| dc.description.abstracteng | The Dom Feliciano Belt in southern Brazil and Uruguay records the superposed tectonic events that led to the assembly of southwestern Gondwana during the Neoproterozoic Brasiliano-Pan-African orogenic cycle. During the course of the orogeny, the belt and associated Precambrian domains were affected by widespread crustal deformation, leading to a complex set of shear zones. This thesis investigates the tectono-thermal history of the main shear zones in the Dom Feliciano Belt and associated terranes. Deformation conditions and the evolution of the shear zones are characterized using structural and microstructural observations, combined with quartz CPO textural analyses. New K-Ar data and a review of the literature are used to constrain this evolution in the geochronological timescale. In addition, the Phanerozoic thermal history of the study area is investigated combining (U-Th)/He analyses on zircon and apatite, thermal modelling, and K-Ar dating of fault gouges. In this way, it is possible to examine the impact of the main Neoproterozoic structures as preferential sites for reactivation.
The oldest terrane boundary in the region is the Ibaré Shear Zone, which records the accretion of the Tonian juvenile São Gabriel Terrane to the Archean-Paleoproterozoic Nico Pérez Terrane as a dextral lateral ramp during SW-verging thrusting. New-K-Ar analyses suggest that it was established at ca. 760 to 740 Ma, and reactivated in the Cryogenian-Ediacaran in narrow sinistral shear zones at cooler conditions, during the formation of the Dom Feliciano Belt. The belt was formed during oblique collision between the Congo and Río de la Plata cratons, together with the Nico Pérez and Luís Alves Terranes, resulting in widespread transpression. This process was probably diachronic, with onset of transcurrent structures being recorded between ca. 650 and 620 Ma in different sectors of the belt, and led to the formation of its main terrane boundary, the Major Gercino-Dorsal do Canguçu-Sierra Ballena lineament. This shear zone system records an intense amount of pure shear and contrasting kinematics along its extension, suggesting local variations to the main horizontal compression and partitioning into different transcurrent vectors. After 600 Ma there is a decrease in wide-scale regional compression, transitioning to localized strike-slip deformation along the main shear zones, suggesting a post-collisional stage. Late ductile reactivations were active until ca. 540-530 Ma.
With the cessation of the orogenic processes, the study area stabilized and achieved an intracratonic position inside Gondwana, experiencing a protracted evolution during the Phanerozoic. Exhumation during the early Paleozoic probably exposed much of the present-day crystalline basement to near-surface conditions, and was followed by regional subsidence during the sedimentation of the Paraná Basin. For most of the belt’s extension, final exhumation was achieved at the latest during the rift stage of the South Atlantic in the Lower Cretaceous, but its northernmost portion records up to 2 km of post-rift exhumation. While recurrent brittle reactivation of Neoproterozoic structures is recorded by the dating of fault gouges, this process is not reflected in the study area’s thermal history. Instead, the main structural control is by transecting fault systems, oriented perpendicular to the South Atlantic coastline. Along the south-southeastern South American passive margin, major reactivation of the inherited structures is predominantly recorded in strongly uplifted regions. | de |