Regional geology of eastern Africa

Abstract Introduction Regional geology of eastern Africa Rukwa Rift Basin Rift stratigraphy, sedimentation and palaeontology Volcanism and geochronology Provenance, drainage patterns and uplift Methods References Acknowledgements Author information Supplementary informationIntroduction Regional geology of eastern Africa Rukwa Rift Basin Rift stratigraphy, sedimentation and palaeontology Volcanism and geochronology Provenance, drainage patterns and uplift Methods References Acknowledgements Author information Supplementary information The East African Rift System transects the anomalously high-elevation Ethiopian and East African plateaux that together form part of the 6,000-km-long African superswell structure. Rifting putatively developed as a result of mantle plume activity that initiated under eastern Africa. The mantle activity has caused topographic uplift that has been connected to African Cenozoic climate change and faunal evolution. The rift is traditionally interpreted to be composed of two distinct segments: an older, volcanically active eastern branch and a younger, less volcanic western branch. Here, we show that initiation of rifting in the western branch began more than 14 million years earlier than previously thought, contemporaneously with the eastern branch. We use a combination of detrital zircon geochronology, tephroand magnetostratigraphy, along with analyses of past river flow recorded in sedimentary rocks from the Rukwa Rift Basin, Tanzania, to constrain the timing of rifting, magmatism and drainage development in this part of the western branch. We find that rift-related volcanism and lake development had begun by about 25 million years ago. These events were preceded by pediment development and a fluvial drainage reversal that we suggest records the onset of topographic uplift caused by the Initiation of the western branch of the East African Rift coeval with the e... http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo1432.html 1 of 6 3/26/2012 10:56 AM African superswell. We conclude that uplift of eastern Africa was more widespread and synchronous than previously recognized. Introduction Abstract Introduction Regional geology of eastern Africa Rukwa Rift Basin Rift stratigraphy, sedimentation and palaeontology Volcanism and geochronology Provenance, drainage patterns and uplift Methods References Acknowledgements Author information Supplementary information The high-elevation (>1,000 m) plateaux of southern and eastern Africa are outstanding, first-order features of the African plate. Despite this, the uplift history and geodynamics of this unique topography remain a subject of debate and continue to challenge traditional plate tectonic concepts1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11. The topographic anomaly is referred to as the African superswell4 and has been attributed to complex patterns of mantle circulation and plume development that initiated ~ 30–40 million years (Myr) ago5, 6. In eastern Africa, the superswell is associated with the East African Rift System (EARS), considerable sections of which are superimposed on large shear zones and sutures within Proterozoic mobile belts, reactivated as rifts during the Palaeozoic era and Cretaceous period5 (Fig. 1a). The superswell developed in concert with the onset of Antarctic glaciation, which together fundamentally altered the African climate7. Regional uplift and formation of the EARS also rerouted and influenced large river systems, including the Nile, Congo and Zambezi12, 13, 14, 15, 16. This in turn resulted in complex and dynamic landscape fragmentation and the development of ecological corridors that, together with climatic shifts, set the stage for the evolution of Africa’s unique fauna, beginning with faunal interchange with Eurasia in the latest Oligocene epoch and leading to the appearance of hominoids/hominins and other groups during the Mio–Pliocene epochs7, 8, 9. Within this broad template, many uncertainties remain regarding the detailed chronology of uplift, volcanism and rifting in eastern Africa, which can be addressed by investigating interior sedimentary basins along the EARS. Figure 1: East African Rift System (EARS). Initiation of the western branch of the East African Rift coeval with the e... http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo1432.html 2 of 6 3/26/2012 10:56 AM a, Image of the EARS modified from the NASA Shuttle Radar Topography Mission collection. Inset map indicates the location of the Rukwa Rift Basin study area within eastern Africa. b, Generalized tectonic/structural map of eastern–central Africa illustrating tectonic elements and their broad ages (age data adapted from refs 48, 49). Note that colours for tectonic terranes used in this map correspond with Figs 3–4. CKB, Choma–Kalambo block; MB, Magondi belt; Gyr, billion-year-old; orange star, volcanic centre. Here, we examine the sedimentary succession preserved within the Rukwa Rift Basin (RRB; Fig. 1), a segment of the western branch of the EARS, to: first, constrain the depositional age of these deposits; second, delimit the timing of rifting and volcanism in the western branch; and third, interpret landscape evolution and drainage development in central–east Africa since the breakup of Gondwana. Our analysis integrates U–Pb detrital zircon geochronology with palaeocurrent analysis to reconstruct sedimentary provenance and unroofing patterns in the basin, coupled with tephroand magnetic stratigraphy of rift-fill deposits, providing a new test of the African superswell hypothesis. Regional geology of eastern Africa