The history of brittle deformation along the south Patagonian Andes (46-51° S): an approach from fault-slip data

Abstract

Stress/strain determinations from rocks deformed under brittle conditions may be achieved through systematic field measurements of diverse structures of tectonic deformation (e.g. fractures, faults, folds). Perhaps the most popular and extensively studied tool in structural geology is the quantitative analysis of fault-slip data. One of the main challenges dealing with this methodology is time-constraint the faulting stage since syn-deformational deposits often are poor in contemporary faults, which are better preserved in the country rock because it is lithified (Sperner and Zweilgel, 2010). For such reason, usually, the measurements are taken in older rocks than the age of the faulting event to increase the population of fault-slip data. Despite it turns out a useful and necessary strategy for statistical purposes, it should be noted, successive deformation stages on a rock mass may drove to kinematic heterogeneities (Marret and Allmendinger, 1990). Thus, as older the rock more deformation phases are recorded, increasing the uncertainty when linking a faulting stage with their respective strain regime. In the South Patagonian Andes (46-51° S), either dynamic or kinematic structural analysis has been applied by several authors (Diraison et al. 2000; Lagabrielle et al. 2004; Barberón et al. 2018; this study), reaching wide covertures of 123 strain/stress derived from more than 2000 pairs of fault plane-striae data. Striking features of this database are the ~67% of tensors were sampled from the pre-Cenozoic rocks, and the overall database show ~61% of strike-slip faulting (Fig. 1). At a first glance, it highlights two main problematic issues to be assessed: (i) The predominance of strike-slip faulting (Fig. 1), seems to be at odds with the classic vision of the Andean orogen in southern Patagonia undergoing accordion-like tectonics by alternating extension-contraction tectonic mode (e.g. Ghiglione et al. 2019). (ii) Despite the fault-slip data was employed as a way to address the Cenozoic tectonics, most of the measurements were obtained from the Paleozoic metasedimentary and Jurassic volcanogenic rocks (i.e. pre-Cenozoic rocks) (Fig. 1). Indeed, it does not allow a good constraint for the age of the faulting stage and only provides a minimum age estimation. Therefore, the analysis carried a high uncertainty to infer Cenozoic strain regimes. Since the problem stated above, turns out necessary to revisit the history of brittle faulting along the South Patagonian Andes to clarify some aspects of the deformation as the dominant faulting class, the orientation of main axes, and their time constraints. Our analysis reveals the direction of the σ3/ T/ λ1 axis for the extensional faulting is radial-like, being NNE-SSW, ESE-WSW, and NW-SE main directions. The direction of the σ1/ P/ λ3 axis is almost E-W-oriented for both the contractional and strike-slip faulting. As the further purpose for differentiating into pre-Cenozoic and Cenozoic faulting, we split the database into temporal classes. Carrying out this procedure and just analyzing those data sampled from Cenozoic rocks, arise that 94% belong to the dip-slip faulting mode in the South Patagonian Andes. This fact is further supported by the findings of Cenozoic syntectonic strata, which have evidenced a stage of Oligocene-early Miocene extensional deformation, followed by a renewed pulse of Andean orogenesis in the middle Miocene (Barberón et al. 2018; Folgurera et al. 2018). Thus, the transpressional deformation model for the Andean orogen in Cenozoic times fails for explaining the available data, fitting better a model of tectonic-mode switching between extensional and contractional deformation pulses.