We present a new gravity map between 45°-70° W and 5°-40° S integrating open source terrestrial gravity data of Argentina with the South American Gravity Model 2004 (SAGM04), a 5 min-arc resolution gravity model. The Bouguer anomaly map reveals a 2,000 km long linear gravity feature from 15° S to 30° S at longitude 55° W, with a steep horizontal gradient separating two gravity domains. The eastern domain is the Paraná basin, with NE-SW trending Bouguer anomalies of-80 mGal in average. The western domain comprises the Chaco-Paraná, Chaco-Tarija and Pantanal basins, with circular positive anomalies of up to 20 mGal in amplitude. Previous seismic studies mapped a thinner crust of less than 35 km in the western domain and the present gravity models indicate a 10 to 20 kg/m³ denser crust. On the other hand, the eastern domain has a thicker crust of more than 40 km. Seismic tomography models also show P-and S-wave velocity reduction in the western domain whereas high-velocity characterises the Paraná basin. These geophysical data indicate that the gravity gradient marks a transition between two distinct lithospheres. The gravity gradient is associated with a tectonic feature referred to as the Western Paraná suture/shear zone. Granites of 530 to 570 Ma ages, located parallel or over the gravity gradient, suggest a Neoproterozoic to Early Cambrian age suture/shear zone, thus approximately synchronous and parallel to the Pampean belt. Sediment corrected residual gravity map and its vertical derivative allow us to define the limits of the Rio Apa, Rio de la Plata and Rio Tebicuary cratons. Their eastern and western limits are the Western Paraná suture and the Pampean belt, respectively. This study unravels Precambrian tectonic elements concealed by the Phanerozoic sedimentary basins adding new constraints for the amalgamation history of SW Gondwana. Research Highlights Gravity map reveals the Neoproterozoic 2,000 km long Western Paraná suture/shear zone Geophysical delimitation of the Rio Apa, Rio Tebicuary and Rio de la Plata cratons Gravity anomalies of the Amazonian and Rio Apa cratons are distinct New tectonic features of SW Gondwana final amalgamation revealed Key Words SW Gondwana; intracontinental basins; cratons; sutures; gravity Research Highlights Gravity map reveals the Neoproterozoic 2,000 km long Western Paraná suture/shear zone Geophysical delimitation of the Rio Apa, Rio Tebicuary and Rio de la Plata cratons Gravity anomalies of the Amazonian and Rio Apa cratons are distinct New tectonic features of SW Gondwana final amalgamation revealed
We conducted a magnetotelluric (MT) study from Paleoproterozoic Rio de la Plata Craton, in Uruguay, toward Paleozoic-Mesozoic Paraná Basin, in Brazil. The 850-km-long MT transect comprises 35 evenly spaced broadband electromagnetic soundings sites. In the Paraná Basin, 11 additional long-period measurements were acquired to extend the maximum depth of investigation. All data were inverted using two-and three-dimensional approaches obtaining the electrical resistivity structure from the surface down to 200 km. The Rio de la Plata Craton is >200-km thick and resistive (~2,000 Ωm). Its northern limit is electrically defined by a lithosphere scale lateral transition and lower crust conductive anomalies (1-10 Ωm) interpreted as a Paleoproterozoic suture at the southern edge of Rivera-Taquarembó Block. The latter is characterized by an approximately 100-km thick and moderate resistive (>500 Ωm) upper mantle. The Ibaré shear zone is another suture where an ocean-ocean subduction generated the 120-km thick and resistive (>1,000 Ωm) São Gabriel juvenile arc. Proceeding northward, a 70-to 80-km thick, 150-km wide, and inclined resistive zone is imaged. This zone could be remnant of an oceanic lithosphere or island arcs accreted at the southern border of Paraná Basin. The MT transect terminates within the southern Paraná Basin where a 150-to 200-km-thick less resistive lithosphere (<1,000 Ωm) may indicate refertilization processes during plate subduction and ocean closure in Neoproterozoic-Cambrian time. Our MT data support a tectonic model of NNE-SSW convergence for this segment of SW Gondwanaland.
À minha orientadora, Prof.ª Dr.ª Naomi Ussami, por inicialmente ter me recebido como aluno e, desde então, por toda a orientação, conselhos e conversas que tivemos ao longo do mestrado. Ao Carlos Alberto Moreno Chaves, pelo programa computacional para o cálculo direto do efeito gravitacional usando prismas retangulares e, principalmente, pela disponibilidade e ajudas frequentes que me prestou. Aos professores do departamento de Geofísica do IAG-USP, em especial:
Contents of this paper were reviewed by the Technical Committee of the 16 th International Congress of the Brazilian Geophysical Society and do not necessarily represent any position of the SBGf, its officers or members. Electronic reproduction or storage of any part of this paper for commercial purposes without the written consent of the Brazilian Geophysical Society is prohibited.
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