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Título

THE ROLE OF TRANSFORM FAULTS, RIDGE JUMPS AND MANTLE UPWELLING IN THE FORMATION OF HYBRID CRUSTS IN THE SOUTH ATLANTIC

Texto do resumo

Major transform faults and associated ridge jumps have long been studied, described and understood as controlling the development of the Brazilian South Atlantic margins. This work aims at performing a further analysis of the crust underlying the Sao Paulo Plateau (SPP) and the Rio Grande Rise (RGR), based on gravity Bouguer and magnetic analysis integrated with recent deep-crustal and 3D seismic data.
Public domain bathymetric data from GEBCO, satellite gravity (Topex-Poseidon) and magnetometry (EMAG2) were used to map the regional structures on SPP and RGR. Density values of 1.3g/cm³; 2.67 g/cm³ and 3.3g/cm³ were applied respectively for the water, crust, and mantle, to calculate the Bouguer anomaly. We used the Oasis montaj software for data analysis and integration and the GM-Sys application for modelling. Integration with deep-crustal 3D seismic data was used to constrain the geophysical models.
The geological interpretation of an E-W section crossing the SPP indicates the presence of crust with peculiar characteristics: an almost unstretched (or thinned) crust, then gradually thinning, changes abruptly to a highly thinned and then to a hyper-stretched crust near the continental to oceanic crust boundary (COB). The highly thinned crust is composed of continental crust covered by extensive volcanic rocks associated with the Early Cretaceous volcanism of the Parana Basin, a large intra-cratonic depression in southern Brazil. At the base of the highly thinned crust a vast area of magmatic underplating can be deduced from interpretations combining potential geophysical data and deep-crustal seismic, indicating the existence of extensive intrusive rocks fed from plutons in the lower crust and upper mantle. All the existing data suggests a vast invasion of the mantle at the external portion of the SPP that was responsible not only for its stretching, but also the crustal warping and topographic anomaly known as the External High of the Santos Basin. Large oil fields are in this elevated region as it served as a catchment for upward migrating hydrocarbons. Similar crustal features can also be interpreted to the east of the SPP on the RGR, where the larger volume of volcanic rocks may be explained by the RGR’s location in an oceanic realm. The integrated interpretation of the whole RGR province, including the results of DSDP wells, supports a large eastern translation of a segment of continental crust along a major fracture zone in association with several ridge jumps. The RGR area was further affected by an Eocene volcanic event, an event like a hot-spot track, which produced a series of volcanos and elevated the area. This volcanic chain, named the Cruzeiro do Sul Lineament, is alkaline in composition (indicating an upper mantle affinity) and can be traced towards the Brazilian continental crust in a SE-NW direction.
Continued geological and geophysical analysis of the São Paulo Plateau and the Rio Grande Rise, suggests the existence of a peculiar crust type: the Hybrid Crust, providing invaluable insights for understanding the dynamics of the Earth’s crust in the western South Atlantic, indicating that Hybrid Crusts may be a far more common phenomenon than previously realized.

Palavras Chave

São Paulo Plateau; Rio Grande Rise; Continent-Ocean Boundary