Transform zones in the South Atlantic rifted continental margins
Integration of seismic, potential field, and borehole data from conjugate basins along the South Atlantic continental margin, particularly the northeastern Brazilian and northwestern African segments, indicates that the rift architecture is controlled by fracture zones that extend from the oceanic crust and penetrate through the continental crust, locally corresponding to Precambrian structures in cratonic regions. The fracture zones may divide the continental margin into several compartments with independent sedimentary depocentres, separate crustal domains along oceanic transforms, and affect the rift architecture by shearing. Oceanic transform zones may leak igneous rocks originated from the mantle.
This work discusses conjugate sedimentary basins in the South Atlantic salt basins, particularly from Jacuípe to Sergipe-Alagoas on the Brazilian side, and from Gabon to Rio Muni on the African side. The following aspects are emphasized: (1) rift depocentres are controlled by border faults subparallel to the margin and by transverse faults that may continue as transform fractures in the oceanic crust; (2) the southernmost segment of the South Atlantic continental margins is characterized by Early Cretaceous volcanic rocks that underlie continental lacustrine Neocomian to Barremian syn-rift sediments; (3) the pre-rift sequences (Mesozoic and Palaeozoic sediments) that underlie the syn-rift depocentres in Gabon and Sergipe/Alagoas are mainly devoid of volcanics; (4) there is seismic evidence of magmatic underplating in the deeper portions of the continental crust, which are expressed by antiformal features locally aligned with transform fractures; (5) basement-involved extensional faults and volcanic activity along leaking transform faults are imaged along several conjugate segments of the margin, particularly along the equatorial margin (Romanche fracture zone); (6) in some segments of the divergent margin, the transition from outer rift blocks to oceanic crust is characterized by wedges of seaward-dipping reflectors with a possible origin associated with emplacement of oceanic ridges; (7) locally, the outermost rift blocks near the continental-oceanic crust boundary seem to be highly eroded by post-rift uplift caused by transform fault shearing or by magmatic underplating; (8) tectonomagmatic episodes climaxed in the Late Cretaceous/Early Tertiary in northeastern Brazil and extended to the Late Tertiary on the West African margin, forming large volcanic complexes along transverse lineaments that affect both oceanic and continental crust.
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Intraplate Strike-Slip Deformation Belts
Intraplate strike-slip deformation belts are common tectonic features, particularly at convergent plate boundaries, where they are produced by both oblique convergence and continental indentation. These lithosphere-scale structures, which also occur in other geodynamic environments such as passive margins, are characterized by complex structural architectures, by the occurrence of large earthquakes, and by the fast uplift and/or subsidence of localized crustal sectors.
Intraplate strike-slip belts can also control the ascent and emplacement of deeply sourced magmas. In some cases, intraplate strike-slip belts link with oceanic fracture zones and transform faults, transferring transform shear from the ridges to the interior of the plates. This evidence has an important impact of the classical concept of transform faulting.
This volume contains 13 papers from an international field of contributors. Studies of intraplate strike-slip deformation belts from Africa, Antarctica, Eurasia, North America and South America are included.