Organitzat conjuntament CSIC-UB
Dominique Frizon de Lamotte, University of Cergy-Pontoise, França
Dimecres, 20 de desembre de 2006,Aula magna de la Facultat de Geologia, a les 12h

Comentaris:The High Atlas belt of Morocco is an intracontinental belt that supports the second highest peak of Africa (Jebel Toubkal, 4165 m). The lack of a significant crustal root under the belt, a consequence of limited shortening (~20 km), implies that another process takes place to explain such elevations.
Geophysical modelling allows us to propose that a lithospheric thinning is located under the belt. In this study we investigate the respective roles of crustal tectonic shortening and asthenospheric processes on the topography of the High Atlas and surrounding areas (Morocco). Lithospheric cross-sections allow us to show that a lithospheric thinning explain 1000 m of topography in the Anti-Atlas, the Central High Atlas, the Middle Atlas. Some of the foreland basins are also affected, like the Souss, Ouarzazate and Missour basins. The thinned area is an elongated stripe running North East / South West and crossing over the main structural domains of Morocco and probably the Africa-Europe plate boundary.
The ages of these processes still remain under debate. In this study, we combine the regional geology data of the High Atlas belt and its borders with new fission track ages to constrain the timing of these events.
The thermochronological results indicate that a denudation phase, in relation with the thinning of the lithosphere, occurred on the Siroua Plateau. The available geological data in the neighbouring basins and on the plateau itself allow us to constrain the age of the eroded material and to propose a Lower to Middle Miocene age for the lithospheric thinning. This largescale uplift is also recorded by the samples in the northern part of the belt and their ages are comprised between 9 and 17 Ma. This event takes place between two main crustal shortening events. The first one is Upper Eocene to Oligocene in age and is recorded by the samples taken in the southern part of the belt, which exhibit a 25 Ma fission track age. The second one occured in Plio- Quaternary times and is still active today.