $T = 350 \pm 50$ °C), the Lower Jurassic Subcordilleran Batholith and equivalents ($t = 190 \pm 20$ Ma, $T = 350 \pm 50$ °C), the Upper Jurassic to Lower Cretaceous North Patagonian Batholith ($t = 135 \pm 35$ Ma, $T = 350 \pm 50$ °C), and Upper Cretaceous shallow intrusions ($t = 80 \pm 20$ Ma and $T = 100 \pm 20$ °C); (2) depositional age of sedimentary rocks such as the Osta Arena Formation (Lower Jurassic) and Cañadon Asfalto Formation (Upper Jurassic); (3) age of the unconformity (Lower Jurassic) on top of the basement; and (4) present day temperature ($10 \pm 10$ °C for all of the rocks). Thermal history simulation results were obtained taking into account the influence of $\alpha$-recoil damage, grain size, and apatite kinetic properties.

## RESULTS

A homogeneous single grain AFT population ($\chi^2$ test passed) was detected in all of the samples, whereas significant intra-sample variability was observed in only two cases among the AHe data despite the large variety of rock types and U-Th-Sm composition (in both these latter samples, apatites derive from Paleozoic rocks from

the foreland). No particular relationships occur between age, eU (eU = [U] + 0.235[Th] + 0.047[Sm]), and grain radius. AFT ages obtained are in all instances older than AHe ages, as is expected given the closure temperatures. The regional picture is characterized by a marked thermochronological age difference between the Patagonian Precordillera and the broken foreland (Figs. 1, 2, and 4). In fact, the samples from the Precordillera show consistent middle Miocene to Pliocene cooling ages. In contrast, AFT and AHe ages in the broken foreland range from $154.4 \pm 6.0$ Ma to $59.3 \pm 4.2$ Ma, and from $109.2 \pm 9.8$ Ma to $26.9 \pm 2.4$ Ma, respectively. Both AFT and AHe ages are always younger than the depositional ages of the sedimentary rocks sampled. This testifies that all samples were affected by a significant heating that almost completely reset both the AFT and AHe systems. It is noteworthy that the samples from the broken foreland display short fission track lengths (Table 1), thus suggesting a long residence in the so-called Partial Annealing Zone (~60–120 °C, Green et al., 1989). Cretaceous granitoids collected in the Patagonian Precordillera record Late Neogene AFT and AHe cooling ages with the exception of three samples characterized by older AFT and/or AHe ages (Fig. 1).

Paleotemperatures were extracted from the best-fit models (Fig. 4; Figs. S1A and S1B<fnref n="1" />) at different times and then plotted on the map of Figure 5. During Early Jurassic times, most of the samples were at shallow depths (at temperatures between 40 °C and 80 °C), whereas some granitoids were emplaced along the so-called Subcordillera Plutonic Belt (red dots in Fig. 5A). Between the Early and early Late Cretaceous, maximum burial temperatures exceeding or very close to the total reset limit for fission tracks in apatite (i.e., ~120 °C; Green et al., 1989) were reached all across the foreland. Soon after, exhumation of the foreland samples started at very slow rates. From the Late Cretaceous, a clear differentiation occurred between the foreland and the orogen. Samples from the latter still lay at greater depth (except for a few intrusions away from the basins and therefore not affected by any burial, which were already close to the surface). The foreland area remained quite stable for the following 70 m.y., whereas the Patagonian Precordillera was affected by rapid cooling during late Miocene–Pliocene times.

