Meso-Cenozoic uplifts on the Atlantic margin of South Morocco

Passive margins are key areas to investigate the relationships between the continental interiors and the marine realm. A careful study of their stratigraphic record is then expected to reveal the complex interplays between subsidence, climate and eustasy (Dauteuil et al., 2013). The eastern passive margin of Central Atlantic initiated in the Early Jurassic and has been subsequently witnessing the evolution of the continental interior during the Meso-Cenozoic drifting of Africa and North America. This passive margin is bounded by the West African Craton to the East, and its geometry and evolution are poorly known (Labails et al., 2009). We have focused our study on the vertical evolution of the onshore part of the basin, in order to improve our knowledge with regards to the dynamics of the basin's infill. The purpose was to identify the main uplift vs. subsidence events impacting the margin during Meso-Cenozoic times and to correlate them to the geodynamic context. We used low-temperature thermochronology on apatites with fission tracks and (U-Th)/He dating to constrain the evolution of the margin during Meso-Cenozoic. These analyses have been performed on samples coming from the onshore basin detrital formations and basement formations from the craton. Modeled thermal histories were then carried through the use of QTQt, a recent program taking into account the most recent developments on apatite thermochronology (Gallagher, 2012). We obtained fission tracks ages ranging from 107×8 Ma to 160×11 Ma and (U-Th)/He ages from 14×1 Ma to 97×9 Ma. The scattered repartition of (U-Th)/He ages is explained by the distribution of effective uranium in the samples and reveal a quite young signal. The fission tracks ages are not so scattered and show a consistent signal. Thermal histories characterize for the first time the polyphased vertical evolution of the basin throughout its Meso-Cenozoic history. Two major steps of exhumation are recorded. First, a Late Jurassic/Early Cretaceous exhumation is demonstrated with the setting of large volumes of detrital deltaic formations all over the basin. After the burial of these formations, a Tertiary final exhumation is also recorded by the very recent helium ages. In accordance with the geological record, these results of thermal modeling emphasize the need for better constrains in the onshore parts of passive margin basins to fully understand the sedimentary dynamics responsible for their present geometry. Dauteuil, O., Deschamps, F., Bourgeois, O., Mocquet, A. & Guillocheau, F., 2013. Post-breakup evolution and palaeotopography of the North Namibian Margin during the Meso-Cenozoic, Tectonophysics, 589, pp.103-115 Gallagher, K., 2012. Transdimensional inverse thermal history modelling for quantitative thermochronology, J. Geophys. Research, 117, doi:10.1029/2011JB008825 Labails, C., Olivet, J.-L. & The Dakhla study group, 2009. Crustal structure of the SW Moroccan margin from wide-angle and reflection seismic data (the Dakhla experiment). Part B: the tectonic heritage, Tectonophysics, 468, (1-4), pp.83-97