The Syn-Rift of the Neuquén Basin (Precuyano and Lower Cuyano Cycle): Review of Structure, Volcanism, Tectono-Stratigraphy and Depositional Scenarios

The Neuquén Basin is constituted by a system of NW-SE rift clusters formed during the Late Triassic–Early Jurassic at the western margin of Gondwana. The rifting is the result of an extensional tectonic regime triggered by the Gondwana break-up that attained a NE-trending direction (λ1) and a magnitude of extension of around 10%. The syn-rift climax occurred synchronously with an important volcanism, shortly after the beginning of crustal extension. The tectono-stratigraphy of the syn-rift indicates that after a short lapse of crustal extension related to mainly epiclastic sedimentation, the rifting was accompanied by widespread volcanism, characterized by sub-alkaline series, with orogenic signature (i.e. Precuyano Cycle). Recent isotope data revealed a purely crustal origin and mantle-derived rocks with high proportion of crustal participation for the magmatic source. The initiation and cessation of the magmatism, as well as the mechanical extension were diachronic through the basin, showing younger ages from North to South. This would have driven the diachronic post-rift initiations that tracked the rift evolution. The combined effect of the ascending global sea level and the diachronic onset of the sag phase determined that the lower Jurassic marine sedimentation (i.e. Lower Cuyano Cycle) had been set either in a syn-rift or post-rift scenarios, depending on its position in the basin. The understanding (in time and space) of the tectono-magmatic-eustacy factors allowed defining a conceptual scheme with the following endmembers recorded in the syn-rift megasequence: (i) continental non-volcano-dominated syn-rift, (ii) continental volcano-dominated syn-rift; (iii) marine volcano-dominated syn-rift, and (iv) marine non-volcano-dominated syn-rift depositional scenarios. The principal controls on the syn-rift stratigraphy were the mechanical subsidence, driven by normal faulting, and the volcanism, which depending on the type and magnitude determined stacking patterns of the infill. In extreme situation volcanic processes captured the tectonic structures triggering volcano-tectonic subsidence events and graben-calderas. The relative sea-level change (eustacy + tectonics) and the climate conditions acted as secondary factors, mainly during the final rifting stage. Structural, volcanic and tectono-stratigraphical features suggest that the Neuquén Basin at its initial stages corresponded to a wide rift, triggered by lithospheric extension driven by far-field stresses, controlling magmatism in a context of a “passive” rift.