Effects of initial rift inversion over fold-and-thrust development in a cratonic far-foreland setting

Abstract

The effects of initial rift inversion over the geometry of a far-foreland intracratonic fold-and-thrust belt are investigated. A total of 25 seismic reflection lines and stratigraphic data from 9 wells are used in association to digital elevation and field geological data, to investigate the geometric relationship between shallow thrust development and initial positive inversion of buried rift faults in a cratonic environment, where inversion tectonics is not common. The study case is conducted in the meeting region of the opposed-verging Brasília and Araçuaí foreland fold-and-thrust Neoproterozoic belts, located in central Brazil, formed in the first stages of West Gondwana amalgamation. Seismic reflection sections are used to map buried paleorift faults and trust ramps. The best 5 sections are also interpreted in detail in terms of fold and thrust geometry and their relationship to the inverted rifts is analyzed. They show high correlation between shallow fold-and-thrust geometry and the shortening of synrift strata, caused by rift inversion. Results indicate that initial positive rift fault reactivation exerts a determinant control over the locus of thrust ramp nucleation, by causing syn-rift strata to overfold and drape fold. The passive folding propagates upwards forming gentle folds in the décollement plane, where thrust ramps tend to nucleate. Back-thrust-forethrust pairs tend to form over elevated horsts, where higher topographic plateaus are preserved. Multiple thrusting and lower relief occur over buried rifts with multiple inverted faults. Strata over non-rifted basement remain practically unaffected by thrusting and folding. Moreover, the development of thrust salients and recesses are demonstrated to be conditioned by the morphology of buried rifts. The study also contributes with a more precise mapping of the buried rift system and thrust system in the area, and brings new insights about the structural evolution of the Brasiliano-Pan Africano orogeny affecting the São Francisco craton basement and covers.