RIFTEAMENTOS SUPERPOSTOS E TECTÔNICA DE INVERSÃO NA BORDA SUDESTE DO CRÁTON DO SÃO FRANCISCO

Abstract

This paper presents a stratigraphical and structural study of the transition zone between São FranciscoCraton and Araçuaí Fold Belt in the Minas Gerais state. Two extensional events with rift formation andsedimentation and one orogenic event can be studied.The first extensional event was dated at 1750-1700 Ma. This event is represented by the N-S Espinhaçobasin formation in a continental rift. Its lower part (Early Sequence) is made of acid to intermediatemetavolcanics and fluvial metasediments, with strong fault-block activity in an initial rift phase (mechanicalsubsidence). The Middle Sequence is composed by quartzite with big cross-bedding and the Upper Sequenceconsists of an alternation of quartzite and metapelitic rocks deposited in a shallow marine setting in asubsequent flexural phase (with termal subsidence).The second extensional phase corresponded to Araçuaí rift formation (1000-900 Ma), with maficdikes swarm in the Espinhaço Supergroup. On the highest uplifted continental glaciogenic facies andglacio-marine sequence (Jequitaí Formation) was deposited. To east, in the Araçuaí basin, they grade intomarine deposits with gravitational sedimentation (debris-flow and turbidites) in a typical prism of passivecontinental margin. Westward, on the craton, the Bambuí Group lies directly on the basement. No equivalentshave been identified in the Araçuaí basin either they have not been deposited or are eroded.The orogenic event (650-550 Ma) is caracterized by zones with asymetric folds showing westwardsvergence separated by thin belts of ductile shearing ( Dp phase). N-S Sp cleavage with stretching lineationto east indicates a westward transport of materials toward the São Francisco Craton. The regional Barrowianmetamorphism is associated to Dp, increasing from anchizone to amphibolite facies. During this orogenicevent the São Francisco Craton and the Araçuaí and Paramirim Fold Belts were formed as individualgeotectonics units.