Evolution tectonothermale du massif hercynien des Rehamna (zone centre-mesetienne, Maroc)

No abstract available in EnglishL’évolution thermique du massif des Rehamna est associée à trois phases de déformation durant lesquelles se sont opérées deux épisodes tectonothermaux successifs : un épisode M1 prograde d’affinité barroviennes et un autre M2 rétrograde.La phase de déformation D1 et la majeure partie de la déformation D2 correspondent à un évènement tectonique compressif qui est responsable de l’engrossissement crustal du massif. A ce régime tectonique en compression s’est surimposé un autre, extensif, durant les stades tarifs-D2. Les stades initiaux de la tectonique extensive coïncident avec le pic thermique régional qui s’est établi dans la zone (ou localités) à staurotide et disthène (Zone de cisaillement de Ouled Zednes et la zone de cisaillement de Kef El Mouneb) ou dans la zone à staurotide (formation des Skhour et Rehamna orientaux). Les conditions du paroxysme thermique régional dans la formation des Skhour sont de 492±5 °C à 8.15±0.01 Kbars dans la zone à grenat, et 560±6 °C à 8.65±0.03 Kbars dans la zone à staurotide ; celles du secteur oriental sont de 420±10 °C à 3.8±0.9 Kbars dans la zone à biotite, 448±7.8 °C à 5.52±0.07 Kbars dans la zone à grenat, et 532±28 °C à 7±0.05 Kbars dans la zone à staurotide.L’épisode tectonothermal rétrograde, correspondant à un refroidissement décompressif, prend ensuite le relais durant les stades tardifs de la tectonique extensive et au cours de la troisième phase de plissement F3. Ces phases de déformation sont à l’origine de la dénudation et du soulèvement de l’édifice structural qui a regagné des conditions relativement superficielles correspondant à la partie supérieur du faciès des schistes verts.Les trajectoires P-T-t, quantifiées à partir des études thermobaromètriques, sont typiques d’une portion de croute moyenne d’un fragment de chaîne de collision qui a subi, conformément à l’évolution structurale par plissement et chevauchement au cours de la phase compressive, un engrossissement crustal de type homogène, auquel est surimposé un amincissement crustal par un régime tectonique extensif et, en fin de compte, un soulèvement, combiné à l’action de l’érosion, d’ampleur modérée

Precambrian isotopic sources of the Anti-Atlas, Morocco

The isotopic data stored in detrital and magmatic zircons are crucial for assessing magma sources, terrane correlation, paleogeography and plate reconstructions. In many cases the comparison of the zircon age and isotope signature of a terrane of unknown provenance with the signature of possible sources, generally old cratonic areas, can resolve questions of origin and paleoposition. Obviously, a precise knowledge of the zircon characteristics of these old areas is essential for reliable comparisons. One of the major sources of sediments of the peri-Gondwanan terranes and of the European Variscan Belt is the West African craton. The northern boundary of this craton is the Pan- African Anti-Atlas belt, which is therefore an ideal place to better constrain the zircon isotopic features of sediments sourced from it. With that aim, we obtained LA-ICM-MS U-Pb and Hf isotopic data of more than 600 zircons separated from six samples of siliciclastic sedimentary rocks from the main Neoproterozoic stratigraphic units of the Anti-Atlas belt, from the Sirwa and Zenaga inliers. The data suggest that the north part of the West African craton formed during three cycles of juvenile crust formation, with variable amount of reworking of older crust. The youngest group of zircons, with a main population clustering around 610 Ma, has a predominantly juvenile character and evidence of moderate mixing with Paleoproterozoic crust, suggesting that the igneous and metamorphic rocks in which the zircons originally crystallized were formed in an ensialic magmatic arc environment. A group of zircons with ages in the range 1.79–2.3 Ga corresponds to the major crust forming event in the West African craton: the Eburnian- Birimian orogeny. The isotopic data indicate that the provenance area should represent a crustal domain that separated from a mantle reservoir at ∼2050–2300 Ma, and further evolved with a time-integrated 176Lu/177Hf of ∼0.013, characteristic of continental crust. The evolution of the Eburnian orogeny is apparently dominated by new crust formation in a magmatic arc environment. The Lower Paleoproterozoic and Neoarchean evolution (2.3–2.75 Ga) involves a group of detrital zircon ages that has not been identified up to now in the igneous or metamorphic rocks of the north West African craton basement. Their Hf isotopic signature points to reworking of juvenile crust mixed with moderate amounts of Archean crust. The significance of these ages is uncertain: they could represent a tectonothermal event not discovered yet in the Reguibat Shield or the zircons can be far-travelled from an unknown source.Peer Reviewe

New U-Pb zircon dating of Late Neoproterozoic magmatism in Western Meseta (Morocco)

