Evolution sedimentaire et tectonique de la partie orientale de massif ancien du Haut-Atlas (Maroc)

SIGLEINIST T 71602 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

The Atlas-Meseta Red Beds basin (Morocco) and the Lower Ordovician rifting of NW-Gondwana

The transition from the Cambrian to Ordovician in Morocco is known to be characterized by a frequent Furongian hiatus, restricted extension of the Tremadocian marine deposits, and frequent unconformities at the base of the transgressive upper Floian deposits. In the present work, we first highlight the occurrence of Fe- and mica-rich, red silty/sandy formations in the Central and Eastern High Atlas between the Middle Cambrian and Upper Floian sequences. In the Tislyt type-locality, a synsedimentary hemigraben structure is defined, within which the red beds show frequent slump folds, debris flows and internal unconformities. The correlation with several coeval series of the Meseta domain allows us to define a shallow marine, ferruginous clastic Atlas-Meseta Red Beds (AMRB) basin during the Tremadocian-early Floian. The AMRB basin extended between the Meseta coastal block and the Anti-Atlas domain, being limited by the fault zones that became the West Meseta shear zone and the South Meseta fault, respectively, in the Variscan orogen. The AMRB basin compares with the coeval rifted basins of the central Iberian and Armorican massifs. The red beds were likely sourced from the east, from both the Precambrian basement and Early Ordovician magmatic rocks, contrary to the Ordovician deposits of the Sahara platform sourced from the south. Subsidence of the AMRB and central Iberian-Armorican basins of the NW-Gondwana border aborted during the Floian, whereas the opening of the Rheic ocean went on more to the west

Aspects and origins of fractured dip-domain boundaries in folded carbonate rocks

We present comparative field studies in folded areas (Southern France. Moroccan Western Atlas and Abruzzo, Italy) giving new insights into fracture distribution within folded rocks of the shallow brittle crust. We show that the curvature in folds formed in brittle mechanical units is usually accommodated by multiple "dip-domain boundaries" (appearing as curvature discontinuities at fold scale) corresponding to relatively narrow and dense fracture zones, striking parallel or slightly oblique to the fold axis. They separate "dip-domains" where curvature is absent or moderate. It is shown that the dip-domain boundaries (which are obvious in the case of kink folds or box-fold anticlines) are currently present as multiple subtle hinges even when the curvature appears continuous at first sight. The nature of dip-domain boundaries is studied, they often cut through the whole thickness of the mechanical units Their internal structure varies, and a non-exhaustive typology is proposed For each type, an interpretative kinematic scenario shows how the dip-domain boundaries could initiate and develop. We suggest two kinds of origins (1) they could correspond to the reactivation of inherited, along-strike fracture zones (opening-mode fracture concentrations such as big joints, fracture corridors. Inherited faults, etc), (2) they could be created as mechanical instabilities during the fold formation (syn-folding origin), in particular through small reverse faults. In both cases, early zones of weakness localize the dip-domain boundaries, and control the Increase in curvature in association with increasing fracture density within the boundary. Because they represent well-defined vertically and axially persistent sub-seismic fracture zones generally limited to the thickness of the folded unit, dip-domain boundaries could enhance the axial permeability of folded and fractured reservoirs

New structural and RSCM thermometric data from the Variscan Orogen of Morocco: insight into the extension-to-compression transition

International audienceThe Western Moroccan Meseta exposes in the Zaian region a pre-12 Visean, Eo-Variscan belt of inclined to recumbent folds detached from their 13 Precambrian basement and embedded in the Variscan Orogen. Using the Raman 14 Spectroscopy of Carbonaceous Material (RSCM) method, we obtained the first 15 quantitative thermometric characterization of the Cambro-Ordovician and un-16 conformable Upper Visean rocks in the area. The pre-Visean fold belt devel-17 oped in high-T conditions (mean value 323±23°C) contrary to the juxtaposed 18 Variscan folds that formed at lower T (209±37°C). The high geotherm of the 19 Paleozoic series during folding is likely inherited from the Late Devonian ex-20 tensional setting, but the fold belt emerged through compression, probably dur-21 ing the Devonian-Tournaisian transition. 2

New structural and RSCM thermometric data from the Variscan Orogen of Morocco: insight into the extension-to-compression transition

International audienceThe Western Moroccan Meseta exposes in the Zaian region a pre-12 Visean, Eo-Variscan belt of inclined to recumbent folds detached from their 13 Precambrian basement and embedded in the Variscan Orogen. Using the Raman 14 Spectroscopy of Carbonaceous Material (RSCM) method, we obtained the first 15 quantitative thermometric characterization of the Cambro-Ordovician and un-16 conformable Upper Visean rocks in the area. The pre-Visean fold belt devel-17 oped in high-T conditions (mean value 323±23°C) contrary to the juxtaposed 18 Variscan folds that formed at lower T (209±37°C). The high geotherm of the 19 Paleozoic series during folding is likely inherited from the Late Devonian ex-20 tensional setting, but the fold belt emerged through compression, probably dur-21 ing the Devonian-Tournaisian transition. 2