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Geoffrey Ruiz, Uni. Neuchatel (Switzerland)
Francois Negro, Uni. Neuchatel (Switzerland)
Jurgen Foeken, SUERC (United Kingdom)
Omar Sadiqqi, Dpt. Geology (Morocco)
Fin Stuart, SUERC (United Kingdom)
Dominique Frizon de Lamotte, Dpt. Geology (France)
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The target area is the WSW- ENE oriented intra-continental Atlas chain in Morocco located between the West Africa Craton and the Betic-Rif system. It is a key natural laboratory encompassing the Pre-Cambrian to recent evolution of the region even though the stratigraphic record is incomplete. The presence of high surface elevations in both the High-Atlas (>4000m) and Anti-Atlas (>2500m) domains to the south is subject to discussions because there is little quantitative data available at present. Phases of uplift are thus ill constrained as places where the associated erosion products were accumulated.
To better constrain the orogenic growth of the Atlas chain, we investigated the time-Temperature paths of bedrocks from two morpho-structural domains that are separated by the narrow Souss Basin, i.e. the High and Anti-Atlas. Pre-Cambrian bedrocks from the Anti-Atlas domain yield old thermochronological Fission-Track ages on both zircon (390-300 Ma) and apatite (180-120 Ma) minerals that are associated with slightly younger (U-Th)/He ages on apatite (120-110 Ma). There are two possible (probably interfingering) interpretations for the preservation of such old thermal record: a) the Anti-Atlas was not affected by the 'Alpine' orogeny and (always) remain 'stable', or b) it is being affected since recent time but no level with such record is yet exposed. Analyses on bedrocks from the Axial Zone of the High-Atlas suggest that exhumation reached 0.2km/my for the Middle Miocene whereas continental series of Cretaceous age from both Sub-Atlas domains indicate total resetting to temperatures greater than 80°C. Using a geothermal gradient of 25°C/km, this suggests that a post Cretaceous sedimentary pile of at least 3 kilometres in thickness is missing. The timing of the erosion of this pile is still unknown but it is being constrained by thermal modelling through Apatite Fission-Track analysis.
Our thermochronological data provide constraints that evidence heterogeneous exhumation history between the different structural domains of the chain. Interestingly, the High and Anti-Atlas are constituted of bedrocks with roughly similar ages (absolute) and lithologies (Pre-Cambrian magmatic series). Hence, additional analyses using thermochronometers with higher temperature of closure (e.g. Fission-Track or (U-Th)/He on zircon) are being performed to investigate when differential exhumation started. The results will allow us to constrain the complete long-term time-temperature paths (i.e. from 270 to 40°C) of these domains and as a result the respective rates of denudation for the uppermost 1-8km, i.e. rates of medium to long-term processes. Comparing these results to existing geological records will enable us to identify the relevant tectonic process that shaped the landscape of this mountain range in the frame of the Africa-Europe convergence.
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