Estudo comparado da arquitetura crustal de dois segmentos de rifte de tipo rico em magmatismo (Bacia de Pelotas) e pobre em magmatismo (Bacia de Almada) na margem leste do Brasil

Abstract

The continental rifted system that initiated in the Late Jurassic in southern Argentina and evolved during the Early Cretaceous in the Brazilian eastern margin heralded the Gondwana break up and the formation of the South Atlantic divergent passive margin. The rifting along the 7,500 Km of the South Atlantic coast was very complex and exhibits important variations in the tectonic styles along the strike. The Pelotas Basin (southern Brazil) and the Almada Basin (northeast Brazil) are endmember examples of the complex processes that formed the south Atlantic continental rift and rifted margins. The Pelotas rift basin is characterized by 1) a wide rift system (~280 Km wide), 2)scarce silicilastic deposits (<1.8 Km thickness), 3) an almost complete absence of evaporates, 4) widespread pre, syn and post-rift volcanisms, and 5) a more symmetric tectonic style of distributed high angle normal faults. A volcano-stratigraphic analysis using high-quality seismic reflection integrated with gravity, magnetics, exploratory boreholes, and published refraction profiles has been done to understand the geological processes that occurred during the rifting and breakup stages. Ten volcanic episodes have been identified and mapped in the extended continental crust of the Pelotas Basin. The magmatic cycle began during the early syn-rift with alkaline/high TiO2 basalts produced at 125 Ma. During the late syn-rift, a series of voluminous tholeiitic/high TiO2 basalts were extruded in the form of SDR wedges. The end of the breakup process was marked by flat-lying tholeiitic/low TiO2 basalts at 118 Ma. The Pelotas Basin constitutes a clear example of a magma rich margin with a magmatic activity displaying temporal and geographical evolution from the continental rifting to the oceanic crust formation. On the contrary, the Almada rift basin is characterized by 1) a narrow rift system (<100 Km wide), 2) an important sedimentary record of pre-rift and syn-rift (up to 3.5 Km thick) deposits, 3) the presence of evaporates, 4) a complete absence of pre and syn-rift volcanism, and 5) an asymmetric tectonic style with the development of major listric fault systems that cross cut the entire crust. At the Almada latitude the breakup of the margin was very asymmetric. The conjugated South Gabon Basin in West Africa is three times larger than the Almada Basin. In the Almada Basin a large and good quality 3D seismic survey allowed the interpretation of the reflection Moho, and distinctive seismic facies interpreted as upper mantle, upper and lower continental crust, and oceanic crust. The integration of deep seismic profiles and 2D gravity forward modeling indicates an important crustal stretching during the rifting phase. The Almada segment can be classified as a magma poor margin with a considerable thinning but without a clear evidence of mantle exhumation. The formation of continental rifts and rifted margins are a fundamental tectonic process mainly governed by the rheological and thermal stage of the lithosphere in extension. To support this study, key published numerical modeling experiments were compiled to explain the different geotectonic processes that operated during the rifting of Pelotas and Almada segments