Late Jurassic back-arc extension in the Neuquen Basin (37°S): Insights from structural, sedimentological and provenance analyses

The Middle Jurassic-Early Cretaceous evolution of the Neuquen Basin is traditionally attributed to a long phase of thermal subsidence. However, recent works have challenged this model. In view of this, we study the Late Jurassic Tordillo Formation, a non-marine depositional unit that marks a shift to regional regression across the basin. Previous studies propose different causes for this regression, including the growth of the magmatic arc in the west, uplift in the south or extension in the north. We studied the Tordillo Formation in sections located at an intermediate position in the Neuquen Basin, in order to understand the tectonic processes active during sedimentation. We present evidence of normal faulting within the Tordillo Formation and the base of the overlying Vaca Muerta Formation. Some of these faults can be attributed as syndepositional. We characterize the Tordillo Formation as part of a distal fan-playa lake depositional system with a contemporaneous western magmatic arc as the main source of sediment. When compared to the Late Triassic-Early Jurassic NE to NNE-oriented rifting, which marks the opening of the Neuquen Basin, the Late Jurassic extension shows a switch in stress orientation; the latter is orthogonal to the north-trending subduction zone. We interpret this change as a renewed phase of back-arc extension induced by slab rollback along with minor distributed intraplate extension prior to opening of the South Atlantic Ocean.