Seismic interpretation of post-aptian magmatic events in the Cabo Frio high, Southeast Brazil, its genesis and its relation with pre-salt alignments / Interpretação sísmica dos eventos magmáticos pós-aptianos no alto de Cabo Frio, sudeste do Brasil, gênese e relação com os lineamentos pré-sal

Sérgio Goulart Oreiro

The continental shelf of the Cabo Frio Area, at the southern part of the Campos Basin, shows unique features in its tectono-sedimentary evolution when compared to other areas of the Campos and Santos basins. The presence of a regular pattern of antithetic faults in the basement and in the syn-rift and the post-rift sedimentary sections, along with important magmatic events in the Late Cretaceous and Early Tertiary sequences (whose climax took place in the earli er portion of the Middle Eocene) are some of these features. This work presents a historical perspective of the human perception of the magmatic events, from the ancient mythic and theologic theories to the current debate between plumers and non-plumers. These informations are needed to allow a better understanding of the conclusions of the present thesis. The present study was based on the analyses of seismic, satellite, magnetic, gravimetric and well data, as well as rock samples collected in dikes cropping out at the Arraial do Cabo Peninsula, the nearest onshore position at the boundary between the offshore Santos and Campos Basins. This thesis has confirmed the results of previous research that had identified two major magmatic pulses, one in the late Cretaceous and the other in the Early Tertiary (~83 and 50 My, respectively). There is also seismic evidence for volcanic edifices formed during the Albian, Maastrichtian and Paleocene times. The analyses of seismic reflection and well log data, in the working area, points to a set of diagnostic features that can lead to the identification of magmatic events and the distinction of intrusive from extrusive rocks, as well as their intercalations with epiclastic sedimentary sequences. Extrusive magmatic events can be recognized in seismic reflection data as chaotic seismofacies with well defined tops and poorly defined bases; these seismofacies are interpreted as a result of interbedding submarine lava flows and epiclastic and volcaniclastic sediments, along with slumps and shallow intrusions (sills). The presence of well shaped conic edifices also points to extrusive magmatism. Dikes and lava flows are recognized by strong positive reflections with abrupt lateral ends. Thicker dolerite sills may be recognized by re flection free zones with strong reflections in their tops and bases. The criteria proposed by this thesis may be used to identify and classify magmatic events in any context where they are associated with sedimentary sequences. The thesis also shows the seismic signatures of nonmagmatic events, which comprise a different range of geologic features that may induce the geophysicists and geologists to interpret them as magmatic events. Among these features, evaporites, turbiditic mounds, mud volcanoes originated by gas escape, slump seismofacies and carbonate deposits are the most important ones. Some alternative interpretations to explain the genesis of this magmatism are presented, which do not take into account the classic model of mantle plumes. The mapping of the main feeder dikes, in 2D and 3D reflection seismic sections, has revealed that they have a predominant SE-NW direction, coincident with the alignment of the Cruzeiro do Sul Deformation Zone, which includes the Jean Charcot submarine mounds and the volcanic mounds on and around the Cabo Frio High. This observation is coherent with the characteristics of the preexisting and reactivated strike-slip faults on land and the orientation of their transtensional component. Moreover, it is shown that the largest volumes of magmatic rocks, intercalated in the sedimentary section, are located at areas of intersections between fault zones trending SE-NW (strike -slip faults) and SW-NE (normal faults), as are the onshore alkaline bodies of the Poços de Caldas Cabo Frio alignment.

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