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The 40-km-wide Araguainha structure is the largest and best-exposed complex impact crater in South America. It was excavated in fl at-lying sediments of the intracratonic Paraná Basin, where target rocks are well exposed and have remained undeformed since the impact event ca. 245 Ma. Despite the excellent state of preservation and exposure, information available on the target rock stratigraphy, post-impact erosion, and morphology of the structure is limited. Our combined fi eld observations and remote sensing analysis demonstrate that Araguainha preserves all features of a shallowly eroded peak-ring crater. The interior of the structure exposes a central peak surrounded by a 5-km-wide annular basin and two main ring features 10-12 and 14-18 km from the center. Analysis of the pre-impact stratigraphy, present morphology, and crater dimensions indicates that excavation related to the transient cavity formation was extensive in the annular basin, but minimal to nonexistent beyond the inner ring feature. The formation of the inner ring feature can be reconciled with compressional stresses during outward collapse of the central uplift and inward slumping of the crater walls. Regional stratigraphic data combined with fi eld observations indicate two periods of post-impact erosion associated with exhumation of the Paraná Basin. We estimate that 250-350 m of fallback deposits and target rocks have been removed by erosion. Erosion also accounted for removal of the proximal ejecta immediately outside the rim of the structure. In contrast to previous suggestions that the impact took place in a shallow-marine environment, our observations are more consistent with an extremely shallow, brackish to freshwater lacustrine environment, with little effect on the developing impact crater.

Abstract-The 40 km wide Araguainha structure in central Brazil is a shallowly eroded impact crater that presents unique insights into the final stages of complex crater formation. The dominant structural features preserved at Araguainha relate directly to the centripetal movement of the target rocks during the collapse of the transient cavity. Slumping of the transient cavity walls resulted in inward-verging inclined folds and a km-scale anticline in the outer ring of the structure. The folding stage was followed by radial and concentric faulting, with downward displacement of kilometer-scale blocks around the crater rim. The central uplift records evidence for km-scale upward movement of crystalline basement rocks from the transient cavity floor, and lateral moment of sedimentary target rocks detached from the cavity walls. Much of the structural grain in the central uplift relates to structural stacking of km-scale thrust sheets of sedimentary strata onto the core of crystalline basement rocks. Outward-plunging radial folds indicate tangential oblate shortening of the strata during the imbrication of the thrust sheets. Each individual sheet records an early stage of folding and thickening due to non-coaxial strains, shortly before sheet imbrication. We attribute this folding and thickening phase to the kilometer-scale inward movement of the target strata from the transient cavity walls to the central uplift. The outer parts of the central uplift record additional outward movement of the target rocks, possibly related to the collapse of the central uplift. An inner ring structure at 10-12 km from the crater center marks the extent of the deformation related to the outward movement of the target rocks.

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