Late Triassic-Early Jurassic extensional tectonics in the Neuquén Basin (Argentina). New insights from stratigraphic and structural analyses of the Chachil depocenter (39°S)

Abstract

The Chachil depocenter is located in the Andean foreland, at ∼39°S. Its origin relates to the syn-rift phase of the Neuquén Basin, a large marine embayment developed along the southwestern margin of Gondwana (30–40°S). In this contribution, we characterize the Late Triassic-Early Jurassic extensional architecture of the Chachil depocenter, based on kinematic data from outcrop-scale normal faults, and a tectonostratigraphic analysis of the syn-rift succession. Our results indicate that the extension and mechanical subsidence were controlled by two major NNW-striking normal faults of opposite polarity, while two NE to ENE-striking faults limited the depocenter to the north and south. Second-order NNW-striking normal faults in the inner sector of the depocenter are related to minor changes in the syn-rift deposits thicknesses. Kinematic data from fourth-order normal faults allow us to characterize the Chachil depocenter as an orthogonal NNW-trending rift developed under ENE extension. These results suggest that the architecture of the rift is related to the reactivation of Paleozoic discrete basement anisotropies. We argue that the local ENE extension in the southwestern area of the Neuquén Basin results from the refraction of the NNE to NE-oriented regional extension obtained for the basin further north. This could be related to the proximity to the N–S southwestern border of Gondwana, where a higher influence of incipient subduction-related processes is expected.