Abordagem preliminar sobre paleotemperatura e evolução do relevo da Bacia do Araripe, Nordeste do Brasil, a partir da análise de traços de fissão em apatita

Resumo

The Araripe Basin, located in the Borborema Province, northeastern Brazil, developed through a sequence of distinct events related to the opening of the South Atlantic. Its geological history is recorded in the preserved stratigraphy, which ranges in age from Paleozoic to Middle Cretaceous. The most conspicuous feature associated with the Araripe Basin is the Araripe Plateau, a large mesa capped by Cenomanian-Albian non-marine sandstones (Exu Formation). At the plateau base, gray to black shales (“Batateiras layers”) represent deposition in a large lake system covering part of northeastern Brazil during Early Aptian. 
Previous researchers have used the preserved geological section in the basin to infer the landscape evolution of the Araripe Plateau, and they have suggested that its uplift and exhumation were caused by widespread epeirogenic movements associated with post-break-up events in the Borborema Province. Independent constraints on the timing and magnitude of uplift in the Araripe Plateau area can now be introduced by this preliminary study using apatite fission track analysis. 
A series of samples along a N-S transect of the Araripe Basin and its neighboring basement provide clear evidence of heating in the past. Results from all samples also show evidence of Late Cenozoic cooling commencing sometime in the last 40 Ma. Maximum paleotemperatures associ-ated with this event are very similar (70º - 90ºC) between samples, consistent with little differen-tial movement across the sampled region.
Fission track results from the Aptian sand-stone sample (Rio da Batateira Formation) indicate an earlier event (pre-40 Ma). However, interpretation of data suggests that any paleo-heating between sediment deposition (112 Ma) and 40 Ma must be associated with paleotempera-tures less than ~100ºC, pointing to minor paleo-burial during the Late Cretaceous in this area. 
The timing of these various events are broad-ly consistent with the timing of events identified in and around the Borborema Plateau using the same method (Morais Neto et al. submitted), which identified two cooling episodes: a Late Cretaceous cooling event commencing sometime between 100 - 90 Ma and a Late Cenozoic cooling commencing between 30 - 0 Ma. This coincidence in the timing of events suggests that these events may be regional in nature and therefore likely to be related to regional exhumation. 
The uplift of the Araripe Basin can be related to epeirogenic movements associated with (or following) the Late Cretaceous cooling event. Thermal maturity presently observed in the “Batateiras layers” suggests that any source rock in the area ceased its thermal evolution due to such process. Present relief is interpreted as the result of subsequent erosion processes. If we assume a paleogeothermal gradient of 30ºC/km (and a surface temperature of 25ºC), we estimate an erosion of ~1.5 km across the Araripe area, associated with the Late Cenozoic cooling event. 
While important parameters on the timing and magnitude of events in this area have been resolved based on only 6 samples, further work involving additional analysis and U-Th/He thermochronology is suggested.