Numerical modeling and 3D-gravity inversion of the Vargeão impact structure formed in a mixed basalt/sandstone target, Paraná Basin, Brazil
Terrestrial impact structures formed in basaltic targets are suitable analogues for analyzing cratering processes in other planetary bodies where basalt is abundant. However, such structures are remarkably scarce on Earth. The Vargeão impact structure in the Santa Catarina state, Brazil, is one of the rare cases of such structures. It was formed in the Paraná-Etendeka Large Igneous Province, in the region that currently corresponds to the Paraná Basin, in southern Brazil. Local stratigraphy comprises volcanic rocks of the Jurassic-Cretaceous Serra Geral Formation, mostly basalts intermixed with rhyodacites, lying on top of sandstones of the Piramboia (Triassic) and Botucatu (Jurassic) formations. We analyzed the dynamics of crater formation through numerical simulations, focusing on the resistance to deformation under high pressure of a very tick basalt (>1 km) layer overlaying clastic sedimentary sequences. We then compared the crater morphology obtained from the model with the results of a 3D-gravity inversion based on the Preconditioned Conjugate Gradient (PCG) algorithm. We used this method to estimate the depth of the sandstones underneath the basaltic layer, and to better understand the complex structural relationship between both lithologies, especially in the central uplift area. Our results showed that the Vargeão crater was likely formed in ca. 130 sec., by a projectile of approximately 1,400 m in diameter, which produced a transient crater of ~14 km diameter, with a subdued topographic high at the center. We also point out that the negative Bouguer anomaly observed at the center was a consequence of the structural uplift of the sandstones by ~500 m, together with pre-Triassic sedimentary sequences underneath the sandstones. Both models support the formation of a final crater with a relatively subtle central peak.