Landscape evolution in Africa during the Cenozoic and Quaternary-the legacy and limitations of Lester C. King

摘要

African landscape evolution since 66 Ma reflects interactions among parts of the Earth system from the Core to the Biosphere. We stress changes in those interactions in three events that have dominated landscape development: (i) a climatic revolution when the circumpolar current and the East Antarctic ice sheet first formed similar to 37 Ma; (ii) a tectonic revolution at similar to 32 Ma dominated by elevation of similar to 30 topographic structural swells continent-wide; and (iii) a second climatic revolution in a Northern Hemisphere cooling event (at similar to 2.7 Ma) that triggered Sahara desert initiation and the beginning of glacial cycles in the Northern Hemisphere (similar to 2.15 Ma). We recognize the following distinct features of the great Afro-Arabian continent (similar to 40 M km(2)) that show its relationship to Earth structure and processes: deep mantle structure and dynamics, plate motion with respect to that structure, especially plate-arrest (similar to 32 Ma). The topographic, erosional, geomorphic, and depositional modifications following that tectonic event were strongly influenced by changes around the continent in oceanic and atmospheric circulation that affected the entire continent. Atmospheric circulation changes since similar to 34 Ma have involved zonal winds, the ITCZ, desert formation and destruction, the evolution of the persistent (since similar to 35 Ma) Antarctic ice sheet, and since similar to 32 Ma of the rapidly cycling Eurasian ice sheets. We explain that a widely supported idea that ancient erosion surfaces have survived at high elevations in Africa is the result of a failure to recognize that the present elevations of the continent's swells are dynamically maintained, but do not display a thermochronological signature because they are young (less than similar to 32 My), so that conduction of heat from shallow convection cells has not yet reached the Earth's surface.(R)