Foreland basin magmatism in the Western Moroccan Meseta and geodynamic inferences

Martin Roddaz (1) , Stéphane Brusset (1) , Jean-Claude Soula (1) , Didier Béziat (1) , Mohamed Ben Abbou , Pierre Debat (1) , Youssef Driouch , Frédéric Christophoul (1) , Ahmed Ntarmouchant , Joachim Déramond (1)

1 Laboratoire de Géochimie

Basaltic magmatism occurs in the Hercynian foreland basin of the Western Moroccan Meseta as pillow lavas flows interbedded with "flysch" deposits and sills of dolerite and gabbro. A sedimentological and tectonic study shows that the deposition of the flysch, and thus the intrusion of the lava flows and sills, was controlled by the northwestward propagation of thrust-related folds in the wedge-top depozone of a syncontractional foreland basin system. The pillow lavas appear as either massive sheets of stacked lava flows or thin lava flows interlayered with the syntectonic turbiditic deposits. The sills are composed of several mafic units (up to six in the most evolved Marziqallal sill) cut through by granophyric veins, which can be interpreted as a result of in situ crystallization. The pillows lava basalts and the dolerites/gabbros are calc-alkaline and cogenetic. A discussion of the possible mechanisms of magma production in subduction related orogens leads us to conclude that this foreland basin magmatism was generated in a retrolithospheric setting. A comparison with the other Early Carboniferous volcanic and plutonic rocks of the Western and Eastern Mesetas shows that calc-alkaline foreland basin magmatism was widespread in the entire Mesetan domain and was probably a result of wet melting of the metasomatised mantle lithosphere of the overriding plate. Because the Moroccan Hercynides are likely to have been a result of continental subduction and the magmatism is observed at a distance of more than 500 km from the suture, it is suggested that the retroforeland basin magmatism was a result of either dehydration of a shallow dipping (∼15°) foregoing oceanic slab, or underplating of the continental lithosphere ending with slab breakoff beneath the foreland basin.

Mots clés: Tectonophysics: Continental contractional orogenic belts; Tectonophysics: Dynamics of lithosphere and mantle-general; Structural Geology: Folds and folding; Mineralogy; Petrology; and Mineral Physics: Minor and trace element composition; Information Related to Geologic Time: Paleozoic; retroforeland basin; pillow lava basalts; mafic sill; calc-alkaline; magmatism; continental subduction; Moroccan Meseta; Hercynian

Domaines: Planète et Univers [physics]; Sciences de la Terre

Martin Roddaz, Stéphane Brusset, Jean-Claude Soula, Didier Béziat, Mohamed Ben Abbou, et al.. Foreland basin magmatism in the Western Moroccan Meseta and geodynamic inferences. Tectonics, 2002, 21, ⟨10.1029/2001TC901029⟩. ⟨insu-03642959⟩