Are the Middle Atlas Mountains of Morocco an Example of Vertical (as well as Horizontal) Strain Partitioning?

Francisco Gomez, Richard Allmendinger, and Muawia Barazangi
Institute for the Study of the Continents, Snee Hall, Cornell University, Ithaca, New York  14853

Ahmed Er-Raji
University Mohammed V, Rabat, Morocco

Mohammed Dahmani
Ministere de L'Energie et des Mines, Rabat, Morocco

Located within the African foreland of the Alpine collisional belt, the Atlas Mountains of North Africa represent a broad zone of deformation accommodated by reactivating zones of crustal weakness inherited from a past episode of rifting.  In the case of the NE-SW trending Middle Atlas Mountains, the reactivated zone is obliquely oriented with respect to the Late Cenozoic regional stress field, resulting in a "transpressive" mountain belt of 2-3 km elevation, approximately 250 km long and 50 km wide.  In the central Middle Atlas region, where folding is confined to a 20 km wide belt, fault kinematic data and other field evidence suggests that oblique deformation is partitioned into strike-slip faulting and orogen-normal shortening.  We further suggest that partitioning deformation between the upper and lower crust may be necessary to reconcile crustal thickening and horizontal shortening within the fold belt.  Cross-section balancing demonstrates approximately 4.5 km of horizontal shortening, and the analysis of geophysical data suggests that this shortening has not produced a significant crustal root beneath the folded Middle Atlas.  Furthermore, estimates of Mesozoic crustal thinning (based on syn-rift stratigraphic data) are inconsistent with the possibility that the lack of a crustal root simply reflects the subsequent thickening of a rifted crust.  It thus appears that crustal thickening does not accommodate all of the shortening.  We suggest an alternative solution:  The upper crust shortens by thickening (faulting and folding) whereas the lower crust responds by deforming laterally.  This model also predicts asperities at the ends of the system which, in the case of the Middle Atlas, may correspond with the enigmatic Neogene Guercif basin.  On a broader note, other reactivated zones of continental transpression (e.g., the Palmyrides of Syria) may also present evidence for vertical strain partitioning.