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Digital archive of theses discussed at the University of Pisa

 

Thesis etd-02052024-152642


Thesis type
Tesi di dottorato di ricerca
Author
CARVALHO LANNES GALVÃO FONSECA, JÚLIA
URN
etd-02052024-152642
Thesis title
2D and 3D seismic constraints on the slope morphology and tectonic structure of the Brazilian Equatorial Margin
Academic discipline
GEO/03
Course of study
SCIENZE DELLA TERRA
Supervisors
tutor Prof.ssa Vannucchi, Paola
correlatore Prof. Iacopini, David
correlatore Prof.ssa Vital, Helenice
Keywords
  • Brazilian Equatorial Margin
  • continental ocean transition
  • continental rifting
  • intermediate margin
  • passive margin
  • rift basin
  • rifted margin
  • seismic interpretation
  • slope morphology
  • submarine channels
Graduation session start date
23/02/2024
Availability
Full
Summary
The Brazilian Equatorial Margin (BEM) is classically interpreted as a transform margin formed during the last phases of the Atlantic rifting of Gondwana. However, its tectonic evolution from the rift phase to the final continental break up has not been constrained.
This thesis presents new findings based on the interpretation of 2D and 3D seismic datasets acquired along the BEM. The datasets expand for ~600 km of the margin and consist of approximately 10.000 km of crustal scale 2D seismic reflection profiles which have been calibrated with drillholes. The cross-examination of crustal-scale structures and age of sediment deposits allowed to determine the style and the timing of the different tectonic phases and to calculate crustal thinning across the entire rift system along the Potiguar and East Ceará Basins (NE Brazil).
Our findings indicate that rifting started ~140-136 My. In the shallow basin extension stopped earlier (late Aptian) than in the deep-water (early Albian) domains. As a result, the shallow basin domains presents minor crustal thinning (~35 thick crust over ~100 km wide) than in the deep-water domains where we identified a ~60 km wide area with 4-8 km thick crust extended in the Late Aptian to Early Albian (116-110 My). The distribution of deformation structures supports a model of rift evolution where deformation is initially distributed while forming the shallow basin, it evolves by focusing, and later on it migrates toward the outer part of the basin to form the deep-water domain. Constraints from seismic reflection data and drillholes help define an abrupt continent to ocean transition (COT) where continental crust is abutted by oceanic crust, and breakup occurred during the early Albian. Basin sedimentation from its onset to the late Aptian is continental and terrigenous, indicating an isolated environment disconnected from the Northern and Southern Atlantic oceans. Sedimentation changes during the late-most Aptian to the early Albian when it becomes marine following the rapid ocean infill.
We also document that rifting across the margin is not dominated by transcurrent deformation, with strike-slip faulting limited to a relatively small region, whereas most of the margin extended by normal faulting deformation during opening.
From the interpretation of the 2D seismic reflection grid it was possible to distinguish abrupt changes in the orientation of the slope. These changes defined three distinct, first order segments along the margin: Southern, Central, and Northern. The different evolution of the three segments throughout the rifting process is analyzed through thickness map of the basement. The Northern segment also displays evidence of heterogeneous presence of magmatism, likely formed during the COT emplacement, which defines second order segments. Our interpretation suggests a spatial correlation between first-order tectonic segmentation and second-order magmatic segmentation with prolongations of fracture zone/transform faults offsets. These findings suggest that some transform faults formed on spreading centers may have originated from continental segmentation during rifting.
Our 3D seismic dataset also allowed to produce the bathymetric map of the Potiguar Basin - 4626km2 - and to analyze processes occurring on this passive margin. The modern seabed of the middle and lower slope of the Potiguar Basin is investigated with the objective of identifying zones of instability along the slope and understand whether the processes shaping this slope are purely gravitational-driven or if neotectonism plays a role. The seabed mapping reveals a complex morphology containing a series of submarine channels, landslides, and gullies. Furthermore, a GIS analysis of the slope area provides quantitative parameters about the submarine channels such as their length, sinuosity, steepness, and the presence of knickpoints in their topographic profiles. The seismic interpretation coupled to the seabed analysis shows that gravitational processes are the main responsible for the current seabed shape. No faults or deformation activity are observed in the Neogene units, which suggests that the tectonic activity has not affected the current slope shape and it did not trigger the mapped shallow sedimentary features. Our results indicate that the areas close to channel walls and landslides escarpments are prone to collapse due to their elevated steepness. Considering the intensity of marine infrastructure present and planned in the area, we suggest that in these regions the construction of submarine structures should be avoided.
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