Tectonic and geometric assessment of inversion systems in the Huincul High, Neuquén Basin (Argentina) – the role of structural inheritance and mechanical stratigraphy

The Neuquén Basin is a major Mesozoic sedimentary depocenter located in the foreland of the Andes Mountains in Argentina. The basin hosts world renown inversion systems that have been the target of georesource exploration for the last three decades. The Huincul High is a structurally and economically prominent ~270km long, E-W trending feature that formed by the accretion of exotic Paleozoic terranes influencing subsequent Mesozoic deformation in the basin. Exploration in Huincul High has been mainly focused on the shallow part of the inversion structures leaving a limited understanding of  deep structural architecture and early tectonic evolution. With this research, for the first time a set of 4 3D seismic reflection surveys covering an area of 1400km2 have been analysed and integrated with stratigraphic information from 15 exploratory wells to provide new insights into the tectonostratigraphic and kinematic evolution of the western reaches of the Huincul High.

Detailed horizon and fault interpretation revealed Late Triassic, isolated, 10-50km long NE-SW to NW-SE trending half grabens. These extensional systems are attributed to the Late Triassic cessation of the Andean subduction to the west and intraplate extension regime ensuing. Thickness map of the Lower Jurassic Los Molles unit shows the development of an extensive ~50km  long ~15km wide NE-SW depocentre at that time. It is proposed that Andean subduction was renewed at that time, moving the Neuquén Basin into a backarc environment with hotter, weaker continental lithosphere thinned by mantle underflow which might have caused ductile flexural sag and minimal brittle faulting.

Prominent NE-SW cylindrical inversion anticlines ~17km across and well-developed harpoon structures are observed in the hangingwall of reactivated  ~50km long, NE-SW trending, extensional faults. Growth strata analysis shows thinning of Middle to Lower Cretaceous strata over the crest of these folds suggesting a phase of  NW-SE compression at this time. This compressional phase is attributed to the increase in Andean subduction rate and shallowing of the subduction dip, as the Neuquén Basin is moved into a foreland setting. Fault displacement analysis suggests that the reactivated faults were formed as separate fault segments at the time of extension in the Late Triassic. Additionally, analysis indicates that faults segments with increased reactivation show prominent hangingwall inversion anticlines.

Dip-steered coherency extractions along the Early Cretaceous Vaca Muerta Formation showed en echelon NW-SE transtensional faults occurring directly above Late Triassic non inverted faults; decoupled by the underlying shaly and mechanically weak Los Molles unit. These observations point to a post-inversion tectonic event that might coincide with reconfiguration of subducted plates changing the principal stress orientation and causing strike slip reactivation.

These results highlight the importance of  structural inheritance of a pre-existing  fault architecture in the development of  consequent inversion, and how mechanically weak units can inhibit fault propagation during the later compressional events, acting as a decoupling layer.  A detailed evolutionary model is proposed for the western reaches of the Huincul High which envisages crustal weakening and thermal sag to explain the thickening of the Early Jurassic strata previous to the main Cretaceous inversion.