Crustal shortening and vertical strain partitioning in the Middle Atlas Mountains of Morocco
Francisco Gomez, Richard Allmendinger, and Muawia Barazangi
Institute for the Study of the Continents, Cornell University, Ithaca, New York
Department de la Géologie, University Mohammed V, Rabat, Morocco
Direction de la Géologie, Ministere de L'Energie et des Mines, Rabat, Morocco
The NE-SW trending Middle Atlas mountains of
Morocco are obliquely oriented within the Late Cenozoic regional stress
field, resulting in deformation that is partitioned into strike-slip faulting
and thrust-related folding. In the central Middle Atlas, thrusting
is confined to a 20 km wide fold belt between two relatively rigid crustal
blocks that are obliquely converging. We suggest that in addition
to strain partitioning observed in plan view, a partitioning of deformation
between the upper and lower crust may be necessary to reconcile estimated
crustal thickening and horizontal shortening within the fold belt.
Cross section balancing based on field observations demonstrates a relatively
modest amount of Cenozoic horizontal shortening (~ 4.7 km) normal to the
fold belt producing 800 meters of structural relief. Yet, geophysical
data suggest this contraction has not produced a significant crustal root
beneath the fold belt, i.e., the belt does not appear to be isostatically
compensated. Assuming all horizontal shortening was accommodated
by crustal thickening beneath the fold belt implies much greater thickening
than is suggested by constraints on the pre-shortened crustal thickness.
It thus appears that thickening does not accommodate all of the contraction.
We suggest one possible solution: The upper crust shortens by thickening
(faulting and folding) whereas the lower crust deforms laterally.