<footnote n="1">¹Supplemental Material. Input files for inverse thermal modeling of the results presented in Figures S1a and S1b. Please visit https://doi.org/10.1130/GSAB.S.13863536 to access the supplemental material, and contact editing@geosociety.org with any questions.</footnote>
<table><caption>TABLE 2. APATITE (U-Th)/HE SINGLE GRAIN DATA</caption><thead><tr><th>Samples</th><th>Latitude (°S)</th><th>Longitude (°W)</th><th>Elev. (m)</th><th>Weight (µg)</th><th>Rs (µm)</th><th>FT</th><th>⁴He (ncc/g)</th><th>U (ppm)</th><th>Th (ppm)</th><th>Sm (ppm)</th><th>Th/U</th><th>eU (ppm)</th><th>AHe age (Ma)</th><th>Corrected age (Ma)</th><th>Error (Ma)</th></tr></thead><tbody><tr><td colspan="18">Data from Savignano et al. (2016)</td></tr><tr><td>G01B</td><td>-42.1928</td><td>-69.1656</td><td>1134.00</td><td>1.70</td><td>38.00</td><td>0.64</td><td>481259.60</td><td>91</td><td>31</td><td>151</td><td>0.30</td><td>98</td><td>40.00</td><td>62.10</td><td>5.60</td></tr><tr><td>G02B</td><td>-42.2658</td><td>-69.4086</td><td>929.00</td><td>1.80</td><td>41.00</td><td>0.72</td><td>159254.40</td><td>18</td><td>59</td><td>82</td><td>3.30</td><td>32</td><td>40.60</td><td>56.50</td><td>5.10</td></tr><tr><td>G02C</td><td>-42.2658</td><td>-69.4086</td><td>929.00</td><td>0.90</td><td>35.00</td><td>0.66</td><td>70897.90</td><td>9</td><td>20</td><td>486</td><td>2.20</td><td>14</td><td>33.00</td><td>49.90</td><td>4.50</td></tr><tr><td>G04A</td><td>-42.3878</td><td>-69.5219</td><td>997.00</td><td>3.80</td><td>53.00</td><td>0.79</td><td>204917.20</td><td>21</td><td>22</td><td>66</td><td>1.00</td><td>27</td><td>62.70</td><td>79.90</td><td>7.20</td></tr><tr><td>G04B</td><td>-42.3878</td><td>-69.5219</td><td>997.00</td><td>1.40</td><td>41.00</td><td>0.71</td><td>257276.90</td><td>32</td><td>33</td><td>97</td><td>1.00</td><td>40</td><td>51.90</td><td>73.60</td><td>6.60</td></tr><tr><td>G04C</td><td>-42.3878</td><td>-69.5219</td><td>997.00</td><td>1.60</td><td>44.00</td><td>0.68</td><td>729467.20</td><td>83</td><td>96</td><td>17</td><td>1.20</td><td>105</td><td>57.30</td><td>83.80</td><td>7.50</td></tr><tr><td>G05C</td><td>-42.3694</td><td>-69.6292</td><td>963.00</td><td>1.80</td><td>46.00</td><td>0.75</td><td>116142.70</td><td>43</td><td>20</td><td>65</td><td>0.50</td><td>47</td><td>20.10</td><td>26.90</td><td>2.40</td></tr><tr><td>G06C</td><td>-42.4444</td><td>-68.7803</td><td>1150.00</td><td>3.50</td><td>55.00</td><td>0.78</td><td>70495.90</td><td>22</td><td>19</td><td>66</td><td>0.90</td><td>27</td><td>21.60</td><td>27.70</td><td>2.50</td></tr><tr><td>G07A</td><td>-42.5597</td><td>-69.0039</td><td>1070.00</td><td>2.60</td><td>52.00</td><td>0.73</td><td>1256944.80</td><td>134</td><td>3</td><td>152</td><td>0.00</td><td>135</td><td>76.50</td><td>104.60</td><td>9.40</td></tr><tr><td>G07C</td><td>-42.5597</td><td>-69.0039</td><td>1070.00</td><td>3.40</td><td>55.00</td><td>0.75</td><td>1587006.90</td><td>159</td><td>9</td><td>66</td><td>0.10</td><td>161</td><td>81.40</td><td>109.20</td><td>9.80</td></tr><tr><td>G08A</td><td>-42.5639</td><td>-69.0042</td><td>1043.00</td><td>1.90</td><td>39.00</td><td>0.69</td><td>113056.00</td><td>15</td><td>8</td><td>133</td><td>0.50</td><td>17</td><td>51.40</td><td>74.50</td><td>6.70</td></tr><tr><td>G09A</td><td>-42.4131</td><td>-69.7822</td><td>995.00</td><td>0.50</td><td>32.00</td><td>0.57</td><td>260075.30</td><td>33</td><td>89</td><td>933</td><td>2.70</td><td>54</td><td>35.10</td><td>61.40</td><td>5.50</td></tr><tr><td>G09B</td><td>-42.4131</td><td>-69.7822</td><td>995.00</td><td>0.90</td><td>33.00</td><td>0.63</td><td>124150.00</td><td>19</td><td>50</td><td>141</td><td>2.70</td><td>31</td><td>32.30</td><td>51.60</td><td>4.60</td></tr><tr><td>G09C</td><td>-42.4131</td><td>-69.7822</td><td>995.00</td><td>1.60</td><td>43.00</td><td>0.68</td><td>149903.50</td><td>19</td><td>67</td><td>58</td><td>3.40</td><td>36</td><td>34.60</td><td>51.10</td><td>4.60</td></tr><tr><td>G10B</td><td>-42.0953</td><td>69.7749</td><td>1060.00</td><td>8.80</td><td>77.00</td><td>0.85</td><td>172830.10</td><td>17</td><td>6</td><td>29</td><td>0.30</td><td>19</td><td>75.10</td><td>88.70</td><td>8.00</td></tr><tr><td>G10C</td><td>-42.0953</td><td>69.7749</td><td>1060.00</td><td>4.50</td><td>60.00</td><td>0.79</td><td>163455.50</td><td>18</td><td>7</td><td>83</td><td>0.40</td><td>19</td><td>67.60</td><td>85.20</td><td>7.70</td></tr><tr><td>G11B</td><td>-42.3114</td><td>-70.0367</td><td>999.00</td><td>1.30</td><td>38.00</td><td>0.69</td><td>89896.40</td><td>17</td><td>22</td><td>98</td><td>1.20</td><td>23</td><td>31.80</td><td>46.40</td><td>4.20</td></tr><tr><td>G11C</td><td>-42.3114</td><td>-70.0367</td><td>999.00</td><td>1.60</td><td>43.00</td><td>0.72</td><td>40621.00</td><td>8</td><td>3</td><td>114</td><td>0.40</td><td>9</td><td>34.10</td><td>47.50</td><td>4.30</td></tr><tr><td>G12A</td><td>-42.0353</td><td>-70.2992</td><td>976.00</td><td>3.90</td><td>53.00</td><td>0.78</td><td>170929.50</td><td>15</td><td>17</td><td>104</td><td>1.10</td><td>19</td><td>71.40</td><td>91.20</td><td>8.20</td></tr><tr><td>G12B</td><td>-42.0353</td><td>-70.2992</td><td>976.00</td><td>3.30</td><td>52.00</td><td>0.76</td><td>348509.60</td><td>50</td><td>13</td><td>159</td><td>0.30</td><td>53</td><td>53.30</td><td>70.20</td><td>6.30</td></tr><tr><td>G13B</td><td>-42.3736</td><td>-69.6039</td><td>973.00</td><td>3.90</td><td>58.00</td><td>0.80</td><td>239706.70</td><td>27</td><td>40</td><td>51</td><td>1.50</td><td>37</td><td>53.30</td><td>66.80</td><td>6.00</td></tr><tr><td>G13C</td><td>-42.3736</td><td>-69.6039</td><td>973.00</td><td>7.00</td><td>70.00</td><td>0.82</td><td>28134.20</td><td>3</td><td>15</td><td>79</td><td>4.40</td><td>7</td><td>30.00</td><td>36.40</td><td>3.30</td></tr><tr><td>G14A</td><td>-42.4253</td><td>-68.8347</td><td>1177.00</td><td>3.90</td><td>58.00</td><td>0.79</td><td>291854.30</td><td>35</td><td>83</td><td>61</td><td>2.40</td><td>56</td><td>43.30</td><td>55.00</td><td>4.90</td></tr><tr><td>G14C</td><td>-42.4253</td><td>-68.8347</td><td>1177.00</td><td>4.10</td><td>54.00</td><td>0.77</td><td>357806.40</td><td>64</td><td>74</td><td>89</td><td>1.20</td><td>82</td><td>36.10</td><td>46.90</td><td>4.20</td></tr><tr><td>G15B</td><td>-42.5861</td><td>-68.8128</td><td>1103.00</td><td>4.10</td><td>41.00</td><td>0.77</td><td>26296.80</td><td>3</td><td>11</td><td>52</td><td>4.30</td><td>5</td><td>39.20</td><td>51.00</td><td>4.60</td></tr><tr><td>G15C</td><td>-42.5861</td><td>-68.8128</td><td>1103.00</td><td>1.60</td><td>45.00</td><td>0.69</td><td>65107.30</td><td>5</td><td>23</td><td>135</td><td>4.30</td><td>11</td><td>44.90</td><td>65.40</td><td>5.90</td></tr><tr><td>G17C</td><td>-41.7119</td><td>-69.6192</td><td>1275.00</td><td>1.10</td><td>35.00</td><td>0.65</td><td>206061.00</td><td>25</td><td>13</td><td>126</td><td>0.50</td><td>28</td><td>58.10</td><td>88.90</td><td>8.00</td></tr><tr><td>G18B</td><td>-42.9306</td><td>-71.2606</td><td>617.00</td><td>2.50</td><td>47.00</td><td>0.73</td><td>42608.50</td><td>103</td><td>83</td><td>208</td><td>0.80</td><td>123</td><td>2.80</td><td>3.90</td><td>0.40</td></tr><tr><td>G19B</td><td>-42.8064</td><td>-71.6608</td><td>576.00</td><td>1.80</td><td>42.00</td><td>0.71</td><td>200378.00</td><td>26</td><td>95</td><td>249</td><td>3.60</td><td>49</td><td>3.30</td><td>4.60</td><td>0.40</td></tr><tr><td>G19C</td><td>-42.8064</td><td>-71.6608</td><td>576.00</td><td>1.30</td><td>39.00</td><td>0.69</td><td>11635.10</td><td>53</td><td>61</td><td>236</td><td>1.10</td><td>68</td><td>1.40</td><td>2.00</td><td>0.20</td></tr><tr><td>G20A</td><td>-42.3847</td><td>-71.2453</td><td>663.00</td><td>1.50</td><td>42.00</td><td>0.67</td><td>204513.00</td><td>11</td><td>50</td><td>164</td><td>4.70</td><td>23</td><td>7.20</td><td>10.60</td><td>1.00</td></tr><tr><td>G20B</td><td>-42.3847</td><td>-71.2453</td><td>663.00</td><td>0.90</td><td>36.00</td><td>0.68</td><td>120817.00</td><td>13</td><td>33</td><td>140</td><td>2.50</td><td>21</td><td>4.50</td><td>6.70</td><td>0.60</td></tr><tr><td>G20C</td><td>-42.3847</td><td>-71.2453</td><td>663.00</td><td>3.30</td><td>51.00</td><td>0.73</td><td>5681.20</td><td>6</td><td>24</td><td>85</td><td>3.80</td><td>12</td><td>3.70</td><td>5.00</td><td>0.50</td></tr><tr><td>G21A</td><td>-42.1672</td><td>-71.3572</td><td>616.00</td><td>0.90</td><td>20.00</td><td>0.61</td><td>41763.50</td><td>51</td><td>25</td><td>152</td><td>0.50</td><td>57</td><td>5.90</td><td>9.80</td><td>0.90</td></tr><tr><td>G21B</td><td>-42.1672</td><td>-71.3572</td><td>616.00</td><td>1.10</td><td>37.00</td><td>0.64</td><td>32364.80</td><td>38</td><td>30</td><td>237</td><td>0.80</td><td>45</td><td>5.80</td><td>9.10</td><td>0.80</td></tr><tr><td>G21C</td><td>-42.1672</td><td>-71.3572</td><td>616.00</td><td>1.90</td><td>44.00</td><td>0.73</td><td>164678.00</td><td>23</td><td>14</td><td>79</td><td>0.60</td><td>26</td><td>5.10</td><td>7.00</td><td>0.60</td></tr><tr><td>G22B</td><td>-42.0303</td><td>-71.5761</td><td>275.00</td><td>1.50</td><td>45.00</td><td>0.69</td><td>33055.70</td><td>84</td><td>185</td><td>70</td><td>2.20</td><td>129</td><td>2.10</td><td>3.10</td><td>0.30</td></tr><tr><td>G22C</td><td>-42.0303</td><td>-71.5761</td><td>275.00</td><td>2.00</td><td>46.00</td><td>0.73</td><td>19459.80</td><td>45</td><td>109</td><td>127</td><td>2.40</td><td>71</td><td>2.20</td><td>3.10</td><td>0.30</td></tr><tr><td