PUBLICATIONS Tectonics RESEARCH ARTICLE Thermochronologic Evidence for Late Eocene 10.1002/2017TC004674 Andean Mountain Building at 30°S This article is a companion to Giambiagi Ana C. Lossada1 , Laura Giambiagi1 , Gregory D. Hoke2 , Paul G. Fitzgerald2, et al. (2017) https://doi.org/10.1002/ 3 3 1 3 4 2017TC004608. Christian Creixell , Ismael Murillo , Diego Mardonez , Ricardo Velásquez , and Julieta Suriano 1IANIGLA, CCT Mendoza, CONICET, Mendoza, Argentina, 2Department of Earth Sciences, Syracuse University, Syracuse, NY, Key Points: USA, 3SERNAGEOMIN, Providencia, Chile, 4IGEBA, Universidad de Buenos Aires, CONICET, Buenos Aires, Argentina • We investigate the exhumation history at the core of the Andes above the flat slab segment by performing Abstract The Andes between 28° and 30°S represent a transition between the Puna-Altiplano Plateau and AFT and (U-Th)/He thermochronology fi • The late Eocene constructional phase the Frontal/Principal Cordillera fold-and-thrust belts to the south. While signi cant early Cenozoic was recognized in the western and deformation documented in the Andean Plateau, deciphering the early episodes of deformation during eastern limits of the High Andes Andean mountain building in the transition area is largely unstudied. Apatite fission track (AFT) and (Frontal Cordillera) at 30°S • We propose a southern continuation (U-Th-Sm)/He (AHe) thermochronology from a vertical and a horizontal transect reveal the exhumation history of the Incaic relief from Puna/Altiplano of the High Andes at 30°S, an area at the heart of this major transition. Interpretation of the age-elevation through the latitude of 30°S profile, combined with inverse thermal modeling, indicates that the onset of rapid cooling was underway by ~35 Ma, followed by a significant decrease in cooling rate at ~30–25 Ma. AFT thermal models also reveal a Supporting Information: second episode of rapid cooling in the early Miocene (~18 Ma) related to rock exhumation to its present • Supporting Information S1 position. Low exhumation between the rapid cooling events allowed for the development of a partial annealing zone. We interpret the observed Eocene rapid exhumation as the product of a previously Correspondence to: A. C. Lossada, unrecognized compressive event in this part of the Andes that reflects a southern extension of Eocene [email protected] orogenesis recognized in the Puna/Altiplano. Renewed early-Miocene exhumation indicates that the late Cenozoic compressional stresses responsible for the main phase of uplift of the South Central Andes also Citation: impacted the core of the range in this transitional sector. The major episode of Eocene exhumation suggests Lossada, A. C., Giambiagi, L., Hoke, G. D., the creation of significant topographic relief in the High Andes earlier than previously thought. Fitzgerald, P. G., Creixell, C., Murillo, I., … Suriano, J. (2017). Thermochronologic evidence for late Eocene Andean mountain building at 30°S. Tectonics, 36. 1. Introduction https://doi.org/10.1002/2017TC004674 The Central Andes are the result of several episodes, or “pulses,” of deformation since the Late Cretaceous. Deciphering the discrete pulses of exhumation and crustal shortening provides crucial constraints on how Received 31 MAY 2017 Accepted 10 OCT 2017 the Andes have evolved in time and space. While Eocene deformation has been widely recognized in the Accepted article online 17 OCT 2017 Puna-Altiplano region, uncertainties remain regarding its southward extent. The Southern Central Andes (~28–36°S) exhibit remarkable along-strike variations in physiography, horizon- tal shortening, crustal thickening, and mean topography (Giambiagi et al., 2012; Isacks, 1988). The mechan- isms invoked to explain the observed trends include variations in subduction dynamics and age and geometry of the slab (Jordan et al., 1983; Ramos, 2010; Ramos et al., 2004; Yáñez & Cembrano, 2004), inherited rheological heterogeneities of the South American plate (Mpodozis & Ramos, 1989; Ramos et al., 2004; Sobolev and Babeyko, 2005), lithospheric strength variations (Oncken et al., 2006; Tassara & Yanez, 2003), and an enhanced climate erosion feedback (Lamb & Davis, 2003), yet it still remains unclear which mechan- ism has the greatest influence. While the Miocene is considered the main phase of Andean orogeny at these latitudes (e.g., Allmendinger et al., 1990; Allmendinger & Judge, 2014; Fosdick, Carrapa, & Ortíz, 2015; Jordan et al., 1983; Suriano et al., 2017), several lines of evidence point toward the importance of Paleogene com- pressive events affecting different sectors of the range. These often overlooked Paleogene events resulted in the initial thickening of the crust and development of high topography, as well as accounting for some