1School of Earth and Environmental Science, the University of Queensland, Brisbane, Qld, Australia.
Australia is sparsely vegetated and the flattest, hottest, most tectonically stable but fastest latitudinally moving continent on Earth; it has migrated from high to low latitudes in ~50 Ma. In contrast, Brazil is a densely vegetated, wet, high relief cratonal terrane that has moved slowly longitudinally along the Equator for the past ~70 Ma. Despite these contrasting underlying geological and geographical characteristics, landscapes in these two regions are remarkably similar, share analogous ancient weathering histories, and are marked by plateaus surrounded by dissected plains that erode similarly and at equivalent rates. In situ cosmogenic isotopes show that plateaus eroded at less than 2 m.Ma-1, while dissected plains erode at 5-20 m.Ma-1. Cosmogenic isotope concentrations in sediments show greater erosion rates, suggesting that erosion focusses preferentially along escarpments. 40Ar/39Ar geochronology on Mn-oxides and (U-Th)/He-4He/3He geochronology on goethite show that ancient weathering profiles blanketing plateaus in both continental areas are as old as ~90-70 Ma. These results are substantiated by in situ cosmogenic 3He concentrations in hematite. Geochronological results for the surrounding plains, on the other hand, indicate that the dissected areas are typically younger than ~30 Ma. Weathering profiles in both continental areas, on opposite sides of the planet, host supergene mineral populations that record analogous and often contemporaneous events of water-rock interactions through time. Importantly, these plateaus have been continuously emergent throughout their entire histories, hosting in the weathering profiles underneath minerals precipitated under contrasting climatic conditions through time. Interestingly, the analogous weathering histories of the two continental areas are mostly recorded in oxides, hydroxides, and clay mineral assemblages. The distribution and abundance of sulphates, carbonates, and silica-minerals, on the other hand, mark significant contrasts between the two landmasses. The most striking contrast is weathering under water deficient conditions in Australia while Brazil weathered under pronounced oversupply of rainwater. It is remarkable that such differences in climatic conditions produced such similar resulting landscapes. This is only possible because in Brazil, iron oxyhydroxides and ferricretes provided the landscape scaffolding that is provided in Australia by silica minerals and silcretes.
Paulo Vasconcelos is a geologist specialised in the development and application of novel geochronological tools for investigating earth and planetary processes.