Transpressive deformation at the boundary between the central and eastern units of the Variscan Jebilet Massif. Implications for the development of the Moroccan Meseta orogen

The Moroccan Meseta orogenic system, part of the Variscides, displays a west-closing curvature that has recently been described as an arc or orocline. The hinge zone of this curvature occurs in the Jebilet Massif of central Morocco, where it is bordered to the west by the Marrakech Shear Zone (MSZ), an NNW-striking sinistral shear zone that forms a terrane boundary between the Eastern and Central Jebilet units, and along which the Sidi Bou Othmane Formation crops out. In order to understand the role of the Moroccan Meseta orocline within the wider Variscan orogen, it is crucial to unravel the tectono-thermal evolution of this terrane boundary. Our results show that deformation evolves from strike-slip along the southern segment of the MSZ, to transpression in its northern segment, and to contraction northwards in the Rehamna Massif. The SBO Formation is intruded by numerous syntectonic pegmatite dykes and affected by a LP-HT metamorphism reaching amphibolite facies conditions. Geothermobarometry of garnet-andalusite-cordierite-bearing metapelites indicates a clockwise pressure–temperature (P-T) path, with peak P-T- conditions at 3.6-3.9 kbar and 565-578°C, followed by a nearly isothermal decompression to P-T conditions of 0.6-1.3 kbar and 530-550°C. 40 Ar/ 39 Ar dating on muscovite obtained on pegmatite and micaschist samples from the SBO Formation shows that metamorphism, shearing deformation and pegmatite magmatism occurred almost simultaneously around 320 Ma. This age, which is older than the age of metamorphism in the Rehamna Massif, as well as the occurrence of strike-slip faults of opposed senses of shear on either side of the Jebilet Massif, indicate that indentation was involved in the development of the Moroccan Meseta curvature. The orocline bending occurred during the dextral transpression between Laurussia and Gondwana, and formation of the Pangea supercontinent.