Beauchamp, W., Allmendinger, R., Barazangi, M., Demnati,
A., El Alji, M., and Dahmani, M.
Inversion tectonics and the evolution of the High Atlas Mountains, Morocco,
based on a geological-geophysical transect
Tectonics, 18, 163-184, 1999.
The High Atlas Mountains of North Africa were
formed over a major intracontinental rift system that had extended from what is
now the Atlantic margin of Morocco to the Mediterranean coast of Tunisia. The
Atlas rift system began in the Triassic and was active through the Jurassic. The
inversion phase of the Atlas rift system began in the Early Cretaceous and extended
into the present. The major uplift phase occurred between 30 and 20 Ma (Oligocene-Miocene)
and corresponds to the Alpine orogenic event. The uplift and inversion of the
Atlas rift system resulted in a shortening of the rift basin by a minimum of 36
km. A restoration of the deformed cross section indicates the original Atlas rift
basin was approximately 113 km wide, comparable to the width of the present-day
Red Sea. Synrift and postrift sedimentary rocks were uplifted by the reactivation
of synrift normal faults, with further shortening along newly formed thin-skinned
thrust faults. Structures formed by the reactivation of synrift faults resulted
in structures with different geometries than those created by newly formed fault-bend
and fault-propagation faults. Shortening across the High Atlas Mountains involved
a partitioning of strain, with the greatest magnitude of shortening occurring
along the margins of the High Atlas Mountains.
An edited version of this paper was published by AGU. Copyright 1998 American
Key Figures and Captions
Figure 15. Schematic cross sections showing the tectonic history
of (a) the Atlas synrift phase, (b) the postrift phase, and (c) the final uplift
and inversion of the Atlas rift system to form the present-day Atlas mountains.
Two regional pin lines are shown which are the equivalent to those in Plate
1 (line a) and (line b). The distance between these pin lines after restoration
is approximately 140 km (a), which yields the original width of the Atlas rift
system. Subsidence during the Late Jurassic to Tertiary is also shown (b). Convergence
between the African and Iberian plates in the Tertiary (Miocene-Oligocene) resulted
in the inversion of the Atlas rift basin by bivergent thrusting along the rift
margins at shallow crustal levels over short distances (~10-14 km). Significantly
less shortening in the interior of the mountain belt indicates shortening is
being achieved at middle to lower crustal levels (c).