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Título

Constraining post-rift tectonics of the SW Atlantic passive margin: the Piquiri fault system, southern Brazil

Texto do resumo

Brittle structures often act as potential controllers of local to regional fluid-flow patterns in the upper continental crust. Calcite mineralization within fractures may be causally related to their opening process, thus recording key information about the timing and tectonic context of mineral growth. Since primary calcite fibers with crack-seal textures on structural fault planes are typically synchronous to fault movement, in-situ calcite U-Pb dating can provide ages of fault (re)activation. The timing and rates of landscape rejuvenation along the southern Brazilian passive margin are mainly controlled by its complex structural framework, lithological variations and climate regimes. After the break-up of southwestern Gondwana, intraplate stress fields were responsible not only for a widespread reactivation of bedrock-inherited zones of weakness, but also for the development of new faults. In this context, erosional processes have played a key role on structurally controlled bedrock channels, with faults and fractures often acting as preferential flow pathways. This is the case of the Ponta Grossa Arch (PGA), an example of a significant tectonic feature with about 600 km of extension that hosted major magmatic activity from Early to Late Cretaceous. We focused our analysis on the Piquiri lineament, the southernmost feature of the PGA, defined by a set of NW-SE structures along the main course of the homonymous river. Absolute time constraints are essential in areas that have undergone a complex tectonic evolution and major landscape changes - as such, detailed morphotectonic analysis were followed by fieldwork and sample collection along potentially reactivated faults. Calcite samples were characterized using Cathodoluminescence (CL) and SEM images to identify potential targeting zones prior to dating. U-Pb isotopic analyses were performed using LA-ICP-MS in the NERC Isotope Geosciences Laboratories (NIGL), British Geological Survey, Nottingham, UK. Morphostructural analysis have revealed that post-rift tectonics in the Piquiri region involves three main deformation pulses: (1) a horizontal NE-SW-oriented σ1, related to N-S to NE-SW dextral and E-W sinistral strike-slip faults; (2) horizontal N-S-trending σ1 - with NW-SE dextral strike-slip and NE-SW sinistral strike slip faults; (3) horizontal E-W-oriented σ1 – including E-W dextral and NW-SE to N-S sinistral strike slip faults. We have obtained two main sets of absolute ages for post-rift faulting in the Piquiri fault system from U-Pb LA-ICP-MS carbonate dating: (1) ~120 Ma and (2) ~80 Ma. Our results implied that the post-rift tectonic evolution of the SW Atlantic margin comprises significant fault reactivations along inherited basement structures linked to the Santonian-Campanian onshore/offshore magmatism that acted as major heating sources for fluid-flow and calcite recrystallization in the region. Our data have also suggested that a transtensional reactivation of the Piquiri fault system at 80 Ma exerted major control on the southernmost deposition of the Caiuá Group, inducing differential erosion across tectonic blocks and drainage captures along the main structures. Changes on intraplate stress fields were interpreted as results of an interaction between local sources (e.g. localized uplift and/or underlying mantle anisotropies) and regional tectonic forces – such as the ongoing Andean orogeny and the westward continental drift of the SW Atlantic margin.

Palavras Chave

fault dating; U-Pb carbonate geochronology; LA-ICP-MS U-Pb dating; Ponta Grossa Arch; Piquiri River