Eric Mercier

The two main steps of the Atlas building and geodynamics of the western Mediterranean

The Atlas system (Morocco, Algeria, and Tunisia) constitutes an important morphologic barrier fringing the Sahara platform. Its structural style changes along strike from a thick-skinned style in Morocco to a thin-skinned one in Algeria and Tunisia. The position relative to the Tell-Rif system is also different in eastern Algeria and Tunisia where the two systems are adjacent and in western Algeria and Morocco where they are separated by large rigid cores (Moroccan Meseta and Algerian High Plateaux). New data, as well as a reappraisal of available data, show that the Atlas build up occurred everywhere during two main phases of late Eocene and Pleistocene-lower Quaternary age, respectively. These phases are clearly distinct and do not represent end points of a progressive deformation. An additional Tortonian event exists in the eastern region where the Tell-Rif is thrusting directly over the Atlas. From Oligocene to middle Miocene the development of the Tell-Rif accretionary prism is coeval to subduction rollback of Maghrebian Tethys lithosphere and related to the opening of the western Mediterranean Sea. For kinematic and chronological reasons this process cannot account for the two specific steps of the Atlas building. They are better explained assuming that they record two jolts in the convergence of Africa with respect to Europe and correspond roughly to the initiation and the cessation of the subduction processes active in the western Mediterranean region.

Geometry and kinematics of the South Atlas Front, Algeria and Tunisia

Abstract The South Atlas Front is the structural boundary between the Sahara platform and the Atlas mountains. It is frequently indicated as a foreland dipping monocline known as the Sahara Flexure. Earlier work generally considered that such a cover structure resulted from direct inversion of basement faults. This assumption is not applicable to the studied area which is situated close to the Algeria-Tunisia border: our study demonstrates a thin-skinned style characterised by ramp-related folds. Ten parallel balanced cross sections give an interpretation for the South Atlas Front from Negrine to Gafsa, about 100 km along strike. The geometry at depth of each cross section is defined by forward modelling of each individual anticline. The depth-to-detachment is defined using the geometric characteristics of the fault-propagation folds well exposed along the deformation front. In all sections, the modelling is consistent with basal decollements located within Cretaceous stratigraphic levels. Lateral changes of the tectonic style accommodated by tear faults are observed. They seem related to lateral variations in the efficiency of the based decollement faults inducing either a jump of the decollement to another level or the development of en-echelon folds. The modelling predicts small rotations of cover sheets separated by large-scale tear faults.

Late-stage evolution of fault-propagation folds: principles and example

Abstract Fault-propagation folding commonly occurs near the fronts of mountain belts. This type of folding may even represent, in some thrust systems, the major mode of deformation. However, fault propagation folds are frequently altered by two kinds of late-stage evolution: breakthrough thrusting or transport on the flat. We model the geometric features of these kinds of fold, in particular the late stage modification, and present the algorithms and equations used in an original program. An application, based on an example from the Atlas mountains of Algeria, illustrates the main features of the model.

Kinematics of large scale tip line folds from the High Atlas thrust belt, Morocco

Abstract The kinematics of several tip line folds situated along the South Atlas Front near Goulmima (Morocco) is established using both forward modelling and analysis of syn-folding deformation. From the Sahara foreland to the Atlas, the two successive major structures forming the mountain front in this area fit geometrically with a ‘generalised fault propagation fold’ model and a model of ‘fault propagation fold’ altered by late transport on the upper flat, respectively. Field evidence shows that the two folds are developed more or less synchronously. The analysis of minor structures helps address the question of migrating hinges vs fixed hinges during the fold’s growth.

Synkinematic emplacement of the Pan-African Ngondo igneous complex (west Cameroon, central Africa)

Abstract The Ngondo Complex is one of the Pan-African plutons intruded in the West Cameroon Pan-African Orogenic Belt. The complex consists of three major groups of rocks: basic to intermediate rocks (diorites, granodiorites and minor gabbros), fined-grained granites and coarsed-grained granites successively emplaced in a metamorphic country rock of amphibolite-facies. Synkinematic emplacement of the complex, in relation with a ductile mega shear zone, is documented by a study of microstructures and foliation patterns which indicate a continuous transition from magmatic to high temperature solid-state deformation. The geometry of the internal foliation trajectories and the joint orientation in the complex suggest that the emplacement of the three groups of rocks was totally controlled by a N30° sinistral shear zone. Emplacement mechanisms, which are related in time and space to a continuum of deformation, may indicate a relative rheological change of the crust from ductile to brittle behaviour.

Magnetic fabrics and oblique ramp-related folding: A case study from the western Taurus (Turkey)

Abstract An analysis of magnetic fabric has been performed in weakly deformed Paleocene-Eocene limestone adjacent to two subparallel ramp-related folds (Akseki and Ormana folds) from the western Taurus (Turkey). The magnetic fabric of tectonic origin records two trends of well-defined magnetic lineation: N160 ° ± 7 ° and N130 ° ± 14 ° in front of the Akseki and Ormana folds, respectively. The trend of the magnetic lineation is oblique to the Akseki fold, whereas in the Ormana fold, magnetic lineation and fold axis are subparallel. The observed change in the trend of magnetic lineation is probably linked to a change of the shortening direction that occurred during the development of the thrust system. This shows that the two subparallel folds result from two different translation vectors.

The thin-skinned style of the South Atlas Front in Central Algeria

Abstract Seismic lines cutting through the southern front of the Sahara Atlas show that the front is not superimposed on a major basement fault. Folded Cretaceous rocks can be observed on these lines to overlie a décollement surface climbing from Triassic to Early Cretaceous, below which flat-lying sediments can be recognized. The structural style is thus interpreted to be thin skinned and the folds underlining the front as ramp- related features. The development of duplexes in the core of some anticlines explains the apparent thickening of Cretaceous and/or Jurassic strata revealed by boreholes. This thickening was previously interpreted to be related to now-inverted extensional half-grabens, a model which cannot now be supported. This new interpretation allows a reassessment of other parts of the Sahara Atlas system, the large-scale structural model for which is that of large half-graben system that has undergone inversion because of shortening between the High Plateau massif and Sahara Platform.