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

Abstract

In active continental margins, mountain building processes have been classically referred to as Andeantype orogenesis (e.g., Uyeda and Kanamori, 1979) wherever the coupling between an oceanic plate subductedbeneath a continent is associated with stress transmission to the upper plate, leading to fold-thrustbelt development and arc magmatism. In these settings, orogenesis is controlled by parameters related toproperties of the upper (continental) and lower (oceanic) plates, such as the upper plate velocity (e.g., Silveret al., 1998; Heuret and Lallemand, 2005; Sobolev and Babeyko, 2005), convergence rates (e.g., Pardo-Casas and Molnar, 1987; Somoza, 1998; Maloney et al., 2013), climate-tectonic feedback (e.g., Lamband Davis, 2003), geometry and subducted bathymetric elements of the oceanic slab (e.g., Gutscher et al.,2000; Folguera and Ramos, 2011), and a variety of processes involving mantle convection dynamics (e.g.,Doglioni et al., 2009; Schellart, 2017). Even though interaction of these processes ultimately defines effectiveinterplate coupling that characterizes the growth and development of a particular orogen, discerning theactive relationship between these factors remains as a challenging task.The Andes are the largest of these cordilleran systems, where variation in crustal shortening and thickeningexpressed through topography and amplitude indicates that orogenesis has not been, either spatiallyor temporarily, in a steady state since its early stages in Jurassic time. On the contrary, even though Andeansegmentation is defined by first-order differences in these features (e.g., Gansser, 1973; Mpodozis andRamos, 1989; Kley et al., 1999; Tassara and Yáñez, 2003), the tectonic parameters behind the alternationof extensional and contractional phases remain unclear. Furthermore, even though this has led to proposalsof a cyclic behavior governing Andean building (e.g., Ramos, 2009; DeCelles et al., 2009, 2015),these approaches describe more adequately the extreme orogenic end members, such as the Altiplano-Punasegments in the Central Andes, rather than providing a holistic characterization of large and segmentedmountain system. The recent proposal of Horton (2018) considers the alternation of tectonic regimes in theCentral and Southern Andes as the result of differential plate coupling, product of distinctive plate convergencerates and slab shallowing episodes, leading to a more integrated perspective of Andean orogenesis.Patagonia, the southern tip of the South American continent from approximately ~39° to ~56°S,shows a southward trend in decreasing topography, width and crustal thickness in the Andes, alongwith a narrowing continental platform to the east (Fig. 1). In the northern segment, ~39?47°S, themain cordillera reaches maximum altitudes of ~2300 m in the active volcanic centers emplaced onits western slope, supported by a ~32-km-thick crust (Tassara and Echaurren, 2012), while in thewestern-central foreland several ~N-S oriented ranges are distributed over an eastward thickeningcrust that reaches ~40 km beneath the cratonic block. The abundant geological record of varioussedimentary, igneous and metamorphic rocks exposed in this area and deformed by an east-vergentfold-thrust belt allows recognition of distinctive tectonic patterns in the Mesozoic and Cenozoicevolution.This chapter is a review of Andean tectonic processes in northern Patagonia, analyzing the mainstructural, petrological, geochronological, and basin-formation events that gave rise to the formation ofa wide and long-lived fold-thrust belt. These parameters will be set in a tectonic framework in order toidentify the principal factors that have governed the Mesozoic and Cenozoic evolution of this area, inan attempt to recognize the roles of the continental upper plate and the different oceanic plates throughoutAndean history. For detailed descriptions of the data presented in this review, the reader is referredto Echaurren et al. (2016, 2017), Fernández Paz et al. (2018), and Folguera et al. (2018).