No abstract

Classical geomorphological models identify regional erosion surfaces and raise hypotheses, currently abandoned but largely untested, about the cumulative effects of tectonics and climate on the evolution of continental landscapes. Weathering geochronology provides the appropriate tool for testing these models. New and published 40 Ar/ 39 Ar geochronology of 346 grains of Mn oxides extracted from weathering profiles representing four regionally recognized landsurfaces in southeastern Brazil reveals that weathering minerals record minimum ages for the surfaces hosting the weathering profiles. Weathering profiles at the highest elevation surfaces are as old as ~ 70 Ma; those at intermediate surfaces formed post ~ 13 Ma; incipient profiles at incised valleys range from ~ 4-1 Ma; and the shallowest and least evolved weathering profiles at coastal plains yield ~ 1.0-0.8 Ma ages. The oldest and highest elevation landsurfaces are blanketed by deep (> 150 m) chemically stratified lateritic profiles; intermediate surfaces host deep saprolites (ca. 40-60 m) capped by regional stone lines; and shallow (5-20 m) and young weathering profiles cover the more deeply dissected parts of the landscape within intracontinental valleys and along the coastal plains. The hierarchy in ages and elevations for the weathering profiles suggests that the landscape in southeastern Brazil evolved through alternation of weathering-prone periods interrupted by periods of uplift and widespread incision. The denudation chronology model derived from weathering geochronology is compatible with the sedimentary record in adjacent offshore sedimentary basins. It is also compatible with erosion rates measured by cosmogenic isotopes for each of the separate landsurfaces. The results reveal that landscapes in the region are not in dynamic equilibrium and that they evolved by episodic uplift followed by denudation along retreating escarpments.

40Ar/ 39 Ar laser incremental-heating analyses of 22 individual grains of supergene cryptomelane from three weathering profiles, up to 400 km apart, in the Rio Doce valley and Barbacena regions at Minas Gerais, Brazil, show that the formation of weathering profiles in these regions is contemporaneous, suggesting a strong weathering event in the Middle to Late Miocene (10-8 Ma). The preservation of these Miocene samples at or near the present surface suggests that either erosion rates have been very low in the region since the Miocene or that a much thicker weathering mantle was present in the region originally. Assuming a constant thickness of weathering profiles in the region throughout the Tertiary, we may calculate weathering front propagation rates of 4-8 m Myr −1 during the past 10 Ma.

The majority of numerical models of landscape evolution in divergent margins are focused on the simulation of margins with simplified lithological control on landscape erosion. However, this approach is insufficient to study the evolution of margins where chemical weathering is an important element increasing rock resistance to physical erosion. One example of this margin is the Borborema Province, northeastern Brazil, where postrift marine sediments are now preserved at elevations ∼700–800 m in up to 1‐km‐high plateaus capped by duricrust layers. The landscape evolution of these uplifted sedimentary basins still eludes explanation. Here we use numerical models that couple weathering, erosion, sedimentation, sea level changes, flexural isostasy, and thermal effects due to lithospheric stretching to simulate the tectonosedimentary evolution of the Borborema Province since the onset of continental stretching during the Lower Cretaceous. These numerical experiments reveal that nearly 70% of the postrift regional uplift observed in the Borborema Province can be explained by differential denudation of the continent and flexural rebound of the lithosphere. The remaining ∼250 m of uplift can be explained by thermal uplift induced by partial erosion of the base of the continental lithosphere under the Borborema Province due to edge‐driven convection, in accordance with the anomalously thin continental lithosphere observed under the Borborema Province. Additionally, the numerical results can explain the regional pattern of fission track ages by the combined effect of differential denudation and flexural rebound in this geometrically complex margin.