Dominique Frizon de Lamotte

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.

Magnetic fabric as a structural indicator of the deformation path within a fold-thrust structure: a test case from the Corbières (NE Pyrenees, France)

Abstract In order to define the deformation path and the distribution of internal deformation in a foreland foldthrust structure, the anisotropy of magnetic susceptibility was measured in 150 samples taken from 11 sites throughout the Vitrollian sequence of the Lagrasse structure (most external Pyrenean Zone). Whereas surrounding rocks seem completely undeformed, the Vitrollian Formation exhibits a complete set of microstructural markers thereby allowing fault striation measurements at sites also sampled for magnetic fabric analysis. Structural analysis shows that two mechanisms acted successively during thrust motion. The first one resulted from a NW-SE layer parallel shortening (LPS). It was responsible for a well defined vertical magnetic foliation parallel to the regional cleavage, in which a vertical magnetic lineation developed in conjunction with an increase in the shortening intensity towards the zone which would subsequently fail, leading to the development of an imbricate thrust system. During this second event, internal deformation was concentrated around the hinge and in the forelimb of the frontal fold. Around the hinge, the magnetic lineation rotated to an horizontal NE-SW trend parallel to the fold axis, indicating a significant component of longitudinal stretching during the development of the frontal ramp-anticline. Subsequently, the magnetic lineation swung again and became parallel to the NW-SE tectonic transport direction beneath out-of-sequence forelimb thrusts. This complex deformation history is not accompanied by a linear increase in the degree of anisotropy. On the contrary, we observe a cycle of construction and destruction of the magnetic fabric during LPS and imbricate thrusting.

The origin of intraplate deformation in the Atlas system of western and central Algeria: from Jurassic rifting to Cenozoic–Quaternary inversion

Abstract Analysis of petroleum exploration data supplemented by paleostress data enabled discussion of the origin of the deformation in the western and central Saharan Atlas (Algeria). This intraplate area has recorded the breakup of Pangea (Late Triassic), the opening of the Maghrebian Tethys (since the Dogger) and subsequently its closure (Oligocene to Present). However, the two periods of strong coupling between Europe and Africa (late Lutetian and Pleistocene), which correspond to rapid uplifts of the Atlas system and important deformations, are not collision-related. They can be correlated to the beginning and the end of the development of the western Mediterranean Sea.

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.

First paleomagnetic evidence for a post-Eocene clockwise rotation of the Western Taurides thrust belt east of the Isparta reentrant (Southwestern Turkey)

Abstract A paleomagnetic study was carried out on 23 sites sampled in Paleocene, Eocene and Lower Miocene sedimentary formations of the Western Taurides belt east of the Isparta reentrant. The paleomagnetic data, based on twelve suitable sites, record a coherent post-Eocene ∼ 40° clockwise rotation throughout the area studied. We interpret this regional rotation as a semi-rigid rotation of the whole Western Taurides belt above a major deep decollement level. Together with previously published data, these new results suggest that the Isparta reentrant would result from diachronous opposite rotations of the two branches: first a late Eocene-Oligocene clockwise rotation of the Akseki-Beysehir Taurides initiating the Tauride arc, and then anticlockwise rotation of the Lycian Taurides, probably during Middle Miocene times.

Magnetic fabrics of Tertiary sandstones from the Arc of Fars (Eastern Zagros, Iran)

An analysis of magnetic fabric (45 sites, 514 samples) has been performed in folded but apparently not penetratively deformed Mio-Pliocene sedimentary rocks (mainly sandstones) from the Arc of Fars (Eastern Zagros, Iran). Different types of magnetic fabrics and shapes of susceptibility ellipsoids were observed from an oblate form reflecting a sedimentary fabric to a prolate form signifying a magnetic lineation of tectonic origin. Classically in such weakly deformed sedimentary rocks the magnetic lineation is perpendicular to the shortening direction and is interpreted as recording a pre-folding layer-parallel shortening. In the studied area, the trends of the magnetic lineation show two clusters oriented ENE-WSW and WNW-ESE. To a first approximation, the magnetic lineation follows the bending of the fold trend. However, a quite systematic obliquity is observed. According to regional geology and to other considerations arising from the magnetic analysis, we consider that the tectonic transport direction is NNW-SSE. The arc pattern observed in the fold and magnetic lineation trends may result either from an inherited structural framework or from post-folding clockwise rotations or both. In any case, our data rule out any interpretation of the whole Arc of Fars as resulting from the superimposition of two non-coaxial folding events.

