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, ga bbros and carbonatites) belonging to the Basal Complex of the Island o f Fuerteventura are described here. They develop typical mylonites wit h their cortege of structures and crystallographic fabrics. An evoluti on from brittle to ductile conditions has been recognized, and interpr eted as being due to the ascent of alkaline magmas and related fluids and subsequent emplacement of weak rocks (carbonatites, syenites) alon g the shear zones. Local metamorphic changes favour the ductile behavi our via a reaction-enhanced softening mechanism. The shear zones are a rranged in a nearly orthorhombic pattern. Kinematic criteria suggest a n extensional tectonic scenario, with a bulk irrotational nonplane def ormation. 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 tentat ively related to the plate-tectonics evolution and mantle anomalies in the northwestern corner of the African plate. (C) 1997 Elsevier Scien ce Ltd.