Pergamon Journal of African Earth Sciences. Vol. 26, No.3, pp. 363-385, 1998 c 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain PII:S0899-5362(98)00021-9 08995362/98 $1900 + 000 Denudation history of the Malawi and Rukwa Rift flanks (East African Rift System) from apatite fission track thermochronology PETER VAN DER BEEK,1.5 EVELYNE MBEDE,2 PAUL ANDRIESSEN3 and DAMIEN DELVAUX 4 'Laboratoire de Geodvnarnique des Chaines Alpines, Institut Dolomieu, 15 rue Maurice Gignoux, 38031 Grenoble cedex, France 2Department of Geology, University of Dar-es-Salaam, Box 35052, Dar-es-Salaam, Tanzania 3Faculty of Earth Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, the Netherlands "Royal Museum for Central Africa, Steenweg op Leuven 13, B3080 Tervuren, Belgium Abstract- Thirty apatite fission track ages and 22 track length measurements are presented from samples of basement rocks flanking the Malawi and Rukwa Rifts (East African Rift System) in order to elucidate the thermotectonic history of the rift flanks. The apatite fission track ages fall in the range 30 ± 15 to 296 ± 10 Ma. The relatively short (11.0-13.2 Jim) mean track lengths and wide (1.3-2.3 Jim) track length distributions suggest a protracted cooling history for the region, spanning Permian (Karoo) to Recent times. Thermal history reconstruction by inverse model calculations of the track length distribution suggests repeated phases of rapid cooling and denudation of the rift flanks at 250-200 Ma, -150 Ma and ::;40-50 Ma. These appear to be linked to the different rifting events in the area and can be correlated with deposition of the different sedimentary units within the basins. Erosion and isostatic rebound have modified the tectonically induced topography around the rifts: the elevation of the footwall flanks is augmented by flexural isostatic rebound, whereas the topography of the hanging wall flanks has been lowered by erosion. The footwall escarpments of the Malawi and Rukwa rifts are erosional features. The highly elevated plateaus flanking the Western Rift represent an erosional surface traditionally referred to as the "Gondwana surface". The apatite fission track results of this study suggest that initial exhumation of the "Gondwana surface" to temperatures around 60-70 oC took place during Karoo times, but that sub-aerial exposure of the surface did not take place until at least the Early Tertiary. © 1998 Resume-Nous presentons 30 ages de traces de fission sur apatite et 22 mesures de longueur de trace de fission pour des echantillons du socle dans les flancs des rifts de Malawi et Rukwa (rift est-africain), dans Ie but de reconstituer I'histoire thermo-tectonique des flancs de ces rifts. Les relativement courtes longueurs moyennes (11.0-13.2 Jim) et larges distributions (1.3-2.3 Jim) des traces de fission suggerent un refroidissement prolonqe pour la region, allant du Permien (Karoo) a la periode recente. La reconstruction de I'histoire thermique par modelisation inverse de la distribution des longueurs de trace de fission suqqere que des phases repetees de refroidissement rapide et de denudation des flancs de rift se sont succedees a 250-200 Ma, -150 Ma et s40-50 Ma. II apparait qu'elles sont reliees a differentes 5Present address: Laboratoire de Geodvnarnique des Chaines Alpines, Institut Dolomieu, 15 rue Maurice Gignaux, 38031 Grenoble cedex, France Journal of African Earth Sciences 363