Nucl. Tracks Radiat. Meas., Vol. 17, No. 3, pp. 339-350, 1990 Int. J. Radiat. Appl. Instrum., Part D Printed in Great Britain 0735-245X/90 $3.00 + .00 1990 Pergamon Press plc AN EARLY CRETACEOUS PHASE OF ACCELERATED EROSION ON THE SOUTH-WESTERN MARGIN OF AFRICA: EVIDENCE FROM APATITE FISSION TRACK ANALYSIS AND THE OFFSHORE SEDIMENTARY RECORD RODERICKW. BROWN,* DEREK J. RUST,t MICHAEL A. SUMMEgFIELD,~" ANDREWJ. W. GLEADOW* and MICHEILC. J. DE WlT~ *Department of Geology, La Trobe University, Bundoora, Victoria 3083, Australia; tDepartment of Geography, School of Earth Sciences, University of Edinburgh, Edinburgh EH8 9XP, U.K.; and :~De Beers Consolidated Mines Ltd, PO Box 47, Kimberley 8300, South Africa (Received 5 September 1988; /n revisedform 17 May 1989) Abstraet--Apatitefission track ages and confined track length distributions have been determined for rock samples from the south-western continental margin of Africa. The apatite ages fall into two groups, one having early Cretaceous ages and mean confined track lengths of ~ 14/~m with very few short tracks, and the other having older ages with confined track length distributions containing a significant proportion of strongly annealed tracks (< 10/am). In any particular area the older apatite ages only occur above a critical threshold elevation, forming a regional pattern in the data and indicating cooling of the upper few kilometres of the crust during the early Cretaceous. This episode of cooling is shown to have been the consequence of an accelerated phase of erosion associated with the early stages of rifting and break-up of Gondwana, and correlates with sedimentation patterns derived from borehole data for the adjacent offshore basin. 1. INTRODUCTION THE ¢tmONOLOGY and nature of landscape develop- ment in southern Africa has stimulated debate since the beginning of the present century, and yet no consensus has been reached (Partridge and Maud, 1988). Major differences in viewpoint result largely from a lack of quantitative information concerning the timing and magnitude of denudation episodes affecting the sub-continent, particularly since the break up of Gondwana. The major discrepancies between the various proposed landscape chronologies (Partridge and Maud, 1987) imply that there are fundamental problems associated with dating ap- proaches used up until now, for example, in the correlation between landsurfaces and unconformities in the offshore sedimentary record (Summerfield, 1985). This situation suggests that the use of a more direct dating technique may help to resolve these discrepan- cies, and with this objective a comprehensive study of the morphotectonic development of southern Africa has been initiated. A mejor component of this study was the application of apatite fission track analysis (AFTA) to rocks collected at a range of elevations across the region. This technique provides a valuable tool for evaluating the thermal history of rocks at temperatures below -.. 125°C and consequently for examining the thermo-tectonic development of the upper few kilometres of the Earth's crust (Gleadow et al., 1986). As a result AFTA can be used to date major periods of denudation directly (Gleadow and Fitzgerald, 1987; Green, 1986; Miller and Duddy, 1989). The aim of the study now underway is to provide quantitative data on the timing and extent of major denudational episodes affecting the southern African sub-continent since the initiation of rifting and the break up of Gondwana. This paper presents the AFTA results obtained from an initial suite of out- crop and borehole samples from the south-western margin, together with previously unpublished bore- hole data from the adjacent offshore basin. 2. RESULTS AND INTERPRETATION Thirty-five outcrop and borehole samples were analysed from the western margin of southern Africa between Cape Town in South Africa and Luderitz in Namibia, a distance of approximately 1000 kin. Nine- teen of the samples were of igneous or metamorphic crystalline basement rocks of Palaeozoic age or older, and six were sedimentary rocks. The outcrop samples were collected, where possible, along transects roughly perpendicular to the present coastline and over a range in elevation from sea level to above the prominent erosional escarpment known as the Great Escarpment (Oilier and Marker, 1985). This feature trends roughly parallel to and at distances of 339