Crustal architecture of South Atlantic volcanic margins

Abstract

Integration of seismic, potential field, and borehole data from the conjugate margins of eastern Brazil and West Africa indicates that the rift architecture varied along strike, and that volcanic episodes (or lack thereof) may substantially affect petroleum exploration of the deep-water provinces. In this chapter we discuss various pairs of conjugate sedimentary basins, from Pelotas and Namibia in the south to the SergipeAlagoas and Rio Muni counterparts in the north. The following aspects are emphasized: (1) rift depocenters are controlled by border faults subparallel to the margin and by transverse faults that may continue as transform fractures in the oceanic crust; (2) in many basins along the southernmost segment of the South Atlantic, Early Cretaceous volcanics underlie continental lacustrine synrift sediments of Neocomian age; (3) in the northern segment of the salt basin, prerift sediments with no volcanic material underlie the synrift sediments; (4) in some segments of the margin, the transition from outer rift blocks to oceanic crust is characterized by wedges of seaward-dipping reflectors with a possible origin associated with the initial phases of oceanic crust emplacement; (5) locally, the outermost rift blocks seem to be highly eroded by postrift uplift caused by shearing or by magmatic underplating; (6) subsequent to the rift sequence, a quiescent period marked by a sag basin above a regional unconformity predated the deposition of Aptian evaporites; (7) the South Atlantic salt basin along both margins was controlled by tectonic and volcanic elements, and locally, salt was deposited directly on the volcanic substratum; (8) volcanic ridges formed before and after salt deposition, and separated portions of the rift and salt basins before the final breakup installed a divergent regime with pure oceanic crust; (9) basement-involved extensional faults, volcanic activity, and enhanced continental margin uplift and denudation are indicative of reactivation of rift-phase faults after salt deposition in some segments of the margin; and (10) tectono-magmatic episodes climaxed in the later Cretaceous-early Tertiary, forming large volcanic complexes along the conjugate margins. Three main episodes of magmatic activity are observed in the South Atlantic salt basins: the Late Jurassic-Early Cretaceous event is related to the Parana-Etendeka flood basalts and the volcanic rocks that occur in the offshore basins; it is followed by thick wedges of volcanic rocks interpreted as seaward-dipping reflectors; and the Late Cretaceous-early Tertiary event is related to hotspot and leaking fracture zones. Fitting together of deep-imaging, multichannel seismic profiles at approximate conjugate margin positions south of the Maceio-Ascension-Kribi Fracture Zone system shows a high degree of asymmetry for the crustal architecture and the rift sequences, suggestive of a simple shear mechanism of lithospheric stretching. Although there is no evidence for synrift volcanism at this segment of the margin, the development of seaward-dipping reflector packages observed in the Brazilian side suggests that magmatic activity in this region may also be asymmetric.