Active tectonism in the intracontinental Middle Atlas Mountains of Morocco : Synchronous crustal shortening and extension
Francisco Gomez, Muawia Barazangi
Institute for the Study of the Continents and Department of Geological Sciences, Cornell University, Ithaca, Snee Hall, New York 14853
Direction de la Geologie, Ministere de L'Energie et des Mines, Agdal-Instituts, B.P. 6208, Rabat, Morocco
Geological field observations are integrated with
digital topography, LANDSAT imagery, and earthquake focal mechanisms to
investigate the Middle and Late Quaternary tectonism in the intracontinental
Middle Atlas mountain belt in northern Morocco. The NE/SW-trending
Middle Atlas Mountains, approximately 80 km in width and about 200 km long,
are part of the Atlas system of northwestern Africa and represent an inverted
rift that developed into an intracratonic mountain system in the foreland
of the Alpine collisional zone. The Middle Atlas is composed of two
provinces, the Folded and Tabular Middle Atlas, representing the palaeo-rift
and a flank of the palaeo-rift, respectively. Evidence for Late Quaternary
tectonism is provided by the analysis of stream morphology in addition
to geological relations. Kinematic analysis of fault-slip data and
earthquake focal mechanisms demonstrate the coexistence of both extensional
and compressional deformation in different areas of the Middle Atlas with
a common sinistral component of slip along NE/SW-striking fault zones.
The compressional features dominate the Folded Middle Atlas, whereas the
extension predominates in the Tabular Middle Atlas. Extension is
also manifested by widespread Middle to Late Quaternary alkali volcanism.
The observed kinematic variations appear to correlate with the Mesozoic
palaeogeography; one possible model may involve tectonic escape.
This suggests that differences in the structures inherited from the Mesozoic
and Palaeozoic may influence the responses of the different regions to
the Cenozoic Alpine collision between Europe and northwest Africa.