Calibrating denudation chronology through 40Ar/39Ar weathering geochronology

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 40Ar/39Ar 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 similar to 70 Ma; those at intermediate surfaces formed post similar to 13 Ma; incipient profiles at incised valleys range from similar to 4-1 Ma; and the shallowest and least evolved weathering profiles at coastal plains yield similar to 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.