of the observed variations in total horizontal shortening. The temporal and spatial growth of the Andes is divided into discrete pulses of shortening, traditionally designated as “tectonic phases,” separated by periods of quiescence or tectonic stability. Major, spatially extensive periods of tectonic activity are well documented during the Cretaceous (the Peruvian phase, ~110–90 Ma; Steinmann, 1929; Jaillard, 1992), the Eocene (the Incaic phase, ~45–35 Ma; Benavides- ©2017. American Geophysical Union. Cáceres, 1999; Charrier et al., 2013; Coira et al., 1982; Hong et al., 2007; Isacks, 1988; Martínez et al., 2016; All Rights Reserved. Moscoso & Mpodozis, 1988; Steinmann, 1929, among others) and the Miocene (the Quechua or LOSSADA ET AL. EOCENE ANDEAN MOUNTAIN BUILDING AT 30°S 1 Tectonics 10.1002/2017TC004674 Pehuenche phase, ~20–10 Ma; Ramos, 1988; Steinmann, 1929; Yrigoyen, 1993). The Eocene compressive phase is widely recognized in the Puna/southern Altiplano, north of 28°S, where it represents the first stage of construction of the Andes (Coutand et al., 2001; del Papa et al., 2004; Elger, Oncken, & Glodny, 2005; Hong et al., 2007; Mpodozis et al., 2005; Oncken et al., 2006; Payrola et al., 2009). In contrast, in the Southern Central Andes, the Neogene is considered the main phase of crustal shortening and tectonic uplift (Giambiagi et al., 2012, 2015; Hoke et al., 2015; Ramos, Cegarra, & Cristallini, 1996; Ramos, Cristallini, & Pérez, 2002), and direct evidence for Eocene deformation is scarce. Evidence of Paleogene deformation in the Andes south of the Altiplano/Puna has been more difficult to constrain due to sparse, poorly dated sedimentary record, and a limited number of thermochronology studies between 28° and 36°S. This lack of evidence leads to the ten- dency to ascribe the growth of the Andes mainly to the Miocene and allows debate to persist regarding the timing of initiation of upper plate shortening in the area. Heterogeneities in crustal thickness and com- position are often proposed as the principal controls on lithospheric strength and hence the total amount of crustal shortening resulting from tectonic stresses. To explore this hypothesis, it is crucial to constrain the spatial and temporal distribution of pre-Miocene orogenic events. Apatite fission track (AFT) thermochronology and apatite (U-Th-Sm)/He (AHe) dating provide a powerful means of identifying and constraining the timing and rate of exhumation of the crust in response to moun- tain building (e.g., Farley, 2002; Fitzgerald et al., 1995; Gallagher, Brown, & Johnson, 1998; Niemi et al., 2013; Reiners & Brandon, 2006). The combination of AFT thermochronology and AHe dating provide complemen- tary but independent constraints on thermal (and hence exhumation) history of the uppermost crust and lead to more robust interpretations than that of a single thermochronometer (e.g., Fitzgerald et al., 2006; Reiners et al., 2003). The vertical profile approach is where samples are collected over significant relief, ideally from ~2 km short-wavelength topography over a short horizontal distance and parallel to structures (e.g., Braun, 2002; Fitzgerald et al., 1995; Huntington et al., 2007). Typically, the vertical profile approach provides more a robust data interpretation than a series of isolated individual samples, although different sampling approaches address different questions. Often the two approaches are paired, and this allows a good control on the spatial distribution of the exhumation. Previous thermochronology studies in the High Andes at 30°S (Cembrano et al., 2003; Rodríguez, 2013) utilized data from individual samples collected at valley level. Our study presents AFT and AHe data and thermal modeling results from a vertical transect collected in the Permian Guanta (Huanta) pluton of the High Andes of central Chile at 30°S. In addition to the Guanta vertical transect, complementary data from a horizontal transect in the vicinity of the vertical profile, and in the Colangüil Range on the eastern slope of the Frontal Cordillera, are presented (Figure 1). We interpret our new data and combine it with existing structural and thermochronology constraints to provide the first strong evidence for a major episode of Eocene exhumation related to crustal shortening in what is today the core of the Andean orogen at 30°S. 2. Tectonic Setting The Andes are a noncollisional compressional orogen formed as the result of crustal shortening since at least the Jurassic, associated with the convergence of the Nazca (Farallones) and the South America plates (Charrier, Pinto, & Rodríguez, 2007; Coira et al., 1982; Mpodozis & Ramos, 1989; Oliveros et al., 2007). At ~30°S the Andes lie within the flat subduction segment (between 28° and 33°S) where the Nazca