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Rapid changes of stress-field in the passive continental margin of southeastern Brazil


, : Rapid changes of stress-field in the passive continental margin of southeastern Brazil. Congres Geologique International, Resumes 33

Cenozoic reactivations of NE to E-W-oriented Neoproterozoic shear zones and associated sedimentation in continental rift basins offer a unique opportunity to study the changes of stress-field in the passive continental margin of southeastern Brazil. Inversion of striae from Paleogene-Neogene faults shows different stress regimes which varies with time. Continental rifting occurred during the Eocene-Oligocene with reactivation of NE to E-W-oriented shear zones as normal faults under NNW-SSE-oriented extension, the deposition of alluvial and lacustrine deposits and extrusion of ankaramite lava-flows. After its installation and syntectonic deposition the rift system was subjected to four phases of deformation. Basin inversion initiated in the early Miocene with left-lateral strike-slip and minor thrust reactivation of NE to E-W shear zones, under a general strike-slip regime with NW-SE extension and local NE-SW compression. Structural highs related to transpression caused the inversion and segmentation of the rift basins. Deposition of fluvial sediments in pull-apart basins took place during the transtension or final relaxation of the compressive stress related to this tectonic phase. Late Miocene fluvial meandering deposits as well as late Pleistocene paleosols, colluvium and stone-line deposits record the second phase of deformation, as right-lateral strike-slip and thrust reactivation of NE to E-W-oriented Neoproterozoic shear zones, as result of a NW-SE compression and NE-SW extension, which occurred during the late Pleistocene-Holocene. During the Holocene the stress regime changed to an E-W to WNW-ESE extension locally responsible for the generation of N-S-trending grabens with syntectonic colluvial and alluvial deposition. The last change of stress field is recorded by an E-W direction of compression which generated E-W normal and N-S reverse faults, and widespread families of ENE-NE and WNW-NW hybrid and shear joints affecting colluvial and alluvial deposits younger than 3,410 yr BP. The rapid variation of stress field in this sector of the passive continental margin of southeastern Brazil may be regionally related to the interplay between the ridge-push and slab-pull of South American plate with respect to the Nazca and African plates, respectively, and the influence of intraplate local stress related to the elevated topography of the continental margin, the loading of the thick sedimentary wedges in the adjoining Santos and Campos basins, and perhaps the sea-level rising after the late Pleistocene glaciation. (Financial support by FAPESP, grant 03/08031-0 and CNPq, grant 304649/2005-8).

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