colspan="18">New data</td></tr><tr><td>G30A</td><td>-43.7439</td><td>-69.8186</td><td>670.00</td><td>12.37</td><td>78.30</td><td>0.85</td><td>126233.00</td><td>4</td><td>39</td><td>76</td><td>9.10</td><td>14</td><td>73.40</td><td>85.90</td><td>0.70</td></tr><tr><td>G30B</td><td>-43.7439</td><td>-69.8186</td><td>670.00</td><td>3.58</td><td>55.60</td><td>0.78</td><td>145207.00</td><td>6</td><td>48</td><td>209</td><td>8.20</td><td>18</td><td>63.00</td><td>80.70</td><td>0.60</td></tr><tr><td>G30C</td><td>-43.7439</td><td>-69.8186</td><td>670.00</td><td>8.72</td><td>71.60</td><td>0.82</td><td>178090.00</td><td>6</td><td>50</td><td>107</td><td>9.20</td><td>18</td><td>81.20</td><td>98.80</td><td>0.80</td></tr><tr><td>G31A</td><td>-43.6650</td><td>-70.0839</td><td>825.00</td><td>5.46</td><td>59.00</td><td>0.79</td><td>0.00</td><td>8</td><td>21</td><td>131</td><td>2.50</td><td>14</td><td>52.30</td><td>66.40</td><td>5.30</td></tr><tr><td>G31C</td><td>-43.6650</td><td>-70.0839</td><td>825.00</td><td>2.95</td><td>43.20</td><td>0.68</td><td>0.00</td><td>11</td><td>45</td><td>102</td><td>4.30</td><td>21</td><td>45.20</td><td>66.40</td><td>5.30</td></tr><tr><td>G31D</td><td>-43.6650</td><td>-70.0839</td><td>825.00</td><td>5.25</td><td>56.40</td><td>0.75</td><td>0.00</td><td>5</td><td>32</td><td>91</td><td>6.40</td><td>13</td><td>41.40</td><td>55.20</td><td>4.40</td></tr><tr><td>G35A</td><td>-43.3244</td><td>-70.8961</td><td>875.00</td><td>4.23</td><td>46.70</td><td>0.72</td><td>22167.00</td><td>5</td><td>18</td><td>80</td><td>3.40</td><td>10</td><td>17.50</td><td>24.30</td><td>1.90</td></tr><tr><td>G35B</td><td>-43.3244</td><td>-70.8961</td><td>875.00</td><td>5.64</td><td>52.40</td><td>0.75</td><td>14432.00</td><td>4</td><td>14</td><td>51</td><td>3.20</td><td>8</td><td>15.10</td><td>20.10</td><td>1.60</td></tr><tr><td>G37A</td><td>-43.5250</td><td>-71.5181</td><td>473.00</td><td>0.54</td><td>27.80</td><td>0.56</td><td>141653.00</td><td>6</td><td>44</td><td>422</td><td>8.00</td><td>16</td><td>60.00</td><td>106.80</td><td>8.50</td></tr><tr><td>G37B</td><td>-43.5250</td><td>-71.5181</td><td>473.00</td><td>0.67</td><td>28.30</td><td>0.60</td><td>149258.00</td><td>9</td><td>31</td><td>343</td><td>3.50</td><td>17</td><td>64.40</td><td>107.80</td><td>8.60</td></tr><tr><td>G38A</td><td>-41.8508</td><td>-71.4219</td><td>627.00</td><td>1.62</td><td>37.40</td><td>0.68</td><td>27338.00</td><td>30</td><td>31</td><td>90</td><td>1.00</td><td>37</td><td>6.00</td><td>8.80</td><td>0.70</td></tr><tr><td>G38B</td><td>-41.8508</td><td>-71.4219</td><td>627.00</td><td>1.65</td><td>39.40</td><td>0.68</td><td>35968.00</td><td>35</td><td>49</td><td>92</td><td>1.40</td><td>46</td><td>6.40</td><td>9.40</td><td>0.80</td></tr><tr><td>G38C</td><td>-41.8508</td><td>-71.4219</td><td>627.00</td><td>2.77</td><td>50.50</td><td>0.72</td><td>16316.00</td><td>25</td><td>39</td><td>93</td><td>1.60</td><td>34</td><td>3.90</td><td>5.40</td><td>0.40</td></tr><tr><td>G39A</td><td>-41.5564</td><td>-71.4806</td><td>731.00</td><td>0.76</td><td>30.30</td><td>0.59</td><td>39981.00</td><td>6</td><td>12</td><td>242</td><td>2.10</td><td>9</td><td>31.60</td><td>53.70</td><td>0.30</td></tr><tr><td>G39B</td><td>-41.5564</td><td>-71.4806</td><td>731.00</td><td>0.76</td><td>26.20</td><td>0.59</td><td>35918.00</td><td>6</td><td>17</td><td>303</td><td>2.90</td><td>10</td><td>23.80</td><td>40.50</td><td>0.20</td></tr><tr><td>G39C</td><td>-41.5564</td><td>-71.4806</td><td>731.00</td><td>0.36</td><td>25.00</td><td>0.53</td><td>115956.00</td><td>15</td><td>31</td><td>601</td><td>2.00</td><td>23</td><td>35.30</td><td>66.20</td><td>0.40</td></tr><tr><td>G40A</td><td>-40.3447</td><td>-71.5064</td><td>898.00</td><td>0.92</td><td>33.70</td><td>0.64</td><td>13772.00</td><td>22</td><td>59</td><td>55</td><td>2.70</td><td>36</td><td>3.20</td><td>5.00</td><td>0.40</td></tr><tr><td>G40B</td><td>-40.3447</td><td>-71.5064</td><td>898.00</td><td>0.60</td><td>29.80</td><td>0.60</td><td>32916.00</td><td>23</td><td>62</td><td>95</td><td>2.60</td><td>38</td><td>7.00</td><td>11.80</td><td>0.90</td></tr><tr><td>G41A</td><td>-41.1061</td><td>-71.4831</td><td>849.00</td><td>0.85</td><td>31.40</td><td>0.65</td><td>103534.00</td><td>60</td><td>43</td><td>177</td><td>0.70</td><td>70</td><td>12.00</td><td>18.50</td><td>0.10</td></tr><tr><td>G41B</td><td>-41.1061</td><td>-71.4831</td><td>849.00</td><td>2.80</td><td>48.70</td><td>0.78</td><td>139480.00</td><td>52</td><td>47</td><td>143</td><td>0.90</td><td>63</td><td>18.00</td><td>23.20</td><td>0.20</td></tr><tr><td>G41C</td><td>-41.1061</td><td>-71.4831</td><td>849.00</td><td>0.60</td><td>28.60</td><td>0.57</td><td>133871.00</td><td>94</td><td>68</td><td>226</td><td>0.70</td><td>110</td><td>9.90</td><td>17.30</td><td>0.10</td></tr><tr><td>G44B</td><td>-41.2089</td><td>-70.1667</td><td>999.00</td><td>2.26</td><td>44.80</td><td>0.69</td><td>63247.00</td><td>9</td><td>1</td><td>95</td><td>0.10</td><td>9</td><td>53.10</td><td>77.10</td><td>6.20</td></tr><tr><td>G44C</td><td>-41.2089</td><td>-70.1667</td><td>999.00</td><td>3.63</td><td>50.40</td><td>0.72</td><td>303657.00</td><td>35</td><td>41</td><td>123</td><td>1.20</td><td>45</td><td>55.40</td><td>76.70</td><td>6.10</td></tr><tr><td>G48A</td><td>-40.8936</td><td>-71.4950</td><td>805.00</td><td>1.79</td><td>42.80</td><td>0.68</td><td>22054.00</td><td>34</td><td>57</td><td>177</td><td>1.70</td><td>48</td><td>3.70</td><td>5.50</td><td>0.40</td></tr><tr><td>G48B</td><td>-40.8936</td><td>-71.4950</td><td>805.00</td><td>2.91</td><td>50.30</td><td>0.75</td><td>23517.00</td><td>31</td><td>51</td><td>136</td><td>1.60</td><td>44</td><td>4.40</td><td>5.80</td><td>0.50</td></tr><tr><td>G48C</td><td>-40.8936</td><td>-71.4950</td><td>805.00</td><td>2.48</td><td>46.50</td><td>0.73</td><td>18720.00</td><td>19</td><td>38</td><td>130</td><td>1.90</td><td>28</td><td>5.30</td><td>7.30</td><td>0.60</td></tr><tr><td>G49A</td><td>-40.4853</td><td>-71.5894</td><td>903.00</td><td>0.74</td><td>32.90</td><td>0.59</td><td>37522.00</td><td>35</td><td>55</td><td>143</td><td>1.60</td><td>49</td><td>6.30</td><td>10.70</td><td>0.90</td></tr><tr><td>G49B</td><td>-40.4853</td><td>-71.5894</td><td>903.00</td><td>1.60</td><td>39.00</td><td>0.65</td><td>23932.00</td><td>23</td><td>58</td><td>134</td><td>2.60</td><td>37</td><td>5.30</td><td>8.20</td><td>0.70</td></tr><tr><td>G50A</td><td>-40.1956</td><td>-71.3614</td><td>976.00</td><td>0.93</td><td>32.60</td><td>0.62</td><td>23618.00</td><td>73</td><td>9</td><td>245</td><td>0.10</td><td>75</td><td>2.50</td><td>4.10</td><td>0.30</td></tr><tr><td>G50B</td><td>-40.1956</td><td>-71.3614</td><td>976.00</td><td>2.48</td><td>48.90</td><td>0.72</td><td>18111.00</td><td>44</td><td>7</td><td>145</td><td>0.10</td><td>45</td><td>3.20</td><td>4.50</td><td>0.40</td></tr><tr><td>G50C</td><td>-40.1956</td><td>-71.3614</td><td>976.00</td><td>1.41</td><td>41.00</td><td>0.66</td><td>451070.00</td><td>106</td><td>31</td><td>284</td><td>0.30</td><td>114</td><td>3.20</td><td>4.80</td><td>0.40</td></tr><tr><td>G51A</td><td>-40.1289</td><td>-71.6456</td><td>791.00</td><td>2.26</td><td>45.90</td><td>0.73</td><td>6474.00</td><td>15</td><td>19</td><td>58</td><td>1.20</td><td>20</td><td>2.70</td><td>3.60</td><td>0.00</td></tr><tr><td>G51B</td><td>-40.1289</td><td>-71.6456</td><td>791.00</td><td>3.73</td><td>53.00</td><td>0.76</td><td>2563.00</td><td>10</td><td>13</td><td>37</td><td>1.30</td><td>13</td><td>1.60</td><td>2.10</td><td>0.00</td></tr></tbody></table>

