Banca de DEFESA: ADRIANE GOMES PINHEIRO PRAXEDES

Uma banca de DEFESA de DOUTORADO foi cadastrada pelo programa.
STUDENT : ADRIANE GOMES PINHEIRO PRAXEDES
DATE: 30/01/2020
TIME: 14:00
LOCAL: PPGG
TITLE:

Geophysical / geological study of the Rio Grande Elevation and adjacent underwater features - South Atlantic

KEY WORDS:

Rio Grande Rise; Magmatism; Extensional tectonics

PAGES: 90
BIG AREA: Ciências Exatas e da Terra
AREA: Geociências
SUMMARY:

This dissertation presents the results of the application of the Seismic Reflection Method together with data from some boreholes in a aseismic ridge in the South Atlantic Ocean called Rio Grande Rise, which is about 1500 km from Rio de Janeiro, in international waters. There are approximately 100 Ma volcanic sills in the oceanic crust, caused by the Tristan - Gough hot spot that would have caused this Elevation. After that, around 97-90 Ma the Meso-Atlantic Ridge would have broken the oceanic crust and the Rise would have become part of the South American plate. Then a phase of pelagic sedimentation begins in shallow water environment on the raised crust by volcanic spill. In the Upper Cretaceous, intense rifting affected the Rise resulting from extensional plate settling forces and the opening movement of the South Atlantic Ocean. In the Middle to Upper Eocene, sedimentary fill of significant thickness continued be deposited. Also in the Eocene expressive magmatism due to pressure relief, caused by the extensional tectonics, occurred in the Rise and forming several guyots and exposing part of the Rise above sea level. When the magmatism ceased between the Eocene and Oligocene, the Rise was cooled, subsidized, eroded, and submerged.

The objective of this dissertation was to reprocess vintage seismic lines in order to obtain better images and new information of tectono-depositional sequences in the Rio Grande Rise. The seismic data have been correlated with a DSDP borehole to obtain seismic stratigraphic interpretations and a sequence of evolutionary stages, based on a priori knowledge, and thus provide more complete explanations of the tectonic, volcanic and sedimentary evolution of the Rise. It also aimed to find correlation points between the Rise and the adjacent continental margin. Four seismic lines were reprocessed using Promax® software. Then, the stratigraphic units described in the borehole close to the lines were correlated. And so it was generated seismic interpretations and evolutionary model of the Rio Grande Rise. Interpretations of three seismic lines located in the vicinity of the Rise were also made.

Our results show that the uplift of the central portion of the Rio Grande Rise was responsible for the appearance of inclined reflectors within the basement, on both sides of the Rise. The interpretation of the sedimentary package shows that the causative extensional pulse of the Cruzeiro do Sul Rift would have been active from the Maastrichtian to the Middle / Upper Eocene. Another rift on the eastern slope of the central portion of the Rise is interpreted, recent reactivation being noted. Faults ranging from acoustic basement to the most superficial units of the sedimentary package plus aborted rift are evidence that this extensional tectonic remains active from the Late Cretaceous to the Recent. Still regarding tectonics, the effects of strike-slip movements along the Porto Alegre Fracture Zone were also observed in the sedimentary package. This fracture zone was imaged in seismic between two guyots. This sinistral strike-slip movement would have displaced the axis of the Cruzeiro do Sul Rift about 90 km eastwards in the Upper Cretaceous. We have also postulated, from volcanic breaches and ash identified in some boreholes, a thermal anomaly of great proportion due to the magmatism that raised the central portion of the Rise. We also observed improved imaging of mounds within the sedimentary package, which are possibly associated with intrusive Eocene volcanism or hydrothermal activity at the base of the sedimentary package and / or gas chimneys due to warming associated with the magmatism. These latter hypotheses are corroborated by depressions of about 500 m in diameter on the ocean floor and polygonal faults that may be related to the presence of fluids and pockmarks. Finally, similarities between seismic facies and extensional tectonic events in the Upper Cretaceous and Eocene volcanism point to a correlated evolution between the Rio Grande Rise and the adjacent continental margin.

This dissertation represents new knowledge about the Rio Grande Rise and its surroundings, which may be useful for future scientific purposes and exploration of mineral resources.

BANKING MEMBERS:
Presidente - 1315614 - DAVID LOPES DE CASTRO
Externo à Instituição - ELLEN DE NAZARÉ SOUZA GOMES - UFPA
Interna - 2218779 - HELENICE VITAL
Externo à Instituição - João Carlos Ribeiro Cruz - UFPA
Interno - 2042405 - MOAB PRAXEDES GOMES