Fernando Mancini

Structural styles of the intracratonic reactivation of the Perimbó fault zone, Paraná basin, Brazil

Abstract The style and origin of intracratonic deformation along the Perimbo fault zone (PFZ) in the Parana basin, Santa Catarina State, southern Brazil, is defined by the integration of outcrop, borehole, aerial photography, and digital terrain modeling data. Typical structures are high-angle strike-slip and oblique-slip normal faults in the Permian sedimentary cover that propagate upward from medium-angle reverse faults in the underlying Precambrian basement. Regional and minor structures suggest blind transtensional faulting and tilting of the overlying strata controlled by the basement heritage. A hypothesis linking deformation between the cover and the basement is proposed on the basis of a structural analysis of a branched fault pattern striking N40–50E and N70–80E. Semi-detailed scale mapping shows that the PFZ has a complex history of polyphase reactivation and is characterized as a plate margin fault in the Proterozoic, evolving to an intracratonic fault in the Phanerozoic, with a main period of reactivation in Permian or Permian–Triassic transition times. The reported data imply that fault reactivation is characterized by normal to left-lateral strike-slip faulting produced by strain propagation from the La Ventana orogenic belt toward the continental interior.

Protracted deformation during cooling of the Paleoproterozoic arc system as constrained by 40Ar/39Ar ages of muscovite from brittle faults: the Transamazonan Bacajá Terrane, Brazil

In the Paleoproterozoic Transamazonas Province, synkinematic granitogenesis has taken place synchronously with compressive tectonic stress. The synkinematic character of the granites is marked by their WNW elongate shape, and by the presence of pervasive and concordant synmagmatic foliation. Ductile shear zones are concordant to the previous regional WNW structures, and tend to be accommodated along contacts between Rhyacian synkinematic granitoids and both Archean orthogneisses and Siderian metabasites. Locally phyllonitic shear zones and brittle-ductile shear zones with cataclasites are oriented subparallel to the preexisting ductile foliation. Late orogenic brittle faults N30E-trending strike-slip faults are either sinistral or destral. 40Ar/39Ar dating of muscovite developed on fault planes gave ages of 1977 ± 8 Ma and 1968 ± 11 Ma. Structural and geochronological data from rocks of the Transamazonas Province permit to conclude that most mylonites and brittle structures were controlled by preexisting structures such as geological contacts and petrographic facies boundaries. Compressive tectonic stress would have initiated at ca. 2100 Ma, since the former magmatic arc (Bacajai complex), still present at 2070 Ma when syntectonic granites were emplaced and remained until 1975 Ma after granite plutonism and regional cooling.

Deformação rúptil em rochas do magmatismo Penatecaua no domo de Monte Alegre (PA)

O domo de Monte Alegre se destaca em meio a planicie do rio Amazonas, em sua margem esquerda, como uma notavel elevacao topografica situada aproximadamente a 10 km a noroeste da cidade homonima, no estado do Para. A partir de sua descoberta, muitas tem sido as hipoteses para a genese do domo, entretanto, somente em 1975, Montalvao e Oliveira associaram a geometria domica as intrusoes toleiticas basicas, que moldaram a estrutura braquianticlinal em forma de elipsoide, com 30 km de comprimento e 20 km de largura. Por meio de pesquisas da morfoestrutura do domo, das camadas sedimentares basculadas e, em particular, dos sistemas de fraturas em rochas intrusivas geradas pelo magmatismo Penatecaua, foi possivel verificar que a atual estrutura braquianticlinal do domo tem origem deformacional devido a pelo menos duas fases de deformacao (F1 e F2) Jurocretacea, que geraram, consequentemente, dois sistemas de cisalhamento ruptil, podendo ser localmente ductil quando a reologia e propicia. Tais fases de deformacao foram correlacionadas, por meio da analise da paleotensao, com regimes tectonicos regionais, como o Diastrofismo Jurua, confirmando-se que tiveram influencia decisiva na deformacao finita do domo.