Flexural modeling of the neoproterozoic Araguaia belt, central Brazil

Flexural modeling of bending of the southern and southeastern borders of the Amazon lithospheric plate under the western border of the Goiás Massif and western Parnaı́ba basin was constrained by 1070 gravity stations between 5°–14°S and 46°–52.5°W. Topography and aeromagnetic data were also used to estimate the loads of the Araguaia thrust belt. A sequence of Bouguer gravity anomaly lows (−80 to −40 mGal) is located over the Araguaia thrust belt and Cenozoic sediments of the Ilha do Bananal basin. Bouguer anomalies over the Amazon craton, to the west of the thrust belt, are higher than −20 mGal. Towards the east, over the Goiás Massif, the São Francisco craton and the Paleozoic to Mesozoic Parnaı́ba basin, anomalies range from −70 to −20 mGal. Comparison between topography and gravity along profiles perpendicular to the cratonic borders and across the Araguaia thrust belt shows that the long-wavelength gravity anomalies are best explained by bending of the Amazon plate caused by loads such as the observed topography, the thrust-sheets of the Araguaia belt and the remnants of ancient island-arc system in the Goiás massif. The thickness of the Araguaia thrust belt together with the Cenozoic sediments was estimated using aeromagnetic data and it ranges from 6 to 8 km. This load was used to calculate the minimum effective elastic thickness Te for the Amazon plate. Te=80 km was estimated by comparing the observed Bouguer anomalies with the gravity anomalies caused by bending of the crust-mantle interface of a broken elastic plate model. These results support the proposition that the Araguaia belt formed during the collision and suture of the Amazon and the São Francisco lithospheric plates, in late Proterozoic times.