Erosion and development of granitic inselbergs in cratonic zones: insights from the Quixadá/Quixeramobim Complex, NE Brazil

The origin of inselbergs is classically linked to long-term episodic exposure, alternating between the deepening of the weathering mantle and the regolith stripping driven by differential erosion. However, a better understanding of denudation patterns and exposures ages in granite landscapes is hindered by key gaps arising with this assumption: i) the absence of sedimentary markers or their dating; ii) the scarcity of quantified denudation rates to reconcile with classical models of evolution in cratonic zones. To address these gaps, in-situ produced ¹⁰Be and ²⁶Al in river-borne sediments, bedrocks, and sedimentary deposits can be compared and analyzed to quantify the role played by differential erosion in the evolution of granitic inselbergs and to unravel complex exposure histories. Previous studies quantifying erosion in granitic landscapes have focused on small inselberg groups or isolated high inselbergs, comparing rock surfaces without integrating sedimentary covers or basin-averaged denudation, thereby limiting regional-scale understanding. In this study, we paired ¹⁰Be and ²⁶Al in quartz from sands and rocks collected across watersheds, colluvial-eluvial sediments, and inselbergs. Denudation rates were combined to reconstruct the history of inselbergs in a tropical semiarid setting.

Using this approach, we selected the Quixadá/Quixeramobim Complex (NE Brazil), a globally significant inselberg field with 561 units, renowned for its diverse granite landforms in cratonic and semiarid areas. These landforms rise 1-410 m above the Surface Sertaneja (160-240 m). Within the plutons, the erosion surface is ~20 m lower than in adjacent regions, where metamorphic rocks and colluvial-eluvial covers are found.

Our results show that colluvial-eluvial covers exhibit higher cosmogenic nuclide concentrations than bedrocks, with values at least twice as high as those in watersheds. Conversely, catchments experience faster erosion than bedrocks. All bedrocks and 60% of the catchments are plotted within the steady-state erosion island, whereas Cenozoic covers display low ²⁶Al/¹⁰Be ratios. Average exposure ages suggest a Pleistocene age (1.0-1.4 Ma) for sedimentary covers. This reflects a general denudation pattern where inselbergs and sedimentary surfaces are preserved while the regional erosion surface is progressively stripped.

Assuming no erosion for sedimentary covers and continuous denudation, the differential denudation obtained between inselberg top and basin-averaged rates indicates that ~20 Ma is required to grow a 175 m-high inselberg, and ~50 Ma for a 400 m-high inselberg. These results highlight the utility of cosmogenic techniques in measuring denudation rates and improving the understanding of processes that govern origin and development of inselbergs.