Recent tectonics in the Ouarzazate basin (Moroccan Atlas): Tectonic uplift and shortening rates from Quaternary markers

Recent studies have shown that the uplift of the moroccan Atlas Mountains resulted from a combination of crustal shortening and mantle thermal processes. Within this framework, to gain insight of the dynamics of these processes in recent times, we have investigated the nature and rates of Quaternary deformation in a case field area, which contains the best dated record of fan and terrace deposits, and a moderate seismicity. The area selected is the northern Ouarzazate basin, at the southern orogenic front ofthe Atlas, where folds and thrusts involve a sequence of stepped terraces where recently obtained Terrestrial Cosmogenic Nuclide surface exposure ages provide a basis to calculate shortening rates for the last —250 ky (since the interglacial peak MIS-9c). Calculated Quaternary shortening rates for individual thrust structures range between 0.13-0.21 mmfy, which are greater that the average for these structures since Miocene times, but not very different from the total shortening rate estimated for the southern Atlas front since then (0.3 mmfy). Seismic reflection profiles show two tectonic styles acting simultaneously within the northern Ouarzazate basin: thick-skinned to the west and thin-skinned to the east. This difference is also reflected on the deformation geometry ofthe Quaternary marker

Rates of Quaternary deformation in the Ouarzazate Basin (Southern Atlas Front, Morocco)

Recent studies have shown that the uplift of the Moroccan Atlas Mountains resulted from a combination of crustal shortening and mantle thermal processes in the interior of the African plate. Within this framework, to gain insight into the dynamics of these processes in recent times, we investigated the nature and rates of the Quaternary deformation in a case field area, namely, the northern Ouarzazate Basin, which contains the best dated records of fan and fluvial terrace deposits of the Atlas system. The area selected is in the southern orogenic front of the High Atlas, where thrusts and fault-related folds have affected a sequence of dated, stepped terraces to varying degrees. Based on cosmogenic nuclide surface exposure dating, we have calculated slow neotectonic deformation rates for the past ~250 ky. We have measured the local rates of structural uplift, yielding 0.2 mm/y in areas of the northern third of the Ouarzazate Basin. We have estimated slip rates at 0.09-0.15 mm/y for the most active fault within the basin. We also document that recent shortening rates (~0.1 mm/y during the last ~250 ky) almost double the average rates from the deformation onset (~0.04 mm/y since the Middle Miocene). These results raise the point of the possible relations between such increases in the deformation rates and the erosional denudation of the Ouarzazate Basin since it became externally drained in the late Pliocene or early Quaternary times.</p