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 eastern Salt Range and South Potwar Basin (Pakistan): a new scenario

The Salt Range of northern Pakistan represents an emergent thrust front along which the Potwar Plateau has been translated southward. Based on the analysis of subsurface data, we have constructed a balanced cross-section exhibiting a frontal fault-bend fold and a set of detachment folds below the Potwar Plateau. From the restored section, we are able to propose a forward kinematic modelling using the THRUSTPACK software. The main characteristics of the model are the alternation of periods during which deformation was concentrated on the frontal thrust (between 10 and 5 Ma and since 1.9 Ma) and an intervening episode during which deformation was distributed along the whole section (between 5 and 1.9 Ma). The latter phase is marked by development of out-of-sequence structures. The episodes of frontal thrust activation are characterised by high denudation rates. On the contrary, out-of-sequence folding occurred during periods of important sedimentary accumulation. The consequences for petroleum exploration are also discussed. However, timing of the petroleum system is strongly dependent on hypotheses made on basal heat flow.

Style of rifting and the stages of Pangea breakup

Pangea results from the progressive amalgamation of continental blocks achieved at 320 Ma. Assuming that the ancient concept of “active” versus “passive” rifting remains pertinent as end-members of more complex processes, we show that the progressive Pangea breakup occurred through a succession of rifting episodes characterized by different tectonic evolutions. A first episode of passive continental rifting during the Upper Carboniferous and Permian led to the formation of the Neo-Tethys Ocean. Then at the beginning of Triassic times, two short episodes of active rifting associated to the Siberian and Emeishan large igneous provinces (LIPs) failed. The true disintegration of Pangea resulted from (1) a Triassic passive rifting leading to the emplacement of the central Atlantic magmatic province (200 Ma) LIP and the subsequent opening of the central Atlantic Ocean during the lowermost Jurassic and from (2) a Lower Jurassic active rifting triggered by the Karoo-Ferrar LIP (183 Ma), which led to the opening of the West Indian Ocean. The same sequence of passive then active rifting is observed during the Lower Cretaceous with, in between, the Parana-Etendeka LIP at 135 Ma. We show that the relationships between the style of rifts and their breakdown or with the type of resulting margins (as magma poor or magma dominated) are not straightforward. Finally, we discuss the respective role of mantle global warming promoted by continental agglomeration and mantle plumes in the weakening of the continental lithosphere and their roles as rifting triggers.

The Salt Diapirs of the Eastern Fars Province (Zagros, Iran): A Brief Outline of their Past and Present

The salt diapirs of the eastern Fars (Zagros, Iran) have been reexamined in light of their relationships to the regional geodynamic history. The present-day surface morphology of salt diapirs can be divided into six types, which we suggest represent different stages in a long and complex history: type A are buried diapirs, type B high relief active diapirs, type C with salt fountain and glacier, type D similar to type C but with more erosion and with out fountain, type E as dead diapirs and empty crater, and type F linear diapirs generally emerging along faults. We show that nearly all the diapirs of the study area were already active prior to Zagros folding either as emergent diapirs forming islands in the Paleogene to Neogene sea or as buried domes initiated at least by the Permian. They have been reactivated by subsequent tectonic events. At the initiation of Zagros folding, the abundance of emergent diapirs close to the present Persian Gulf coast line weakened the entire sedimentary cover and facilitated the preferential localization of the deformation in a narrow zone. Then, salt-cored detachment folding in the whole eastern Fars Zagros Fold-Thrust Belt reactivated the preexisting domes and allowed salt movement along faults.