Notes: Rs—sphere equivalent radius of hexagonal crystal with the same surface/volume ratio; FT—geometric correction factor for age calculation; eU—effective uranium concentration.

DISCUSSION

Best-fit thermal histories derived from the inverse modeling of samples collected in the foreland show maximum burial conditions at temperatures higher than 110 °C (Figs. 5 and

6A). The present-day average geothermal gradient is estimated at 34 ± 11°C/km for the upper crust in the north Patagonian Cordillera based on regional surface heat flow measurements and thermal conductivities for granitoids (Hamza and Muñoz, 1996; Seipold, 1998; Muñoz, 1999;

Thomson, 2002; Adriasola et al., 2006; Sachse et al., 2016). Nevertheless, the geothermal gradient in the north Patagonian Precordillera and its foreland may have changed over time, especially due to magmatic events, slab window opening, or crustal thinning. Although the magmatic arc
<table><caption>TABLE 2. APATITE (U-Th)/HE SINGLE GRAIN DATA</caption><thead><tr><th>Samples</th><th>Latitude (°S)</th><th>Longitude (°W)</th><th>Elev. (m)</th><th>Weight (µg)</th><th>Rs (µm)</th><th>FT</th><th>⁴He (ncc/g)</th><th>U (ppm)</th><th>Th (ppm)</th><th>Sm (ppm)</th><th>Th/U</th><th>eU (ppm)</th><th>AHe age (Ma)</th><th>Corrected age (Ma)</th><th>Error (Ma)</th></tr></thead><tbody><tr><td colspan="18">Data from Savignano et al. (2016)</td></tr><tr><td>G01B</td><td>-42.1928</td><td>-69.1656</td><td>1134.00</td><td>1.70</td><td>38.00</td><td>0.64</td><td>481259.60</td><td>91</td><td>31</td><td>151</td><td>0.30</td><td>98</td><td>40.00</td><td>62.10</td><td>5.60</td></tr><tr><td>G02B</td><td>-42.2658</td><td>-69.4086</td><td>929.00</td><td>1.80</td><td>41.00</td><td>0.72</td><td>159254.40</td><td>18</td><td>59</td><td>82</td><td>3.30</td><td>32</td><td>40.60</td><td>56.50</td><td>5.10</td></tr><tr><td>G02C</td><td>-42.2658</td><td>-69.4086</td><td>929.00</td><td>0.90</td><td>35.00</td><td>0.66</td><td>70897.90</td><td>9</td><td>20</td><td>486</td><td>2.20</td><td>14</td><td>33.00</td><td>49.90</td><td>4.50</td></tr><tr><td>G04A</td><td>-42.3878</td><td>-69.5219</td><td>997.00</td><td>3.80</td><td>53.00</td><td>0.79</td><td>204917.20</td><td>21</td><td>22</td><td>66</td><td>1.00</td><td>27</td><td>62.70</td><td>79.90</td><td>7.20</td></tr><tr><td>G04B</td><td>-42.3878</td><td>-69.5219</td><td>997.00</td><td>1.40</td><td>41.00</td><td>0.71</td><td>257276.90</td><td>32</td><td>33</td><td>97</td><td>1.00</td><td>40</td><td>51.90</td><td>73.60</td><td>6.60</td></tr><tr><td>G04C</td><td>-42.3878</td><td>-69.5219</td><td>997.00</td><td>1.60</td><td>44.00</td><td>0.68</td><td>729467.20</td><td>83</td><td>96</td><td>17</td><td>1.20</td><td>105</td><td>57.30</td><td>83.80</td><td>7.50</td></tr><tr><td>G05C</td><td>-42.3694</td><td>-69.6292</td><td>963.00</td><td>1.80</td><td>46.00</td><td>0.75</td><td>116142.70</td><td>43</td><td>20</td><td>65</td><td>0.50</td><td>47</td><td>20.10</td><td>26.90</td><td>2.40</td></tr><tr><td>G06C</td><td>-42.4444</td><td>-68.7803</td><td>1150.00</td><td>3.50</td><td>55.00</td><td>0.78</td><td>70495.90</td><td>22</td><td>19</td><td>66</td><td>0.90</td><td>27</td><td>21.60</td><td>27.70</td><td>2.50</td></tr><tr><td>G07A</td><td>-42.5597</td><td>-69.0039</td><td>1070.00</td><td>2.60</td><td>52.00</td><td>0.73</td><td>1256944.80</td><td>134</td><td>3</td><td>152</td><td>0.00</td><td>135</td><td>76.50</td><td>104.60</td><td>9.40</td></tr><tr><td>G07C</td><td>-42.5597</td><td>-69.0039</td><td>1070.00</td><td>3.40</td><td>55.00</td><td>0.75</td><td>1587006.90</td><td>159</td><td>9</td><td>66</td><td>0.10</td><td>161</td><td>81.40</td><td>109.20</td><td>9.80</td></tr><tr><td>G08A</td><td>-42.5639</td><td>-69.0042</td><td>1043.00</td><td>1.90</td><td>39.00</td><td>0.69</td><td>113056.00</td><td>15</td><td>8</td><td>133</td><td>0.50</td><td>17</td><td>51.40</td><td>74.50</td><td>6.70</td></tr><tr><td>G09A</td><td>-42.4131</td><td>-69.7822</td><td>995.00</td><td>0.50</td><td>32.00</td><td>0.57</td><td>260075.30</td><td>33</td><td>89</td><td>933</td><td>2.70</td><td>54</td><td>35.10</td><td>61.40</td><td>5.50</td></tr><tr><td>G09B</td><td>-42.4131</td><td>-69.7822</td><td>995.00</td><td>0.90</td><td>33.00</td><td>0.63</td><td>124150.00</td><td>19</td><td>50</td><td>141</td><td>2.70</td><td>31</td><td>32.30</td><td>51.60</td><td>4.60</td></tr><tr><td>G09C</td><td>-42.4131</td><td>-69.7822</td><td>995.00</td><td>1.60</td><td>43.00</td><td>0.68</td><td>149903.50</td><td>19</td><td>67</td><td>58</td><td>3.40</td><td>36</td><td>34.60</td><td>51.10</td><td>4.60</td></tr><tr><td>G10B</td><td>-42.0953</td><td>69.7749</td><td>1060.00</td><td>8.80</td><td>77.00</td><td>0.85</td><td>172830.10</td><td>17</td><td>6</td><td>29</td><td>0.30</td><td>19</td><td>75.10</td><td>88.70</td><td>8.00</td></tr><tr><td>G10C</td><td>-42.0953</td><td>69.7749</td><td>1060.00</td><td>4.50</td><td>60.00</td><td>0.79</td><td>163455.50</td><td>18</td><td>7</td><td>83</td><td>0.40</td><td>19</td><td>67.60</td><td>85.20</td><td>7.70</td></tr><tr><td>G11B</td><td>-42.3114</td><td>-70.0367</td><td>999.00</td><td>1.30</td><td>38.00</td><td>0.69</td><td>89896.40</td><td>17</td><td>22</td><td>98</td><td>1.20</td><td>23</td><td>31.80</td><td>46.40</td><td>4.20</td></tr><tr><td>G11C</td><td>-42.3114</td><td>-70.0367</td><td>999.00</td><td>1.60</td><td>43.00</td><td>0.72</td><td>40621.00</td><td>8</td><td>3</td><td>114</td><td>0.40</td><td>9</td><td>34.10</td><td>47.50</td><td>4.30</td></tr><tr><td>G12A</td><td>-42.0353</td><td>-70.2992</td><td>976.00</td><td>3.90</td><td>53.00</td><td>0.78</td><td>170929.50</td><td>15</td><td>17</td><td>104</td><td>1.10</td><td>19</td><td>71.40</td><td>91.20</td><td>8.20</td></tr><tr><td>G12B</td><td>-42.0353</td><td>-70.2992</td><td>976.00</td><td>3.30</td><td>52.00</td><td>0.76</td><td>348509.60</td><td>50</td><td>13</td><td>159</td><td>0.30</td><td>53</td><td>53.30</td><td>70.20</td><td>6.30</td></tr><tr><td>G13B</td><td>-42.3736</td><td>-69.6039</td><td>973.00</td><td>3.90</td><td>58.00</td><td>0.80</td><td>239706.70</td><td>27</td><td>40</td><td>51</td><td>1.50</td><td>37</td><td>53.30</td><td>66.80</td><td>6.00</td></tr><tr><td>G13C</td><td>-42.3736</td><td>-69.6039</td><td>973.00</td><td>7.00</td><td>70.00</td><td>0.82</td><td>28134.20</td><td>3</td><td>15</td><td>79</td><td>4.40</td><td>7</td><td>30.00</td><td>36.40</td><td>3.30</td></tr><tr><td>G14A</td><td>-42.4253</td><td>-68.8347</td><td>1177.00</td><td>3.90</td><td>58.00</td><td>0.79</td><td>291854.30</td><td>35</td><td>83</td><td>61</td><td>2.40</td><td>56</td><td>43.30</td><td>55.00</td><td>4.90</td></tr><tr><td>G14C</td><td>-42.4253</td><td>-68.8347</td><td>1177.00</td><td>4.10</td><td>54.00</td><td>0.77</td><td>357806.40</td><td>64</td><td>74</td><td>89</td><td>1.20</td><td>82</td><td>36.10</td><td>46.90</td><td>4.20</td></tr><tr><td>G15B</td><td>-42.5861</td><td>-68.8128</td><td>1103.00</td><td>4.10</td><td>41.00</td><td>0.77</td><td>26296.80</td><td>3</td><td>11</td><td>52</td><td>4.30</td><td>5</td><td>39.20</td><td>51.00</td><td>4.60</td></tr><tr><td>G15C</td><td>-42.5861</td><td>-68.8128</td><td>1103.00</td><td>1.60</td><td>45.00</td><td>0.69</td><td>65107.30</td><td>5</td><td>23</td><td>135</td><td>4.30</td><td>11</td><td>44.90</td><td>65.40</td><td>5.90</td></tr><tr><td>G17C</td><td>-41.7119</td><td>-69.6192</td><td>1275.00</td><td>1.10</td><td>35.00</td><td>0.65</td><td>206061.00</td><td>25</td><td>13</td><td>126</td><td>0.50</td><td>28</td><td>58.10</td><td>88.90</td><td>8.00</td></tr><tr><td>G18B</td><td>-42.9306</td><td>-71.2606</td><td>617.00</td><td>2.50</td><td>47.00</td><td>0.73</td><td>42608.50</td><td>103</td><td>83</td><td>208</td><td>0.80</td><td>123</td><td>2.80</td><td>3.90</td><td>0.40</td></tr><tr><td>G19B</td><td>-42.8064</td><td>-71.6608</td><td>576.00</td><td>1.80</td><td>42.00</td><td>0.71</td><td>200378.00</td><td>26</td><td>95</td><td>249</td><td>3.60</td><td>49</td><td>3.30</td><td>4.60</td><td>0.40</td></tr><tr><td>G19C</td><td>-42.8064</td><td>-71.6608</td><td>576.00</td><td>1.30</td><td>39.00</td><td>0.69</td><td>11635.10</td><td>53</td><td>61</td><td>236</td><td>1.10</td><td>68</td><td>1.40</td><td>2.00</td><td>0.20</td></tr><tr><td>G20A</td><td>-42.3847</td><td>-71.2453</td><td>663.00</td><td>1.50</td><td>42.00</td><td>0.67</td><td>204513.00</td><td>11</td><td>50</td><td>164</td><td>4.70</td><td>23</td><td>7.20</td><td>10.60</td><td>1.00</td></tr><tr><td>G20B</td><td>-42.3847</td><td>-71.2453</td><td>663.00</td><td>0.90</td><td>36.00</td><td>0.68</td><td>120817.00</td><td>13</td><td>33</td><td>140</td><td>2.50</td><td>21</td><td>4.50</td><td>6.70</td><td>0.60</td></tr><tr><td>G20C</td><td>-42.3847</td><td>-71.2453</td><td>663.00</td><td>3.30</td><td>51.00</td><td>0.73</td><td>5681.20</td><td>6</td><td>24</td><td>85</td><td>3.80</td><td>12</td><td>3.70</td><td>5.00</td><td>0.50</td></tr><tr><td>G21A</td><td>-42.1672</td><td>-71.3572</td><td>616.00</td><td>0.90</td><td>20.00</td><td>0.61</td><td>41763.50</td><td>51</td><td>25</td><td>152</td><td>0.50</td><td>57</td><td>5.90</td><td>9.80</td><td>0.90</td></tr><tr><td>G21B</td><td>-42.1672</td><td>-71.3572</td><td>616.00</td><td>1.10</td><td>37.00</td><td>0.64</td><td>32364.80</td><td>38</td><td>30</td><td>237</td><td>0.80</td><td>45</td><td>5.80</td><td>9.10</td><td>0.80</td></tr><tr><td>G21C</td><td>-42.1672</td><td>-71.3572</td><td>616.00</td><td>1.90</td><td>44.00</td><td>0.73</td><td>164678.00</td><td>23</td><td>14</td><td>79</td><td>0.60</td><td>26</td><td>5.10</td><td>7.00</td><td>0.60</td></tr><tr><td>G22B</td><td>-42.0303</td><td>-71.5761</td><td>275.00</td><td>1.50</td><td>45.00</td><td>0.69</td><td>33055.70</td><td>84</td><td>185</td><td>70</td><td>2.20</td><td>129</td><td>2.10</td><td>3.10</td><td>0.30</td></tr><tr><td>G22C</td><td>-42.0303</td><td>-71.5761</td><td>275.00</td><td>2.00</td><td>46.00</td><td>0.73</td><td>19459.80</td><td>45</td><td>109</td><td>127</td><td>2.40</td><td>71</td><td>2.20</td><td>3.10</td><td>0.30</td></tr><tr><td