Thermochronology of South America passive margin between Uruguay and southern Brazil: A lengthy and complex cooling history based on (U–Th)/He and fission tracks

Thermochronology studies carried out in crystalline rocks on the South America passive margin reveal distinct cooling patterns along the coast. While most of the margin presents syn-to post-rift final exhumation, the region between parallels 29° and 35° S presents primarily pre-rift exhumation. This stretch of the coast corresponds to Uruguay and southern Brazil, where the Rio de La Plata Craton and the Dom Feliciano Belt occur. Previous studies in the area suggest temperatures below 200 °C since the early Paleozoic, and a complex thermotectonic history during the Phanerozoic. Here we present the first zircon fission track ages for both craton and belt; they range between 562 and 280 Ma and corroborate an early Paleozoic cooling of the basement, likely related to regional exhumation after the Brasiliano/Pan-African Cycle (Neoproterozoic-Cambrian) and the formation of West Gondwana. The thermal history after this cooling phase is a matter of debate. Therefore, we compiled all thermochronometry ages available for the region to evaluate the current hypotheses for the basement exhumation. We suggest that this region went through protracted and continuous cooling during the Paleozoic-Mesozoic, until South Atlantic opening in the Jurassic-Cretaceous. The basement was exposed to near surface conditions (T ≤ 60 °C) in the Mesozoic, and the magmatism associated with the breakup likely caused a geothermal disturbance, which may be responsible for the highly dispersed apatite (U–Th)/He ages observed in previous studies. Basement exhumation after South Atlantic opening was minimal in the region. The key to constrain the low temperature thermotectonic history of both craton and belt appears to be a better comprehension of the long-term effects of accumulation of radiation damage within thermochronometers, and to quantify the effects of protracted cooling and minor reheating events on apatites and zircons with variable uranium content.