MSc thesis: Geophysical investigation of the Ceará rise in the brazilian equatorial margin – a continental crust or an oceanic crust?

(2017) Victor do Couto Pereira



The classification of the Brazilian Equatorial Margin concerning rifting, lithosphere breakup and volcanism processes is still controversial. Consequently, the origin and evolution of oceanic plateaus, highs, ridges and rises located on rifted margins such as the Ceará Rise on the Brazilian Equatorial Margin are misunderstood. The studies published over the past 40 years have suggested two geological scenarios for the Ceará Rise: a continental and an oceanic crust. We have interpreted a 2D vertical cross section that extends through the continental area down to the oceanic floor crossing the Ceará Rise by using seismic and gravity data. In this cross section, the main elements are: seawater, sediments, crust and mantle layers, continent-ocean transition (COT), Moho interface and Ceará Rise. We assume that the water, sediments and mantle layers are homogeneous media with known densities. We also assume a lateral density variation within the crustal layer. Aiming at investigating the position of the COT and the crustal density of the Ceará Rise, the geometry of the sedimentary layer is deduced from our interpretation of ultra-deep seismic imaging. We have investigated the Moho depth along this cross section by using Airy isostatic compensation model and seismic interpretation. The 2D gravity disturbance modeling computed by using either the isostatic Moho or the seismic Moho allows investigating the COT and the crustal density of the Ceará Rise. The gravity disturbance modeling from isostatic Moho supports neither the Ceará Rise as a huge oceanic crust accumulation nor an abrupt COT because it produces poor data fitting. However, the isostatic Moho over the "normal" oceanic crust comprehended in the interval from COT to the Ceará Rise yields an acceptable data fitting. Under the hypotheses of continental crust to the Ceará Rise and of an exhumed subcontinental mantle domain, the seismic Moho yields an acceptable data fitting over the Ceará Rise and over the region from the continental area to COT. However, the seismic Moho over the "normal" oceanic crust yields a poor data fitting. We have proposed a hybrid modeling that joins the isostatic and seismic Mohos under the hypothesis of continental crust to the Ceará Rise. In such model, the isostatic Moho is used over the "normal" oceanic crust and the seismic Moho is used over the Ceará Rise and from the continental area to COT. Hence, the hybrid modeling supports the Brazilian Equatorial Margin as a magma-poor rifted margin. Moreover, the hypotheses of the Ceará Rise as an abandoned continental fragment and a COT with mantle exhumation must be accepted because these hypotheses together with a hybrid modeling produce an acceptable fitting of observed gravity disturbance.