Inversion tectonics of intracontinental ranges: High and Middle Atlas, Morocco

Abstract

ZusammenfassmlgDer Hohe und der Mittlere Atlas sind intrakontinentale Gebirge im mobilen Vorland des mediterranen Rif-Orogens. Ihre Entwicklung weist drei Perioden auf: Die erste (Perm-Bathonium) kulminierte im Lias mit der Bildung von Riftgr~iben entlang spfitvariskischer Bruchzonen und Tholeiit-Ergtissen. Im Intervall Callovium-Eoz~in deuten tektonische Beruhigung und geringere Sedimentation auf eine allmfihliche Abktihlung der Lithosphfire hin. Seit dem Oligoz~in steht die Region unter Kompression. Der Hohe und der Mittlere Atlas haben sich zeitgleich mit den Hauptphasen der Kompression im Rif herausgehoben. Refraktionsseismische Untersuchungen haben einen fiachen Lagenbau der Kruste mit mehreren low-velocity-Zonen aufgewiesen, deren tiefste mit einer Zone hoher elektrischer Leitf~ihigkeit zusammenffillt und als bedeutende Abscherungszone gedeutet wird.Aus der geotektonischen Entwicklung des Hohen und des Mittleren Atlas und aus der heutigen Krustenstruktur wird folgendes Modell abgeleitet: In der frtthmesozoischen Rift-Phase wurde die Kruste fiber den Mantel-Aufw61bungen dutch Zerrungsbrtiche und durch Zergleiten an subhorizontalen Scherfl~ichen ausgedfinnt. W~ihrend der k~inozoischen Kollisionen im Rif-Atlas wurden diese Scherfl~ichen dann gegenlfiufig bewegt, die Riftgraben-Ftillungen dabei bis zu geringer Krustenverdickung eingeengt und anschlieBend herausgehoben. Die Inversion der beiden Atlas-Gebirge ist somit nicht nur isostatisch bedingt, sonderu auch durch Aufpressung bei thick-and-thin-skinned-Tektonik verursacht. AbstractThe High and Middle Atlas are intracontinental mountain belts situated within the mobile foreland of the Mediterranean Rif orogen. They developed in three stages. The first period (Permian -Bathonian) culminated during the Lias with extended rift grabens and tholeiite extrusions. From Callovian to Eocene, the tectonic activity and the rates of sedimentation were reduced, both pointing to a cooling of the lithosphere. Since the Oligocene, the whole region is submitted to compressional stress. The High and the Middle Atlas were uplifted within two phases, which were correlated with main phases of Rif orogenesis. Refraction seismic measurements have recently revealed there a flat layered structure of the crust with several low velocity zones. The deepest one coincides with a layer of high electric conductivity, which is interpreted as a zone of detachment.From the geotectonic evolution of the High and Middle Atlas and from the structure of the crust, the following model was deduced: During Early Mesozoic rifting, the crust on top of the mantle elevations was thinned by both extensional fracturing and by gliding along intracrustal detachment planes. During the Cenozoic collisions of the Rif, these shear planes were reactivated by thrusting in opposite directions. Compressional deformation of the graben fillings led now to a moderate thicken-*) Authors' address: E GmsE, V. JACOBSHAGEN,

Electrical signature of modern and ancient tectonic processes in the crust of the Atlas mountains of Morocco

“…In this model, Ayarza et al (2005) infer crustal-scale thrusting affecting the Moho and penetrating into the mantle. This interpretation differs from previous ones based on seismic refraction and magnetotelluric data (Wigger et al, 1992;Giese and Jacobshagen, 1992) in which the thrusts were rooted at intracrustal detachment level below the Middle and High Atlas, and all the shortening is transferred to the Rif Chain in northern Morocco. All prior studies indicate that below the High Atlas there is no detectable crustal root, and therefore all models require a buoyancy contribution from subcrustal levels to support the topography of the Atlas mountains.…”

New numerical and theoretical model to characterize the upper crustal structure of the Moroccan atlas from wide-angle seismic reflection data

“…Located at more than 600 km of the Africa-Europe plate boundary, the High and Middle Atlas intracontinental mountain chains feature a really high topography (up to 4000 m). However, shortening, which is basically related to the convergence between Africa and Europe, is moderate, generally less than 20%, [1]- [6].…”

“…All these features, together with the presence of an intriguing alkaline volcanism and a high heat flow [5], make the understanding of the geological evolution of the Atlas Mountains a matter of great interest in the Earth Science research community [1]- [6].…”

Deep-rooted “thick skinned” model for the High Atlas Mountains (Morocco). Implications for the seismic Eurasia-Africa plate boundary region

“…The Atlas Mountains (Middle and High Atlas) of Morocco is considered as an intracontinental range (Mattauer et al, 1977;Laville, 1985;Giese and Jacobshagen, 1992;Beauchamp et al, 1999;Piqué et al, 2002) located in the foreland of the Mediterranean Alpine belt (Betic Cordillera, Rif, Tell, e.g., fig. 1).…”

“…Unfortunately, at present, we have no subsurface data on the prolongation of this "SAF" farther north. Do the flat-lying faults which constitute the overthrust continue far towards the north, under the Atlas, as suggested by Giese and Jacobshagen (1992), or are they actually rooted in a steeply dipping fault system which represents the edge of the pre-existing Mesozoic trough? (El Harfi, 2001;Chorowicz et al, 2001;Piqué et al, 2002).…”