Morphostratigraphic constraints and low temperature thermochronology: Lessons from a review of recent geological and geomorphological studies in northeast Brazil

摘要

In the last decade, detailed geomorphological analyses of northeast Brazil led the authors to publish a model of landscape development in which long-term landform evolution was driven by regional swell-like uplift post-dating Early Cretaceous intracontinental rifting and the formation of the Atlantic passive margin in Aptian times. The post-Cenomanian uplift caused an inversion of Cretaceous basins and generated a landscape in which the most elevated landforms correspond either to resistant post-rift sedimentary cover, or to residuals of Cretaceous rift shoulders, above a low erosion surface. Since no evidence of a former post-Cenomanian sedi-mentary cover of significant thickness was found outside the coastal fringe, we could evaluate the uplift to 600 m at most and the mean erosion rates to 10 m/Ma or less. However, according to models based upon the results of thermochronological analyses (apatite fission tracks analysis-AFTA), two slices of 1000 m to 2500 m would have been deposited over the present pile in the south part of the study area, respectively in Campanian and Oligocene-Miocene times, before being totally removed. We examine here the diverging scenarios of geomorphic evolution respectively based upon morphostratigraphy (our model) and upon low temperature thermochronol-ogy, submitting them to available evidence provided by an updated and geographically extended review of geomorphological and sedimentological data, and trying to decipher some of the reasons that might lead to disputable geomorphic interpretations. We stress the fundamental importance of taking into account all the available results of geomorphological and geological approaches in any interpretation of thermochronological and other analytic methods used for reconstructing long-term landscape evolutions. This is one of the conditions for reinforcing the trust one can have in their results, which may bring complementary or unique information, peculiarly in places where sedimentary or volcanic markers are missing.