Tectonic Control of the Campos-Santos Basins, Offshore Brazil -- A Discussion
By
F. R. M. Pires1, L. A. M. da Costa2
(1) UFRJ - Federal University of Rio de Janeiro, N/A, Brazil (2) CPRM - Companhia de Pesquisa de Recursos Minerais, N/A, Brazil
Several tectonic events were responsible for generations of the oil fields
in the Campos-Santos Basins, offshore Brazil. Distensive processes probably
started as early as the end of Paleozoic, beginning of Triassic. However,
tectonic activity was recorded approximately 130Ma and developed up to nowadays.
The brittle regime
which prevailed during those events produced ENE-WSW
prominent fracture system, which aligned the coastal flat-lying gneisses at Rio
de Janeiro. Diabase and phonolite dikes and plugs were emplaced along some of
the fractures. Sills occur in the proximities of the joints. Thermal output
generated by the magmatic activity was favorable for hydrocarbon formation
and/or migration. These system imparted characteristic magnetic anomalies
detected by the aeromagnetometric surveys. Almost orthogonally disposed to the
fracture system there is another set of fractures, which produced a basculated
type of structure, by a rotation along one of the
fault
lines. This structure
has been designaled as resequent tilt-blocks. From onshore, there are sets of
splays diverging from the Paraiba do Sul Megashear Zone and related structures.
Reactivation of the splays contributed significantly to the opening of the
basins and sub-basins during the Cretaceous. A graben-horst system was
developed, and several troughs filled by the high sedimentation rates dominated
by turbidites. Salt diapirism, probably triggered by tectono-magmatic activities
and/or gravitational differences, initially developed in depocenters, have
facilitated the formation of the oil traps.