CONICET | Buscador de Institutos y Recursos Humanos

ABSTRACT The study area is located in the northern sector of the Neuquén basin, one of the most important oil and gas bearing basins of southern South America. Our study was focused on the Atuel depocenter, which sedimentary infill records the first stages of the Neuquén basin opening during Late Triassic to Early Jurassic times in response to regional extension processes. In Neogene times the Andean orogeny deformed and uplifted the Atuel depocenter infill. As a result, well exposed sections are now cropping out in the northern sector of the Malargüe fold and thrust belt. The study area is located in the northern sector of the Neuquén basin, one of the most important oil and gas bearing basins of southern South America. Our study was focused on the Atuel depocenter, which sedimentary infill records the first stages of the Neuquén basin opening during Late Triassic to Early Jurassic times in response to regional extension processes. In Neogene times the Andean orogeny deformed and uplifted the Atuel depocenter infill. As a result, well exposed sections are now cropping out in the northern sector of the Malargüe fold and thrust belt. The study area is located in the northern sector of the Neuquén basin, one of the most important oil and gas bearing basins of southern South America. Our study was focused on the Atuel depocenter, which sedimentary infill records the first stages of the Neuquén basin opening during Late Triassic to Early Jurassic times in response to regional extension processes. In Neogene times the Andean orogeny deformed and uplifted the Atuel depocenter infill. As a result, well exposed sections are now cropping out in the northern sector of the Malargüe fold and thrust belt. Our main goal was to confirm or discard the hypothesis of an oblique rifting mechanism for the opening of the Atuel depocenter. This was previously proposed on the basis of a bimodal orientation of the main normal faults. We put special emphasis on the kinematic evolution of the depocenter by integrating the analysis of small-scale fault-slip data with timing and orientation of the main normal faults. Our study was mainly based on field data obtained from detailed geological and structural mapping. We carefully mapped the Upper Triassic to Lower Jurassic units that constitute the synrift infill of the depocenter. We identified syntectonic elements related to the extensional deformation: normal faults of a diverse range of scales, roll-over folds, forced folds (hanging-wall synclines), growth strata and angular or progressive unconformities. We recognized more than 300 medium to small scale normal faults with displacements ranging from few centimeters to several meters, widely distributed in a number of localities within the study area. After restoring the data to their original position (pre-Andean folding) we processed the normal faults and their kinematic indicators in order to obtain the orientation of the main axes of the strain ellipsoid for each location. We compared the obtained values and evaluated possible regional, local and temporal changes. Finally, we integrated the results of the strain analysis with data from the main normal faults. From the kinematic analysis of small-scale fault-slip data we have found a mean NE stretching direction (Az 045º) for the synrift phase of the Atuel depocenter during the Late Triassic to Early Jurassic. This stretching direction is oblique to the general NNW orientation of the depocenter, which in turn is coincident with the trend of the main normal faults (Az 170º). The orientation of the principal faults, which developed early in the history of the subbasin, is similar to the Upper Paleozoic basement fabric, suggesting that they formed as the result of normal or oblique-normal slip reactivation of previous structures. Our results seem to favor the hypothesis that the Atuel depocenter developed as an oblique rift, with a mean NE stretching direction oblique to the NNW general trend of the sub-basin (α ≈ 55º, being α the angle between the rift trend and the extension direction).