colspan="18">New data</td></tr><tr><td>G30A</td><td>-43.7439</td><td>-69.8186</td><td>670.00</td><td>12.37</td><td>78.30</td><td>0.85</td><td>126233.00</td><td>4</td><td>39</td><td>76</td><td>9.10</td><td>14</td><td>73.40</td><td>85.90</td><td>0.70</td></tr><tr><td>G30B</td><td>-43.7439</td><td>-69.8186</td><td>670.00</td><td>3.58</td><td>55.60</td><td>0.78</td><td>145207.00</td><td>6</td><td>48</td><td>209</td><td>8.20</td><td>18</td><td>63.00</td><td>80.70</td><td>0.60</td></tr><tr><td>G30C</td><td>-43.7439</td><td>-69.8186</td><td>670.00</td><td>8.72</td><td>71.60</td><td>0.82</td><td>178090.00</td><td>6</td><td>50</td><td>107</td><td>9.20</td><td>18</td><td>81.20</td><td>98.80</td><td>0.80</td></tr><tr><td>G31A</td><td>-43.6650</td><td>-70.0839</td><td>825.00</td><td>5.46</td><td>59.00</td><td>0.79</td><td>0.00</td><td>8</td><td>21</td><td>131</td><td>2.50</td><td>14</td><td>52.30</td><td>66.40</td><td>5.30</td></tr><tr><td>G31C</td><td>-43.6650</td><td>-70.0839</td><td>825.00</td><td>2.95</td><td>43.20</td><td>0.68</td><td>0.00</td><td>11</td><td>45</td><td>102</td><td>4.30</td><td>21</td><td>45.20</td><td>66.40</td><td>5.30</td></tr><tr><td>G31D</td><td>-43.6650</td><td>-70.0839</td><td>825.00</td><td>5.25</td><td>56.40</td><td>0.75</td><td>0.00</td><td>5</td><td>32</td><td>91</td><td>6.40</td><td>13</td><td>41.40</td><td>55.20</td><td>4.40</td></tr><tr><td>G35A</td><td>-43.3244</td><td>-70.8961</td><td>875.00</td><td>4.23</td><td>46.70</td><td>0.72</td><td>22167.00</td><td>5</td><td>18</td><td>80</td><td>3.40</td><td>10</td><td>17.50</td><td>24.30</td><td>1.90</td></tr><tr><td>G35B</td><td>-43.3244</td><td>-70.8961</td><td>875.00</td><td>5.64</td><td>52.40</td><td>0.75</td><td>14432.00</td><td>4</td><td>14</td><td>51</td><td>3.20</td><td>8</td><td>15.10</td><td>20.10</td><td>1.60</td></tr><tr><td>G37A</td><td>-43.5250</td><td>-71.5181</td><td>473.00</td><td>0.54</td><td>27.80</td><td>0.56</td><td>141653.00</td><td>6</td><td>44</td><td>422</td><td>8.00</td><td>16</td><td>60.00</td><td>106.80</td><td>8.50</td></tr><tr><td>G37B</td><td>-43.5250</td><td>-71.5181</td><td>473.00</td><td>0.67</td><td>28.30</td><td>0.60</td><td>149258.00</td><td>9</td><td>31</td><td>343</td><td>3.50</td><td>17</td><td>64.40</td><td>107.80</td><td>8.60</td></tr><tr><td>G38A</td><td>-41.8508</td><td>-71.4219</td><td>627.00</td><td>1.62</td><td>37.40</td><td>0.68</td><td>27338.00</td><td>30</td><td>31</td><td>90</td><td>1.00</td><td>37</td><td>6.00</td><td>8.80</td><td>0.70</td></tr><tr><td>G38B</td><td>-41.8508</td><td>-71.4219</td><td>627.00</td><td>1.65</td><td>39.40</td><td>0.68</td><td>35968.00</td><td>35</td><td>49</td><td>92</td><td>1.40</td><td>46</td><td>6.40</td><td>9.40</td><td>0.80</td></tr><tr><td>G38C</td><td>-41.8508</td><td>-71.4219</td><td>627.00</td><td>2.77</td><td>50.50</td><td>0.72</td><td>16316.00</td><td>25</td><td>39</td><td>93</td><td>1.60</td><td>34</td><td>3.90</td><td>5.40</td><td>0.40</td></tr><tr><td>G39A</td><td>-41.5564</td><td>-71.4806</td><td>731.00</td><td>0.76</td><td>30.30</td><td>0.59</td><td>39981.00</td><td>6</td><td>12</td><td>242</td><td>2.10</td><td>9</td><td>31.60</td><td>53.70</td><td>0.30</td></tr><tr><td>G39B</td><td>-41.5564</td><td>-71.4806</td><td>731.00</td><td>0.76</td><td>26.20</td><td>0.59</td><td>35918.00</td><td>6</td><td>17</td><td>303</td><td>2.90</td><td>10</td><td>23.80</td><td>40.50</td><td>0.20</td></tr><tr><td>G39C</td><td>-41.5564</td><td>-71.4806</td><td>731.00</td><td>0.36</td><td>25.00</td><td>0.53</td><td>115956.00</td><td>15</td><td>31</td><td>601</td><td>2.00</td><td>23</td><td>35.30</td><td>66.20</td><td>0.40</td></tr><tr><td>G40A</td><td>-40.3447</td><td>-71.5064</td><td>898.00</td><td>0.92</td><td>33.70</td><td>0.64</td><td>13772.00</td><td>22</td><td>59</td><td>55</td><td>2.70</td><td>36</td><td>3.20</td><td>5.00</td><td>0.40</td></tr><tr><td>G40B</td><td>-40.3447</td><td>-71.5064</td><td>898.00</td><td>0.60</td><td>29.80</td><td>0.60</td><td>32916.00</td><td>23</td><td>62</td><td>95</td><td>2.60</td><td>38</td><td>7.00</td><td>11.80</td><td>0.90</td></tr><tr><td>G41A</td><td>-41.1061</td><td>-71.4831</td><td>849.00</td><td>0.85</td><td>31.40</td><td>0.65</td><td>103534.00</td><td>60</td><td>43</td><td>177</td><td>0.70</td><td>70</td><td>12.00</td><td>18.50</td><td>0.10</td></tr><tr><td>G41B</td><td>-41.1061</td><td>-71.4831</td><td>849.00</td><td>2.80</td><td>48.70</td><td>0.78</td><td>139480.00</td><td>52</td><td>47</td><td>143</td><td>0.90</td><td>63</td><td>18.00</td><td>23.20</td><td>0.20</td></tr><tr><td>G41C</td><td>-41.1061</td><td>-71.4831</td><td>849.00</td><td>0.60</td><td>28.60</td><td>0.57</td><td>133871.00</td><td>94</td><td>68</td><td>226</td><td>0.70</td><td>110</td><td>9.90</td><td>17.30</td><td>0.10</td></tr><tr><td>G44B</td><td>-41.2089</td><td>-70.1667</td><td>999.00</td><td>2.26</td><td>44.80</td><td>0.69</td><td>63247.00</td><td>9</td><td>1</td><td>95</td><td>0.10</td><td>9</td><td>53.10</td><td>77.10</td><td>6.20</td></tr><tr><td>G44C</td><td>-41.2089</td><td>-70.1667</td><td>999.00</td><td>3.63</td><td>50.40</td><td>0.72</td><td>303657.00</td><td>35</td><td>41</td><td>123</td><td>1.20</td><td>45</td><td>55.40</td><td>76.70</td><td>6.10</td></tr><tr><td>G48A</td><td>-40.8936</td><td>-71.4950</td><td>805.00</td><td>1.79</td><td>42.80</td><td>0.68</td><td>22054.00</td><td>34</td><td>57</td><td>177</td><td>1.70</td><td>48</td><td>3.70</td><td>5.50</td><td>0.40</td></tr><tr><td>G48B</td><td>-40.8936</td><td>-71.4950</td><td>805.00</td><td>2.91</td><td>50.30</td><td>0.75</td><td>23517.00</td><td>31</td><td>51</td><td>136</td><td>1.60</td><td>44</td><td>4.40</td><td>5.80</td><td>0.50</td></tr><tr><td>G48C</td><td>-40.8936</td><td>-71.4950</td><td>805.00</td><td>2.48</td><td>46.50</td><td>0.73</td><td>18720.00</td><td>19</td><td>38</td><td>130</td><td>1.90</td><td>28</td><td>5.30</td><td>7.30</td><td>0.60</td></tr><tr><td>G49A</td><td>-40.4853</td><td>-71.5894</td><td>903.00</td><td>0.74</td><td>32.90</td><td>0.59</td><td>37522.00</td><td>35</td><td>55</td><td>143</td><td>1.60</td><td>49</td><td>6.30</td><td>10.70</td><td>0.90</td></tr><tr><td>G49B</td><td>-40.4853</td><td>-71.5894</td><td>903.00</td><td>1.60</td><td>39.00</td><td>0.65</td><td>23932.00</td><td>23</td><td>58</td><td>134</td><td>2.60</td><td>37</td><td>5.30</td><td>8.20</td><td>0.70</td></tr><tr><td>G50A</td><td>-40.1956</td><td>-71.3614</td><td>976.00</td><td>0.93</td><td>32.60</td><td>0.62</td><td>23618.00</td><td>73</td><td>9</td><td>245</td><td>0.10</td><td>75</td><td>2.50</td><td>4.10</td><td>0.30</td></tr><tr><td>G50B</td><td>-40.1956</td><td>-71.3614</td><td>976.00</td><td>2.48</td><td>48.90</td><td>0.72</td><td>18111.00</td><td>44</td><td>7</td><td>145</td><td>0.10</td><td>45</td><td>3.20</td><td>4.50</td><td>0.40</td></tr><tr><td>G50C</td><td>-40.1956</td><td>-71.3614</td><td>976.00</td><td>1.41</td><td>41.00</td><td>0.66</td><td>451070.00</td><td>106</td><td>31</td><td>284</td><td>0.30</td><td>114</td><td>3.20</td><td>4.80</td><td>0.40</td></tr><tr><td>G51A</td><td>-40.1289</td><td>-71.6456</td><td>791.00</td><td>2.26</td><td>45.90</td><td>0.73</td><td>6474.00</td><td>15</td><td>19</td><td>58</td><td>1.20</td><td>20</td><td>2.70</td><td>3.60</td><td>0.00</td></tr><tr><td>G51B</td><td>-40.1289</td><td>-71.6456</td><td>791.00</td><td>3.73</td><td>53.00</td><td>0.76</td><td>2563.00</td><td>10</td><td>13</td><td>37</td><td>1.30</td><td>13</td><td>1.60</td><td>2.10</td><td>0.00</td></tr></tbody></table>