We present new U-Pb zircon ages from magmatic rocks of the Western Meseta, part of the Moroccan Variscan belt. The Neoproterozoic–Cambrian stratigraphy in the region of Goäida (Aguelmous massif, SE of Moroccan Central Massif) consists of limestones with conglomeratic and felsic volcaniclastic levels, pelites and mafic volcanic rocks assigned to the Cambrian which unconformably overlie rhyolites, andesites and rhyodacites and felsic tuffs associated with the Aguelmous granite of probable Neoproterozoic age. The Neoproterozoic–Cambrian stratigraphy of the region of Sidi Ali is roughly similar and also includes a volcanic-sedimentary complex with limestones, arkoses rhyolites and conglomerates. These conglomerates contain pebbles of granite and rhyolite, whose source may be the Neoproterozoic basement. In order to constrain the age of the Precambrian felsic magmatism we sampled and dated zircons (LA-ICPMS) from the Goäida granite and a pebble of granite included in the conglomeratic levels of the volcanicsedimentary complex at Sidi Ali dome (central Rehamna massif). In the Goäida granite, zircon ages are Ediacaran ranging between ~ 610 Ma and ~ 540 Ma, with a discordia upper intercept age of 598±32 Ma (MSWD=0.04) that could be interpreted as the age of intrusion. However, if we consider only the two youngest ages we obtain a Concordia age of 590±3 Ma (MSWD= 0.34). In the Sidi Ali pebble sample the majority of zircon ages are Cryogenian–Ediacaran in the range ~ 640–600 Ma, with the youngest yielding a Concordia age of 609±2 Ma (MSWD=0.04), indicating the intrusion age of the granite from which the pebble derived. These results support the existence of Cadomian/Pan-African magmatism in the Western Meseta of Morocco, characterized by the intrusion of granites at ~ 609 Ma and ~ 590 Ma. This result is important for studies of sedimentary provenance and to improve paleogeographic reconstructions of the northern margin of Gondwana during the Neoproterozoic

Experimental investigation of surface roughness effect on fatigue performance of AISI 1045 carbon steel and fatigue limit prediction

In fatigue life, surface integrity perform a major role to determine the fatigue lifetime. It could differ greatly among specific cases even for the same solicitations. Indeed, the manufacturing process carries modifications on surface state, residual stresses and the microstructure which affects the fatigue behavior of mechanical parts.This article investigate the effect of surface roughness on the fatigue strength of AISI 1045 carbon steel obtained by the turning process. The work carried out consists in testing in uniaxial fatigue different batches of specimens which have a controlled surface state. The S-N curves obtained present a strong dependence between fatigue performance and surface state. Based on the surface topography, a model for predicting fatigue limit will be adopted

Experimental investigation of surface roughness effect on fatigue performance of AISI 1045 carbon steel and fatigue limit prediction

In fatigue life, surface integrity perform a major role to determine the fatigue lifetime. It could differ greatly among specific cases even for the same solicitations. Indeed, the manufacturing process carries modifications on surface state, residual stresses and the microstructure which affects the fatigue behavior of mechanical parts. This article investigate the effect of surface roughness on the fatigue strength of AISI 1045 carbon steel obtained by the turning process. The work carried out consists in testing in uniaxial fatigue different batches of specimens which have a controlled surface state. The S-N curves obtained present a strong dependence between fatigue performance and surface state. Based on the surface topography, a model for predicting fatigue limit will be adopted

Tectonic evolution of the Rehamna metamorphic dome (Morocco) in the context of the Alleghanian-Variscan orogeny

Structural and 40Ar/39Ar geochronological investigations of the Rehamna Massif (Meseta, Moroccan Variscan belt) provide new constraints on the tectonic evolution of the Alleghanian-Variscan orogen during the Upper Paleozoic. Three main tectonic events have been recognized: (1) Southward thrusting of an Ordovician sequence over the Proterozoic basement, its Cambrian sedimentary cover, and the overlying Devono-Carboniferous basin. This event caused subhorizontal shearing and prograde Barrovian metamorphism of the buried rocks. (2) Continuous shortening resulting in the development of a synconvergent extrusion of metamorphosed units to form a dome elongated E-W. This was responsible for synconvergent detachment of the Ordovician upper crustal sequence. The timing of these two episodes is constrained to 310–295 Ma by cooling and metamorphic amphibole and mica ages (3) A NW-WNW convergence in a direction orthogonal to the previous one and characterized by the accretion of the Rehamna dome to the continental basement in the east. Based on 40Ar/39Ar cooling ages from a syntectonic granitoid and its host rocks and metamorphic 40Ar/39Ar ages from greenschist facies mylonite, the timing of this event falls between 295 and 280 Ma. The end of the Variscan orogeny in the Moroccan Meseta is constrained by the 40Ar/39Ar cooling age of a posttectonic pluton dated at ~275 Ma. The tectonic events highlighted in Morocco coincide with the late Variscan-Alleghanian tectonic evolution of southern Europe and North America and can be correlated with the global reorganization of plates that accompanied suturing of Pangaea at around 295 Ma.Peer reviewe