Print this pageStretching and Thinning of the Upper Lithosphere and Continental-Oceanic Crustal Transition in Southeastern Brazil
Interpretation of recently acquired deep (down to 25 km) PSDM seismic reflection lines in the Santos, Campos and Espírito Santo basins (offshore southeastern Brazil), controlled with gravity and magnetic modeling, allowed for the first time a direct visualization of the extension and thinning modes of the uppermost lithosphere and of the different ways through which continental crust passes into oceanic crust in this segment of the South Atlantic passive margin. The deep seismic data indicate a different picture from earlier works based only on satellite-derived gravity data. Features described in the modern concepts for the evolution of hyper-extended deep rifted margins related to stretching, thinning, exhumation, oceanization and oceanic spreading phases were especially sought out. Although most of such characteristics are present in the studied area they are not present in all of it, and several other features should be incorporated in future revisions of these novel ideas.
Extension seen on those lines is clearly depth-dependent with major thinning taking place in the ductile lower crust. Stretching (brittle deformation via high-angle planar rotational faults) is broadly distributed in the upper crust. Thinning of the continental crust is taken up mostly by the ductile lower crust by decoupling from the upper crust and upper mantle through multiple major decollement zones. Exhumation of the lower crust is indicated in several seismic profiles while mantle exhumation seems to be restricted to some areas. Hence, most of the continental-oceanic transition takes place between lower continental and oceanic crustal settings.
The results obtained indicate that several common assumptions may not be correct. For instance, a mid-crust strong reflection previously interpreted either as the basement or the Moho may correspond to the Conrad Discontinuity (the upper limit of the ductile lower crust). The Moho is rarely a distinct reflection. It is better marked at the base/end of the numerous anastomosing reflections of the ductile lower crust. In the oceanic crust a strong Moho reflection, 12-14km deep, is locally interpreted below landward- and basinward-dipping reflectors. Regardless of syn-rift volcanism, break-up correlated seaward-dipping reflectors and diverse intrusive/extrusive magmatism are often observed.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil