Long-term evolution of the West African Transform margin : estimates of denudation from Benin using apatite thermochronology

Abstract : The Benin continental margin was formed during the breakup of Gondwana through oblique rifting along transform faults. The evolution of topography following breakup directly impacts the evolution of sedimentary basins, which has major implications for hydrocarbon exploration in the region. Quantitative constraints on erosion across Benin are limited to the Cenozoic, based on analysis of dissected lateritic paleolandscapes. To resolve the Mesozoic erosion history, we have obtained apatite fission track and single-grain (U-Th-Sm)/He data from 18 samples collected across a 600-km-long transect through Benin. We invert these data, including available geological and geomorphic constraints, to obtain time-temperature paths, which are used to estimate magnitudes of denudation over the last 200 Myr. Our study suggests denudation was focussed over a c. 300-km-long seaward sloping limb of the marginal upwarp and at the southern margin of the interior Iullemmeden Basin from 140-100 Ma with lower magnitudes of denudation characterising the continental interior and post-Cretaceous evolution of the margin. Models are consistent with modest burial (c. 1 km) of the Iullemmeden Basin between 120 and 85 Ma, and of the continental margin between 85 and 45 Ma. By the Eocene, the first-order relief of Benin had developed with regional erosion rates <20m/Myr since then.