Notes: Rs—sphere equivalent radius of hexagonal crystal with the same surface/volume ratio; FT—geometric correction factor for age calculation; eU—effective uranium concentration.

DISCUSSION

Best-fit thermal histories derived from the inverse modeling of samples collected in the foreland show maximum burial conditions at temperatures higher than 110 °C (Figs. 5 and

6A). The present-day average geothermal gradient is estimated at 34 ± 11°C/km for the upper crust in the north Patagonian Cordillera based on regional surface heat flow measurements and thermal conductivities for granitoids (Hamza and Muñoz, 1996; Seipold, 1998; Muñoz, 1999;

Thomson, 2002; Adriasola et al., 2006; Sachse et al., 2016). Nevertheless, the geothermal gradient in the north Patagonian Precordillera and its foreland may have changed over time, especially due to magmatic events, slab window opening, or crustal thinning. Although the magmatic arc
<figure><img src="image_5.png" /></figure>

**Figure 4.** Time-temperature history from QTQt for all samples subdivided according to their locations (Patagonian Precordillera or broken foreland, left and right columns, respectively), age, and lithology are shown. Colored solid lines show the best-fit, time-temperature model of each sample (each diagram includes samples with similar age and lithology). Gray dotted lines show 95% confidence intervals for each group of samples. Boxes show initial constraints: deposition time for sedimentary samples, high-temperature box for emplacement age of intrusive rocks, and age of the unconformity between the top of the basement and overlying formations. Thin dotted lines show boundaries of the Partial Annealing Zone.

migrated through time either toward the eastern foreland or the trench (Fig. 2), thermal modeling does not show any particular thermal event that could be potentially related to the shifts of the magmatic arc. However, within the general framework of progressive cooling since the Late Cretaceous, a significant change in the near-surface geothermal gradient—perhaps at a local scale—cannot be completely ruled out. Thus, given that a precise estimation of paleo-geothermal gradients is not possible, we can only roughly estimate a regional burial of 3–4 km across the foreland, assuming variations around an average geothermal gradient of ~30 °C/km. Such a burial may be easily explained by the thickness of the Jurassic–Lower Cretaceous sedimentary succession deposited in the half-graben systems forming the Cañadón Asfalto Rift Basin (Zafarana and Somoza, 2012; Figari et al., 2015). Significantly higher values of burial, which are not easily explainable by sediment thickness alone, are not likely as they would imply lower geothermal gradients, which is actually not consistent with the Jurassic–Early Cretaceous back-arc setting. The samples from the Patagonian Precordillera were collected close to the thick successions deposited in the Piltriquitrón and Rio Mayo Basins (Cesari, 1977; Ghiorzi, 1979) for which basin subsidence was related to back-arc extension triggered by ongoing slab roll-back and slab steepening (Fig. 6B; Horton, 2018). Also, for these samples, an exhumation of 2.5–4 km may be envisaged.

Regional exhumation recorded by our data is consistent with the unconformity recognized between the Jurassic–Lower Cretaceous succession and the synorogenic Upper Cretaceous deposits (Gianni et al., 2015). This regional exhumation was favored by the strong increase in coupling of the subducting plates and the overriding plate
<figure><img src="image_7.png" /></figure>

**Figure 4.** Time-temperature history from QTQt for all samples subdivided according to their locations (Patagonian Precordillera or broken foreland, left and right columns, respectively), age, and lithology are shown. Colored solid lines show the best-fit, time-temperature model of each sample (each diagram includes samples with similar age and lithology). Gray dotted lines show 95% confidence intervals for each group of samples. Boxes show initial constraints: deposition time for sedimentary samples, high-temperature box for emplacement age of intrusive rocks, and age of the unconformity between the top of the basement and overlying formations. Thin dotted lines show boundaries of the Partial Annealing Zone.

migrated through time either toward the eastern foreland or the trench (Fig. 2), thermal modeling does not show any particular thermal event that could be potentially related to the shifts of the magmatic arc. However, within the general framework of progressive cooling since the Late Cretaceous, a significant change in the near-surface geothermal gradient—perhaps at a local scale—cannot be completely ruled out. Thus, given that a precise estimation of paleo-geothermal gradients is not possible, we can only roughly estimate a regional burial of 3–4 km across the foreland, assuming variations around an average geothermal gradient of ~30 °C/km. Such a burial may be easily explained by the thickness of the Jurassic–Lower Cretaceous sedimentary succession deposited in the half-graben systems forming the Cañadón Asfalto Rift Basin (Zafarana and Somoza, 2012; Figari et al., 2015). Significantly higher values of burial, which are not easily explainable by sediment thickness alone, are not likely as they would imply lower geothermal gradients, which is actually not consistent with the Jurassic–Early Cretaceous back-arc setting. The samples from the Patagonian Precordillera were collected close to the thick successions deposited in the Piltriquitrón and Rio Mayo Basins (Cesari, 1977; Ghiorzi, 1979) for which basin subsidence was related to back-arc extension triggered by ongoing slab roll-back and slab steepening (Fig. 6B; Horton, 2018). Also, for these samples, an exhumation of 2.5–4 km may be envisaged.

Regional exhumation recorded by our data is consistent with the unconformity recognized between the Jurassic–Lower Cretaceous succession and the synorogenic Upper Cretaceous deposits (Gianni et al., 2015). This regional exhumation was favored by the strong increase in coupling of the subducting plates and the overriding plate
<figure><img src="image_6.png" /><figcaption>Figure 5. (A–F) Reconstructed Early Jurassic to Pliocene paleotemperature evolution for the samples collected in northern Patagonia is shown. These diagrams indicate a similar thermal history for the Patagonian Precordillera and the broken foreland during most of the Mesozoic. The samples from the broken foreland reached low temperatures, and therefore near-surface conditions, during the Late Cretaceous. A differentiation between these areas persisted throughout the Cenozoic.</figcaption></figure>

(Horton and Fuentes, 2016), which also resulted in regional uplift (Fig. 6C; e.g., Mescua et al., 2013). Strong coupling is related to slab shallowing induced by the change of absolute motion of the South American Plate, which became west directed (Maloney et al., 2013). Overriding plate kinematics exert a first-order control on slab dip evolution (Cerpa et al., 2018). This is dominant over oceanic subducting plate convergence as demonstrated in the Central Andes, where an increase in deformation occurred during stages of convergence rate decrease (Oncken et al., 2006). Slab shallowing is recognized to produce

significant foreland uplift, as is observed along the present day flat-slab in central Chile (Ramos et al., 2002) and Peru (Bishop et al., 2018). Examples in the geological record include the Late Cretaceous to early Eocene, low-angle subduction in the Laramide orogen, USA (Jordán et al., 1983; Fan and Carrapa, 2014) and Paleocene low-angle subduction in Alaska, USA (Finzel et al., 2011). In northern Patagonia, exhumation of the overriding plate during the Late Cretaceous may have been further enhanced by the subduction of mid-ocean ridges, further triggering shallow subduction and possibly also opening

of a slab window (Aragón et al., 2011). This latter may have added a further component of foreland uplift (Ávila and Dávila, 2020; Guil-laume et al., 2009; Fig. 6D). The integrated uplift and coeval erosion may account for the cooling ages recorded over the whole northern Patagonia region and particularly in the broken foreland. Although a significant spread of cooling ages characterizes the foreland samples (Fig. 2), almost all of them record exhumation during upper plate shortening, around the Late Cretaceous de-formation peak and through the early Paleocene (Fig. 6D). Actually, few single grain ages from
<figure><img src="image_8.png" /><figcaption>Figure 5. (A–F) Reconstructed Early Jurassic to Pliocene paleotemperature evolution for the samples collected in northern Patagonia is shown. These diagrams indicate a similar thermal history for the Patagonian Precordillera and the broken foreland during most of the Mesozoic. The samples from the broken foreland reached low temperatures, and therefore near-surface conditions, during the Late Cretaceous. A differentiation between these areas persisted throughout the Cenozoic.</figcaption></figure>

