Volcanism in the Paraná basin, one of the largest continental flood basalt provinces on Earth, was contemporaneous with the Early Cretaceous opening of the South Atlantic Ocean. Problematic is the lack of crustal extension associated with this flood basalt event, although three Palaeozoic–Jurassic extensional episodes have been reported in parts of the basin. Gravity measurements at 185 new stations were established to constrain the geometry and magnitude of Bouguer anomalies that characterise the poorly understood western portion of the Paraná basin, and to interpret new and existing data in light of its poly-phase extensional history. The Bouguer map of the Paraná basin is characterised by two gravity lows (up to −100 mGal) in the eastern and western portions with a relative high in its centre (around −50 mGal). The spectral characteristics of the N–S-oriented western gravity lows and flanking regions suggest density contrasts at 7.4–4 km, which we interpret as variable depth to basement beneath the Palaeozoic–Cretaceous sedimentary and Cretaceous volcanic rocks. Forward modelling (2.5 D) of two profiles across the N–S-trending western gravity low shows that the low can be explained by shallow, steeply deepening structures, bounding what we believe to be a rift basin buried beneath the Mesozoic flood basalts and sedimentary strata. Comparison with other gravity data, heat flow, contour maps of crustal extension, basement and structural maps all support our interpretation of a pre-Cretaceous rift basin. We also compare our results to Palaeozoic–Mesozoic rift basins in Brazil, the North Sea and Central Africa with similar gravity signatures and better subsurface control. These comparisons also support a 3–5 km-deep, thick rift sequence beneath the western Paraná basin sequences.
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