A review on the evolution of the Patagonian orocline in the context of South America-Antarctica interactions from a kinematics perspective

Abstract

The Patagonian Orocline, defi ned as the curvature were the southernmost Andes progressively change their strike from a N-S direction towards a W-E orientation, is one of the most conspicuous features of the entire Andean chain. The kinematic evolution of this present-day geometrical structural and geological feature has been under discussion for almost 100 years, since the early proposals from Alfred Wegener (1929), who suggested a drifting process involving the separation between Antarctica-South America plates and Scotia Sea islands. Carey (1958) named the orogenic bent the “Patagonian Orocline,” a nomenclature that assumes a large counterclockwise rotation of an originally straight mountain chain. The critical position of the bend situated among tectonic plate boundaries and ophiolitic remnants, within a context of superposed strike-slip and compressional modes of deformation, motivates re-evaluation of its evolution considering multiple factors. Nevertheless, resolving the correct origin of the Patagonian Orocline should provide insightful clarity into important tectonic processes, such as the closure of the Rocas Verdes Basin, opening of the Drake Passage and widespread strike-slip plate boundaries in the southernmost Andes.However, the tectonic processes involved in the formation of the curvature are still under discussion, and the alternative to orocline bending, i.e., a primary curvature, remains a viable kinematic explanation. Multiple datasets have been gathered and used to tackle this problem, with most kinematic data derived from paleomagnetism and fault slip data. Estimation of tectonic rotations from characteristic remanent magnetization (ChRM) provides the most direct information used to test the oroclinal bending theory. In this sense, the most recent conducted paleomagnetic studies show no direct correlation between magnitude of tectonic rotations and distance to the theoretical pivot point (Poblete et al. 2014). Notably, tectonic rotations follow a S to N geographical counterclockwise progression pattern in the Fuegian Andes (Rapalini et al. 2016), in which the southern domain (i.e., Patagonian Batholith) underwent the largest rotation (up to 90º), and the basement domain has been rotated around 30º, whereas the external domain of the fault-thrust belt indicates negligible rotation. Other studies based on structural kinematic information, such as shortening directions from fault kinematics and overall deformation mode, indicate that Cenozoic deformation was characterized by crustal scale strike-slip faulting (Diraison et al. 2000), whereas a concave-to-foreland indenter can explain brittle deformation patterns in the fold-thrust belt (Ghiglione and Cristallini 2007). Taken together, these most recent paleomagnetic and structural studies point to a two-step evolution: Late Cretaceous oroclinal bending during Rocas Verdes Basin closure followed by Cenozoic shortening of an orogenic arc (Poblete et al. 2014, Maffi one et al. 2015), similar to an early proposal from Burns et al. (1980).We present a tectonic-kinematic model considering structural mapping of faults and lineaments that takes into account paleomagnetic and fault slip data and earthquake focal mechanisms. Our models emphasize the importance of strike-slip faulting, associated restraining and releasing belts, and the known age of basement highs and pull-apart basins, to present a new model for the evolution of the Patagonian Orocline.