Kinematic Evolution of the Southern Andean Orogenic Arc

Abstract

In the classic book ‘Origin of Continents and Oceans’ written 40 years before the plate tectonics theory was postulated, Alfred Wegener suggested that map-view curvatures in orogenic belts were the result of the motion of continents. In the mid-1950s, Carey coined the term ‘orocline’ to indicate a curved mountain belt formed by secondary bending of an originally straight orogen. Oroclines are genetically different from primary arcs, which are curved mountain belts whose curvature is primary and not due to secondary tectonic processes. A typical example of curved mountain belt is the southernmost segment of the Andean Cordillera, where the regional N–S trend of the Patagonian Andes sharply changes to ESE–WNW in the Fuegian Andes south of 53°S. Given the complex tectonic history of this region and the paucity of geological constraints, the nature, timing of deformation, and kinematics of formation of this orogenic bend have long been debated since its first definition by Carey as ‘Patagonian orocline.’ The dispute revolves around the question whether the southernmost Andes is an orocline or a primary arc. Implications for both options are significant and directly related to the understanding of fundamental tectonic processes operating at plate boundaries. More specifically, unraveling the tectonic evolution of the curved segment of the southernmost Andes is key to understand the geodynamic evolution of this region and the complex interaction between South America, Scotia, and Antarctica plates. Based on paleomagnetic, structural, and magnetic fabric data gathered in the last four decades, the orogenic curvature of the southernmost Andes does not specifically fit in any of the classic definitions of curved mountain belts, as it rather represents a ‘hybrid’ curved belt. While the outer side of the curvature (inner structural domains) seems to represent an orocline (or a progressive arc), the inner part of the curvature (external structural domains) developed as a primary arc throughout the Cenozoic. For this reason, the more generic term of ‘Patagonian Arc’ has recently been proposed as a more suitable name to describe the curved segment of the southernmost Andes. While oroclinal bending of the external part of the arc is generally associated with the closure of the Rocas Verdes marginal basin in Late Cretaceous–Early Paleocene times, the mechanisms of formation of the primary arc represented by the Magallanes fold and thrust belt are unclear. A recent model proposed that slip-partitioning mechanisms along preexisting fault controlled the formation of the Magallanes fold and thrust belt by enabling propagation of ~N-ward and ~ENE-ward contraction in the Fuegian and Southern Patagonian Andes, respectively. Future studies are, however, still needed to better constrain the timing and kinematics of formation of the Patagonian Arc.