Morphologic evolution of a Semiarid Relief: The case of Chapada do Araripe (Ceará, Brazil)

The Brazilian northeastern is composed of a mix of Precambrian crystalline rocks and Mesozoic sedimentary basins. In this region, the landscape evolution model is illustrated by a great regional uplift after the cretaceous rift followed by regional denudation. The most resistant rocks from sedimentary Cretaceous basins and the old hard granites support the actual existent high hills. However, the regional landscape evolution scenarios left behind some questions, according to the AFT data, about 1000-2500 m of sediments were deposed and eroded after Cenomanian, resulting in erosion rates higher than 60 m/Myr after Miocene. These scenarios are opposed to the model based on morpho-stratigraphic observations that suggest erosion rates below 10 m/Myr. Surrounded by large planation surfaces, the Chapada do Araripe (Chapada = plateau), located on the border of the states of Ceará, Piaui, and Pernambuco, is part of the drainage divide of 3 large Brazilian watersheds, and is the higher representant of the post cretaceous uplift in the Borborema structural province, with late Aptian–Albian marine sediments (Santana Group) at elevations between 700-800 m a.s.l. That combination of characteristics makes the Chapada do Araripe a landscape with huge potential to assist the understanding of the landscape evolution over the last 70 Myers in the Brazilian northeast. One of the most visible characteristics of the chapada is the difference between the sides. Meanwhile, the north side (Jaguaribe basin) has a steeper relief the south face (São Francisco basin) has longer and less sloping slopes. This abstract is part of a Ph.D. thesis that aims to illustrate the regional relief perspective from a combination of cosmogenic nuclides and digital topographic analysis. The following discussions are based on digital elevation model derivates indices, longitudinal profiles analysis, and fieldwork observations. The north side of the Chapada (Jaguaribe drainage basin) shows indices that indicate a relief with greater geomorphic variations in most of the basin. In contrast, the south side (São Francisco drainage basin) shows smaller variations, with lower altimetric amplitudes and average slopes. These observations can be seen on both the morphometric indices and longitudinal profiles. In the longitudinal profiles, the curves show superior proximity of the equilibrium profile in the streams located in the São Francisco basin when compared to those located in the Jaguaribe basin. From the 290 knickpoints identified, only 15% are in the São Francisco basin. On both sides, 65% of the knickpoints are linked to lithological changes or faults, evidencing the landscape's strong presence of lito-structural controls. The application of morphometric indexes allowed detailing the two sides of the Chapada, contributing to the morphological characterization. Ongoing analyzes at LN2C-ASTER to measure basin-wide denudation rates using in situ produced ¹⁰Be and ²⁶Al around the Chapada will complement the results found in this work and will allow a better background to the discussion of the plio-quaternary relief evolution.