Evolução geotectônica da elevação do Rio Grande com base em dados gravimétricos e magnéticos

Abstract

The Rio Grande Rise (RGR) is located on the South American plate and occupies an immense area of the western portion of the southern segment of the South Atlantic Ocean, representing, along with the Walvis Chain in the African plate, a typical example of magmatic events associated with interaction of a hotspot with the mesoceanic ridge. The questions about the RGR have been the focus of many researches, however, the information regarding their formation and internal structuring are still very limited. In this sense, this research proposed the use of geophysical data of global models (bathymetric, gravimetric and magnetic) for a wide scale investigation of this submarine elevation and for a spatio-temporal reconstruction of elevation in the midst of the formation of the ocean floor and the interaction of the Dorsal mesoceanic with the plume Tristan da Cunha - Gough. The use of enhancement techniques on the geophysical data allowed some analysis on the complex tectonic evolution of the RGR. The simplified model of geotectonic evolution showed that for the elevation, formed between 100 and 55 Ma, the main conditioning factors for the morphological difference of its segments were the rearrangement of the tectonic plates, during the greater change of the poles of rotation of the same ones (≈84 Ma), the instability of the accretion system and the magmatic contribution of the hot spot Tristan da Cunha. These factors, acting together, caused some segments of the mesoceanic ridge to rearrange to near the hot spot and change their main orientation SSW-NNE to N-S. Finally, based on the distribution of the magnetic lineaments observed in the Analytical Signal Inclination (ASI) and Horizontal Gradient Amplitude (HGA) maps, a more accurate mapping of the Ocean Fracture Zones (OFZs), which cut the entire RGR, evidenced that numerous inflections along the Cruzeiro do Sul Rift, observed in the gravimetric and bathymetric data, coincide with the positioning of these OFZs. These inflections may represent zones of preexisting transformer faults that were reactivated during rift formation (Paleogene-Neogene), in response to the combination of transcurrent dextral forces on the OFZs and a dextral transcurrent component in the transtensive stresses that formed the Cruzeiro do Sul Rift. This rift was divided into two large NW-SE orientation segments, which aligned with the Cox and Meteor fractures zones. These two large segments were further subdivided into smaller compartments, visibly rotated counterclockwise. In addition, the origin of the Jean Charcot seamounts chain in its southern segment is possibly associated with tectonic reactivations in areas of intersection between OFZs and old aborted spreading centers with NE-SW orientation.