Executive Summary:

Title: Structural evolution of the Algerian Saharan Atlas

The Saharan Atlas of Algeria is an intracratonic mountain belt developed at the southern margin of Tethys. Inversion, transpression and thin skinned thrusting have all been invoked to explain the structure of this area. A new structural model is developed for this mountain belt using Landsat TM images, seismic, well and gravity data together with scaled analogue models.

Landsat TM scenes jointly with existing geological maps were used to interpret regional and individual structures across the Saharan Atlas. This mountain belt is segmented across NNW-SSE diffuse zones. They delimit 3 main structural domains across which occur changes in the width of the folded zone. The folds are en-échelon mostly double plunging and segmented. They are dominantly oriented N45_ east which is oblique to the general N70_ east orientation of the Saharan Atlas. A large proportion of folds are asymmetric and show random changes in polarity along strike. The dominant fold population has a length of 7.5 km and a width varying from 2.5 km to 5 km.

Gravity and aeromagnetic data confirm the deep origin of the segmentation observed across the Saharan Atlas. A seismic data set limited to the flanks of this mountain belt was interpreted. It shows Triassic-Jurassic extensional structures which are partly inverted. The shortening was locally accommodated through intra-Cretaceous thrust-related folds.

A systematic set of analogue models experiments were carried out to simulate the 3D geometry of segmented rift systems and their contraction. Segmented en-échelon faults were produced during the rifting stage. They change in polarity along strike across well developed accommodation zones characterised by overlapping conjugate fault arrays. The contraction of these rift systems resulted in a very limited reactivation of the previous extensional faults. New thrust faults were produced with the increase in contraction. They develop at the tips of the underlying extensional faults. There are striking similarities between analogue models of contracted oblique rift systems and the Saharan Atlas.

Two main tectonic stages can be identified in the evolution of the Saharan Atlas :

• Oblique rifting from the Triassic to Jurassic which was induced by the SE drift of the African plate. The NE-SW Hercynian faults were reactivated as extensional faults and the NNW-SSE Pan-African grain acted as broad zones of deformation which segmented the Saharan Atlas.

• Progressive inversion of this rift architecture which started from the Upper Senonian and reached its paroxysm during the Middle to Upper Eocene. The Triassic evaporites and other Cretaceous ductile layers played a major role in the deformation of this mountain belt. Local strain partitioning resulted in minor strike slip faults.