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
University Mohammed V, Rabat, Morocco
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.