3D Gravity modeling of impact structures in basaltic formations in brazil: Part ii – Vista alegre, paraná

Abstract

This study characterized the subsurface framework of the Vista Alegre impact structure in terms of a 3D mass density model obtained from forward gravity modeling, constrained by petrophysical and geological data from a previously published work. Like the nearby Vargeão impact structure, Vista Alegre is a complex impact structure formed in basaltic lava flows of the Serra Geral Formation with a central uplift exposing sandstones of Piramboia/Botucatu Formations and impact breccias. A 3D mass density model is proposed, consisting of five subsurface layers: polymictic breccias, shocked/fractured basalts, basalts (Serra Geral), shocked sandstones (Piramboia/Botucatu) and a lower layer of pre-Triassic sedimentary rocks. The central region containing the fractured polymictic breccias and shocked target rocks (basalt and sandstone) is~100 m thick. The target rocks are deformed at depths of up to ~1 km, which represents the basal contact of the sandstones of the Piramboia/Botucatu Formations with Pre-Triassic strata. Values of structural uplift (~650 m), central uplift diameter (~3.5 km) and depth of excavation (~400 m) inferred from our model are fairly consistent with theoretical values, thus supporting a meteoritic impact nature. The Vista Alegre model is in agreement with the density model for the nearby Vargeão impact crater and provides new insights into the formation of impact structures in basaltic targets, with potential implications for the study of other planetary surfaces.