(Horton and Fuentes, 2016), which also resulted in regional uplift (Fig. 6C; e.g., Mescua et al., 2013). Strong coupling is related to slab shallowing induced by the change of absolute motion of the South American Plate, which became west directed (Maloney et al., 2013). Overriding plate kinematics exert a first-order control on slab dip evolution (Cerpa et al., 2018). This is dominant over oceanic subducting plate convergence as demonstrated in the Central Andes, where an increase in deformation occurred during stages of convergence rate decrease (Oncken et al., 2006). Slab shallowing is recognized to produce

significant foreland uplift, as is observed along the present day flat-slab in central Chile (Ramos et al., 2002) and Peru (Bishop et al., 2018). Examples in the geological record include the Late Cretaceous to early Eocene, low-angle subduction in the Laramide orogen, USA (Jordán et al., 1983; Fan and Carrapa, 2014) and Paleocene low-angle subduction in Alaska, USA (Finzel et al., 2011). In northern Patagonia, exhumation of the overriding plate during the Late Cretaceous may have been further enhanced by the subduction of mid-ocean ridges, further triggering shallow subduction and possibly also opening

of a slab window (Aragón et al., 2011). This latter may have added a further component of foreland uplift (Ávila and Dávila, 2020; Guil-laume et al., 2009; Fig. 6D). The integrated uplift and coeval erosion may account for the cooling ages recorded over the whole northern Patagonia region and particularly in the broken foreland. Although a significant spread of cooling ages characterizes the foreland samples (Fig. 2), almost all of them record exhumation during upper plate shortening, around the Late Cretaceous de-formation peak and through the early Paleocene (Fig. 6D). Actually, few single grain ages from
<figure><img src="image_7.png" /></figure>

**Figure 6.** Cartoons show interpreted geodynamic evolution and migrations of the magmatism revealing slab geometry variations. (A) Regional subsidence linked to pronounced slab rollback and Gondwana rifting during Jurassic-Early Cretaceous times (PB—Piltriquitrón Basin, CAB—Cañadon Asfalto Basin, SB—Subcordilleran Batholith). (B) Late Cretaceous shallowing of the subducting slab and increase of plate coupling (PBF—Patagonian Broken Foreland, NPB—North Patagonian Batholith). (C) Paleogene opening of a slab window, enhancing uplift of the overriding plate (PB—Pilcaniyeu Belt). (D) Arc retreating to the trench between the Oligocene (EMB—El Maitén Belt) and the early Miocene (CMB—coastal magmatic belt) coeval with distal retroarc volcanism (SM—Somuncura plateau) and retroarc quiescence to moderate subsidence from the late Paleogene to the early Miocene. (E) Increased interplate coupling induced by plate reorganization, triggering exhumation of the Patagonian Precordillera (and possibly modest deformation of the broken foreland).

<figure><img src="image_8.png" /></figure>

the Gastre-Navidad area yielded AHe ages spanning through the early Eocene and the Oligocene (Savignano et al., 2016). However, discrepancies among replicates make a reliable evaluation of thermal history not possible.

The lack of late Eocene-early Miocene exhumation both in the foreland and in the mountain range is a clear signal of tectonic quiescence—or even local subsidence—which we relate to plate decoupling due to the onset of slab rollback and re-steepening of the subduction geometry (Fig. 6E). This was induced by a sharp reduction in average trenchward velocity and/or by subduction of the former oceanic segment of the Farallon plate and a related decrease in buoyancy (Aragón et al., 2011; Maloney et al., 2013). Coeval retroarc magmatism was widespread (Pilcaniyeu Belt, El Maitén Belt, and Somuncura Plateau; De Ignacio et al., 2001; Echaurren et al., 2016; Iannelli et al., 2018).

Cooling ages and thermal histories derived from thermal modeling indicate distinct evolution patterns for the Andean chain and the broken foreland during the Neogene. Indeed, middle Miocene-Pliocene exhumation is solely recorded by our data in the Patagonian Precordillera, although the cooling pattern is not homogeneous throughout the area investigated. In fact, a few samples collected from sub-volcanic bodies show a rapid cooling following emplacement at shallow depths during the Cretaceous (Adriasola et al., 2006). Other samples were affected at the

same time by minor heating, which was possibly related to the intrusion of the North Patagonia Batholith. While Late Cretaceous-Paleocene regional shortening was related to flat-slab subduction, Neogene deformation in the Patagonian Precordillera occurred without major changes in the subduction geometry (Echaurren et al., 2019). Plate coupling increase at this time was rather controlled by plate reorganization due to higher convergence rates and a mostly orthogonal subduction induced by the break-up of the Farallon Plate at ca. 23 Ma (Somoza, 1998; Lonsdale, 2005). On the other hand, our data do not record any significant Neogene exhumation in the broken foreland. Although substantial middle Miocene-Pliocene shortening was also suggested in this sector (Bilmes et al., 2013), the lack of any thermochronology signal since the Paleogene confirms that Neogene deformation was minor (Savignano et al., 2016) and unable to produce significant vertical movement. Based on inverse modeling, we propose that the thermal history of the foreland over the last 70 m.y. was characterized by regional cooling that was not affected by any particular thermal and/or important deformation event (with the exception of a few local areas characterized by some heating related to volcanic activity). Available low-temperature thermochronological data from the southern Andean foreland at 47-49°S (Deseado Massif; Fernández et al., 2020) suggest that such a nearly steady-state slow cooling may have characterized the whole Patagonian foreland since the Late Cretaceous.

## CONCLUSIONS

AFT and AHe data, as well as temperature-time history modeling, suggest spatio-temporal variations of exhumation associated with alternating phases of plate-scale regional shortening and tectonic quiescence in Patagonia. Kilometer-scale subsidence during the Jurassic resulted from protracted slab rollback and a rifting phase in the eastern retroarc. Exhumation of the Patagonian Precordillera and the broken foreland related to slab shallowing started during late Early Cretaceous to Paleocene times and was enhanced by the deformation of preexisting grabens located between the Patagonian Andes and the North Patagonian Massif. Thermochronological data in the foreland show minor subsidence from late Eocene to early Miocene times. A marked contrast is observed between the broken foreland samples, which were already at low temperatures during the Paleogene, and the Precordilleran granite samples, which were affected by a late reheating phase that was possibly linked to Miocene arc magmatism. Middle Miocene-Pliocene exhumation is then exclusively recorded in the

<figure><img src="image_9.png" /></figure>
<figure><img src="image_9.png" /></figure>

**Figure 6.** Cartoons show interpreted geodynamic evolution and migrations of the magmatism revealing slab geometry variations. (A) Regional subsidence linked to pronounced slab rollback and Gondwana rifting during Jurassic-Early Cretaceous times (PB—Piltriquitrón Basin, CAB—Cañadon Asfalto Basin, SB—Subcordilleran Batholith). (B) Late Cretaceous shallowing of the subducting slab and increase of plate coupling (PBF—Patagonian Broken Foreland, NPB—North Patagonian Batholith). (C) Paleogene opening of a slab window, enhancing uplift of the overriding plate (PB—Pilcaniyeu Belt). (D) Arc retreating to the trench between the Oligocene (EMB—El Maitén Belt) and the early Miocene (CMB—coastal magmatic belt) coeval with distal retroarc volcanism (SM—Somuncura plateau) and retroarc quiescence to moderate subsidence from the late Paleogene to the early Miocene. (E) Increased interplate coupling induced by plate reorganization, triggering exhumation of the Patagonian Precordillera (and possibly modest deformation of the broken foreland).

the Gastre-Navidad area yielded AHe ages spanning through the early Eocene and the Oligocene (Savignano et al., 2016). However, discrepancies among replicates make a reliable evaluation of thermal history not possible.

The lack of late Eocene-early Miocene exhumation both in the foreland and in the mountain range is a clear signal of tectonic quiescence—or even local subsidence—which we relate to plate decoupling due to the onset of slab rollback and re-steepening of the subduction geometry (Fig. 6E). This was induced by a sharp reduction in average trenchward velocity and/or by subduction of the former oceanic segment of the Farallon plate and a related decrease in buoyancy (Aragón et al., 2011; Maloney et al., 2013). Coeval retroarc magmatism was widespread (Pilcaniyeu Belt, El Maitén Belt, and Somuncura Plateau; De Ignacio et al., 2001; Echaurren et al., 2016; Iannelli et al., 2018).

Cooling ages and thermal histories derived from thermal modeling indicate distinct evolution patterns for the Andean chain and the broken foreland during the Neogene. Indeed, middle Miocene-Pliocene exhumation is solely recorded by our data in the Patagonian Precordillera, although the cooling pattern is not homogeneous throughout the area investigated. In fact, a few samples collected from sub-volcanic bodies show a rapid cooling following emplacement at shallow depths during the Cretaceous (Adriasola et al., 2006). Other samples were affected at the

same time by minor heating, which was possibly related to the intrusion of the North Patagonia Batholith. While Late Cretaceous-Paleocene regional shortening was related to flat-slab subduction, Neogene deformation in the Patagonian Precordillera occurred without major changes in the subduction geometry (Echaurren et al., 2019). Plate coupling increase at this time was rather controlled by plate reorganization due to higher convergence rates and a mostly orthogonal subduction induced by the break-up of the Farallon Plate at ca. 23 Ma (Somoza, 1998; Lonsdale, 2005). On the other hand, our data do not record any significant Neogene exhumation in the broken foreland. Although substantial middle Miocene-Pliocene shortening was also suggested in this sector (Bilmes et al., 2013), the lack of any thermochronology signal since the Paleogene confirms that Neogene deformation was minor (Savignano et al., 2016) and unable to produce significant vertical movement. Based on inverse modeling, we propose that the thermal history of the foreland over the last 70 m.y. was characterized by regional cooling that was not affected by any particular thermal and/or important deformation event (with the exception of a few local areas characterized by some heating related to volcanic activity). Available low-temperature thermochronological data from the southern Andean foreland at 47-49°S (Deseado Massif; Fernández et al., 2020) suggest that such a nearly steady-state slow cooling may have characterized the whole Patagonian foreland since the Late Cretaceous.

