Sequential development of the metamorphic aureole beneath the Ronda peridotites and its bearing on the tectonic evolution of the Betic Cordillera

Abstract

At Sierra Alpujata (western Betic Cordillera, Spain), the Ronda peridotites overlay a metamorphic sole of felsic composition formed during the hot overthrusting of the ultrabasic rocks over the Ojén nappe. The metamorphic sole evolved from an initial eclogite facies metamorphism ( P ≥ 1.7 GPa; T = 790°C) to low-pressure ( P ≤ 300 MPa) high-temperature ( T ∼ 600°C) conditions. The active movement zone migrated downwards in the metamorphic sole as temperatures decreased during this exhumation process: high temperatures led to strain localization along the 200-m-thick ductile shear zone that forms the uppermost level of the metamorphic sole, and brittle to ductile deformations achieved at temperatures below 500°C are concentrated in the basal contact of the metamorphic sole. The maintenance of a common kinematics, with ENE-trending stretching lineations and top-to-the-ENE shearing, along the high-temperature and the low-temperature shear zones suggests that the main part of the exhumation process was performed before the overall north-south convergence of the Iberian and African plates. The subduction of the Ojén nappe, a wedge of subcontinental lithosphere, below the orogenic lherzolites of Ronda probably gave rise to the syn-collisional exhumation of the metamorphic sole and then to the slab breakoff and sinking of the subducted lithospheric mantle.