Shear zones as a result of intraplate tectonics in oceanic crust: the example of the Basal complex of fuerteventura (Canary Islands)

Ductile shear zones affecting igneous rocks (pyroxenites, syenites, gabbros and carbonatites) belonging to the Basal Complex of the Island of Fuerteventura are described here. They develop typical mylonites with their cortege of structures and crystallographic fabrics. An evolution from brittle to ductile conditions has been recognized, and interpreted as being due to the ascent of alkaline magmas and related fluids and subsequent emplacement of weak rocks (carbonatites, syenites) along the shear zones. Local metamorphic changes favour the ductile behaviour via a reaction-enhanced softening mechanism. The shear zones are arranged in a nearly orthorhombic pattern. Kinematic criteria suggest an extensional tectonic scenario, with a bulk irrotational nonplane deformation. The long axis of the finite deformation ellipsoid is E-W and horizontal. The available data allow us to establish this deformation as being from the Late Oligocene to the Early Miocene. This is tentatively related to the plate-tectonics evolution and mantle anomalies in the northwestern corner of the African plate.