Abstract
In Eastern South America, a series of fault-bounded sedimentary basins that crop out from Southern Uruguay to Southeastern Brazil were formed after the main collisional deformation of the Brasiliano Orogeny and record the tectonic events that affected the region from the Middle Ediacaran onwards. We address the problem of discerning the basin-forming tectonics from the later deformational events through paleostress analysis of more than 600 fault-slip data, mainly from the Camaquã Basin (Southern Brazil), sorted by stratigraphic level and cross-cutting relationships of superposed striations, and integrated with available stratigraphic and geochronological data. Our results show that the Camaquã Basin was formed by at least two distinct extensional events, and that rapid paleostress changes took place in the region a few tens of million years after the major collision (c.a. 630 Ma), probably due to the interplay between local active extensional tectonics and the distal effects of the continued amalgamation of plates and terranes at the margins of the still-forming Gondwana Plate. Preliminary paleostress data from the Castro Basin and published data from the Itajaí Basin suggest that these events had a regional nature.
Highlights
The tectonic evolution of Precambrian terrains has long been a major theme of scientific research, but the tectonic record present in the unmeta morphosed and low-grade sedimentary and volcanic successions of the Proterozoic and Archean is frequently underestimated
We present orientation data on brittle faults with slickensided fibers and striations of each major unit of the Camaquã Basin, as well as some younger units, in order to establish the chronology of the deformational events and to distinguish the directions of paleostress axes related to basin formation from those that were responsible for its later deformation
Independent evidence suggests that the extensional events were responsible for the subsidence cycles, and that the strike-slip faulting events are related to younger deformation: (1) the oldest extensional event, with ENE-WSW to NESW σ3, is found only in the lower stratigraphic levels and predates the first identified strike-slip event, as revealed by dikes feeding syn-depositional volcanism; (2) a similar extensional event, with E-W σ3, was responsible for syn-sedimentary faulting in the Late Ediacaran Santa Bárbara Group; (3) the units generated during the last subsidence cycle of the basin, in the Early Cambrian, are not affected by the main compression and record an extensional event prior to an event of strike-slip faulting, which shows a different orientation of stress axes from the previous
Summary
The tectonic evolution of Precambrian terrains has long been a major theme of scientific research, but the tectonic record present in the unmeta morphosed and low-grade sedimentary and volcanic successions of the Proterozoic and Archean is frequently underestimated. In this context, the Ediacaran to Early Cambrian system of fault-bounded basins that crops out from Southern Uruguay to Southeastern Brazil is an ideal object for paleostress analysis since these basins show no regional metamorphism or ductile deformation, and it is possible to identify and date, through geochronological analysis of several volcanic units, distinct events of subsidence bounded by unconformities. The Ediacaran to Early Cambrian system of fault-bounded basins that crops out from Southern Uruguay to Southeastern Brazil is an ideal object for paleostress analysis since these basins show no regional metamorphism or ductile deformation, and it is possible to identify and date, through geochronological analysis of several volcanic units, distinct events of subsidence bounded by unconformities This brings the possibility of determining an absolute chronology for the tectonic events that occurred during the basin evolution. These interpretations are based mostly on the presence of strike-slip and inverse faults affecting the sedimentary successions, but no evidence for a syn-depositional nature of this deformation has been presented to date
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