ASPECTOS TECTÔNICOS FUNDAMENTAIS DO PEOTEROZOICO MÉDIO NA AMAZÔNIA BRASILEIRA

Abstract

The varied volcanic, plutonic and sedimentary rocks of Middle Proterozoic age are related to an extensional thermo-tectonic event which affected ali the Brazilian Amazon region .Thegeometry and cinematics of the structural features and the basin fillings allowed to recognize two major extensional compartments. Several sets of NW-SE to WNW-ESE-trending normal faults with dips towards NE in the Eastem Compartrnent are observed, which formed many half-grabens; some normal faults are inclined to SW and form local symmetric grabens, like that of the Cachimbo. The transfer faults have consistent NE-SW direction, parallel to the extension axis. The K-Ar and Rb-Sr radiometric data point out younger ages from NE to SW, what is seen as due to the propagation of the normal faults by footwall collapse in that sense. The Westem Compartrnent has normal faults sets with E-Wdirection, mostly dipping to N. The transfer faults have N-S trend, parallel to the direction of the extension axis. As the Iithologic units present younging from S to N, one may infer the development ofthe fault system by hanging-wall collapse in that sense. The Eastem and Westem Compartrnents are separatedby the Boa Vista-Caracarai Transfer Zone. The normal faults of both compartments and the Boa Vista-Caracarai Transfer Zone have coincident trends with the direction of earlier basement anisotropies. Such observation supports a reactivation of pre-existent structures as an important factor for the extensional movements and lithospheric thinning. The nature and volume of magmatic rocks associated to the several basins indicate that the lithospheric stretching factor was higher than 2, but lower than 5, and it did not reach the value necessary for generation of oceanic crust. The extensional event inverted to a compressive one at the beggining of the Upper Proterozoic, more noticeable in the Westem Compartrnent. The normal faults were reactivated as upthrusts and the transfer zones as lateral ramps.