## CONCLUSIONS

AFT and AHe data, as well as temperature-time history modeling, suggest spatio-temporal variations of exhumation associated with alternating phases of plate-scale regional shortening and tectonic quiescence in Patagonia. Kilometer-scale subsidence during the Jurassic resulted from protracted slab rollback and a rifting phase in the eastern retroarc. Exhumation of the Patagonian Precordillera and the broken foreland related to slab shallowing started during late Early Cretaceous to Paleocene times and was enhanced by the deformation of preexisting grabens located between the Patagonian Andes and the North Patagonian Massif. Thermochronological data in the foreland show minor subsidence from late Eocene to early Miocene times. A marked contrast is observed between the broken foreland samples, which were already at low temperatures during the Paleogene, and the Precordilleran granite samples, which were affected by a late reheating phase that was possibly linked to Miocene arc magmatism. Middle Miocene-Pliocene exhumation is then exclusively recorded in the
Andean chain, whereas coeval deformation in the broken foreland was rather limited and anyway insufficient to produce noticeable exhumation. Besides allowing us to define the articulated exhumation pattern characterizing the Andean orogen and its foreland between 40°S and 45°S, altogether our results unravel with unprecedented resolution the significant magnitude of Jurassic to Early Cretaceous burial and Late Cretaceous to present unroofing of northern Patagonia.

ACKNOWLEDGMENTS

Funding for this work was provided by the University of Padova (Progetto di Ateneo 2015, CPDA158355) and the Universidad Nacional de Río Negro (PICT-FONCyT 2013-2916, PIP-CONICET 330, U.N. Comahue). S. Boesso, R. Pinna, and L. Bór- dier are thanked for sample preparation and for their assistance during the analyses. The thoughtful and con- structive review by B. Horton and the useful comments of the associate editor are gratefully acknowledged. B. Guillaume is thanked for his useful comments.

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Andean chain, whereas coeval deformation in the broken foreland was rather limited and anyway insufficient to produce noticeable exhumation. Besides allowing us to define the articulated exhumation pattern characterizing the Andean orogen and its foreland between 40°S and 45°S, altogether our results unravel with unprecedented resolution the significant magnitude of Jurassic to Early Cretaceous burial and Late Cretaceous to present unroofing of northern Patagonia.

ACKNOWLEDGMENTS

Funding for this work was provided by the University of Padova (Progetto di Ateneo 2015, CPDA158355) and the Universidad Nacional de Río Negro (PICT-FONCyT 2013-2916, PIP-CONICET 330, U.N. Comahue). S. Boesso, R. Pinna, and L. Bór- dier are thanked for sample preparation and for their assistance during the analyses. The thoughtful and con- structive review by B. Horton and the useful comments of the associate editor are gratefully acknowledged. B. Guillaume is thanked for his useful comments.

REFERENCES CITED

Adriasola, A.C., Thomson, S.N., Brix, M.R., Hervé, F., and Stöckhert, B., 2006, Postmagmatic cooling and late Ce- nozoic denudation of the North Patagonian Batholith in the Los Lagos region of Chile, 41-42 15' S: Inter- national Journal of Earth Sciences, v. 95, p. 504-528, https://doi.org/10.1007/s00531-005-0027-9.

Anselmi, G., Gamba, M.T., and Panza, J.L.A., 2004, Hoja Geológica 4369-IV Los Altares: Servicio Geológico Minero Argentino. Instituto de Geología y Recursos Minerales.

Aragón, E., D'Eramo, F., Castro, A., Pinotti, L., Brunelli, D., Rabbia, O., Rivalenti, G., Varela, R., Spakman, W., and Demartis, M., 2011, Tectono-magmatic re- sponse to major convergence changes in the North Patagonian suprasubduction system; the Paleogene subduction-transcurrent plate margin transition: Tecto- nophysics, v. 509, p. 218-237, https://doi.org/10.1016/j.tecto.2011.06.012.

Aragón, E., Pinotti, L., Fernando, D., Castro, A., Rabbia, O., Coniglio, J., Demartis, M., Hernando, I., Cavarozzi, C.E., and Aguilera, Y.E., 2013, The Farallon-Aluk ridge collision with South America: Implications for the geo- chemical changes of slab window magmas from fore- to back-arc: Geoscience Frontiers, v. 4, p. 377-388, https://doi.org/10.1016/j.gsf.2012.12.004.

Ardolino, A., Lizuin, A., Salani, F., and Pezzuchi, H., 2011, Mapa geológico preliminar de la Hoja 4369-II: Gan Gan, provincia del Chubut. Programa Nacional de Car- tas Geológicas de la República Argentina, escala, v. 1.

Ávila, P., and Dávila, F.M., 2020, Lithospheric thinning and dynamic uplift effects during slab window formation, southern Patagonia (45°-55° S): Journal of Geody- namics, v. 133, no. 101689, https://doi.org/10.1016/j.jog.2019.101689.

Bilmes, A., D'Elia, L., Franzese, J.R., Veiga, G.D., and Hernández, M., 2013, Miocene block uplift and basin formation in the Patagonian foreland: The Gaste Basin, Argentina: Tectonophysics, v. 601, p. 98-111, https://doi.org/10.1016/j.tecto.2013.05.001.

Bishop, B.T., Beck, S.L., Zandt, G., Wagner, L.S., Long, M.D., and Tavaera, H., 2018, Foreland uplift during flat subduction: Insights from the Peruvian Andes and Fitzcarrald Arch: Tectonophysics, v. 731, p. 73-84, https://doi.org/10.1016/j.tecto.2018.03.005.

Burtner, R.L., Nigrini, A., and Donelick, R.A., 1994, Ther- mochronology of Lower Cretaceous source rocks in the Idaho-Wyoming thrust belt: American Association of Petroleum Geologists bulletin: Tectonophysics, v. 78, p. 1613-1636.

Butler, K.L., Horton, B.K., Echaurren, A., Folguera, A., and Fuentes, F., 2020, Cretaceous-Cenozoic growth of the

Patagonian broken foreland basin, Argentina: Chro- nostratigraphic framework and provenance variations during transitions in Andean subduction dynamics: Journal of South American Earth Sciences, v. 97, no. 102242, https://doi.org/10.1016/j.jsames.2019.102242.

Cerpa, N.G., Guillaume, B., and Martinod, J., 2018, The inter- play between overriding plate kinematics, slab dip and tectonics: Geophysical Journal International, v. 215, p. 1789-1802, https://doi.org/10.1093/gji/ggy365.

Cesari, O., 1977, Informe de ensayo inicial del pozo YPF. Ch. PRM. es-1. Sección: Consideraciones estratigráfi- cas: Unpublished report of YPF.

Charrier, R., Pinto, L., and Rodríguez, M.P., 2007, Tec- tonostratigraphic evolution of the Andean Orogen in Chile, in Moreno, T., and Gibbons, W., The Geology of Chile: London, UK, Geological Society, London, p. 21-114.

Cloos, M., 1993, Lithospheric buoyancy and collisional orogenesis: Subduction of oceanic plateaus, conti- nental margins, island arcs, spreading ridges, and seamounts: Geological Society of America Bulletin, v. 105, p. 715-737, https://doi.org/10.1130/0016- 7606(1993)105<0715:LBACOS>2.3.C02.

Cobbold, P.R., and Rossello, E.A., 2003, Aiptan to recent compressional deformation, foothills of the Neuquén Basin, Argentina: Marine and Petroleum Geology, v. 20, p. 429-443, https://doi.org/10.1016/S0264- 8172(03)00077-1.

Coira, B., Kay, S.M., and Viramonte, J., 1993, Up- per Cenozoic magmatic evolution of the Argentine Puna—A model for changing subduction geometry: Inter- national Geology Review, v. 35, p. 677-720, https:// doi.org/10.1080/00206819309465552.

Cucchi, R.J., Busteros, A.G., Lema, H.A., Dalponte, M.R., Espejo, P.M., and Franchi, M., 2001, Hoja Geológica 4169-II Los Menucos: Servicio Geológico Minero Ar- gentino, Instituto de Geología y Recursos Minerales.

Cucchi, R.J., Espejo, P.M., and González, J., 1998, Hoja Geológica 4169-I, Piedra del Agüila, provincias del Neuquén y Río Negro, Programa Nacional de Cartas Geológicas de la República Argentina, scale 1:250,000: SEGMAR (Buenos Aires, Servicio Geológico Minero Argentino. Instituto de Geología y Recursos Minerales) Boletín—Instituto de Estudios de Población y Desar- rollo (Dominican Republic), v. 242, p. 1-74.

Cúneo, R., Ramezani, J., Scasso, R., Pol, D., Escapa, I., Zav- vattieri, A.M., and Bowring, S.A., 2013, High-precision U-Pb geochronology and a new chronostratigraphy for the Cañadón Asfalto Basin, Chubut, central Patagonia: Implications for terrestrial fauna and floral evolution in Jurassic: Gondwana Research, v. 24, p. 1267-1275, https://doi.org/10.1016/j.gr.2013.01.010.

De Ignacio, C., López, I., Oyarzun, R., and Márguez, A., 2001, The northern Patagonia Somuncura plateau ba- salts: A product of slab-induced, shallow asthenospher- ic upwelling?; Terra Nova, v. 13, p. 117-121, https:// doi.org/10.1046/j.1365-3121.2001.00326.x.

Dunkl, I., 2002, TRACKKEY: A Windows program for cal- culation and graphical presentation of fission track data: Computers & Geosciences, v. 28, p. 3-12, https://doi.org/10.1016/S0098-3004(01)00024-3.

Eagles, G., 2007, New angles on South Atlantic opening: Geophysical Journal International, v. 168, p. 353-361, https://doi.org/10.1111/j.1365-246X.2006.03206.x.

Echaurren, A., Folguera, A., Gianni, G., Orts, D., Tas- sara, A., Encinas, A., Giménez, M., and Valencia, V., 2016, Tectonic evolution of the North Patagonian Andes (41°-44°S) through recognition of syntonic strata: Tectonophysics, v. 677, p. 99-114, https://doi.org/10.1016/j.tecto.2016.04.009.

Echaurren, A., Oliveros, V., Folguera, A., Ibarra, F., Creixell, C., and Lucassen, F., 2017, Early Andean tectonomag- matic stages in north Patagonia: Insights from field and geochemical data: Journal of the Geological Society, v. 174, p. 405-421, https://doi.org/10.1144/jgs2016-087.

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