GRAVITY SIGNATURE OF THE WESTERN PARANA BASIN, BRAZIL

Volcanism in the Parana basin, one of the largest continental flood ba salt provinces on Earth, was contemporaneous with the Early Cretaceous opening of the South Atlantic Ocean. Problematic is the lack of crust al extension associated with this flood basalt event, although three P alaeozoic-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 char acterise the poorly understood western portion of the Parana basin, an d to interpret new and existing data in light of its poly-phase extens ional history. The Bouguer map of the Parana basin is characterised by two gravity lows (up to -100 mGal) in the eastern and western portion s with a relative high in its centre (around -50 mGal). The spectral c haracteristics of the N-S-oriented western gravity lows and flanking r egions suggest density contrasts at 7.4-4 km, which we interpret as va riable depth to basement beneath the Palaeozoic-Cretaceous sedimentary and Cretaceous volcanic rocks. Forward modelling (2.5 D) of two profi les 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 basalt s and sedimentary strata. Comparison with other gravity data, heat flo w, contour maps of crustal extension, basement and structural maps all support our interpretation of a pre-Cretaceous rift basin. We also co mpare our results to Palaeozoic-Mesozoic rift basins in Brazil, the No rth Sea and Central Africa with similar gravity signatures and better subsurface control. These comparisons also support a 3-5 km-deep, thic k rift sequence beneath the western Parana basin sequences. (C) 1998 E lsevier Science B.V. All rights reserved.