TWO PHASES OF CONTINENTAL SUBDUCTION IN THE BETIC OROGEN, WESTERN MEDITERRANEAN

New Lu-Hf growth ages on garnet from high-P rocks in the Betic Cordillera of S Spain indicate two stages of subduction of continental crust, separated or accompanied by large-scale intra-orogenic extension. The Betic orogen includes two major metamorphic complexes, made up mainly of continental crustal rocks, separated by an extensional detachment fault. The structurally higher complex (Alpujarride) shows early high P/T metamorphism followed by a thermal event that accompanied rapid crustal extension in the period 24-18 Ma, leading to the formation of the Alboran Sea basin. The lower complex (Nevado-Filabride) shows eclogite facies metamorphism followed by isothermal decompression.

Garnets from mafic and pelitic garnet granulites structurally overlying the Ronda peridotite massif in the Alpujarride complex give Lu-Hf ages of 36.6±1.2 and 55.5±5.3 Ma respectively, suggesting an Eocene phase of crustal thickening. The difference in age may reflect partial resetting during the early Miocene thermal maximum, which reached 800°C, or a difference in garnet growth rates in rocks of different composition. An Eocene age of contractional deformation is supported by Ar-Ar white mica growth ages as old as 48 Ma in low-grade pelites from the Alpujarride Complex.

Eclogites and pelitic schists from the underlying Nevado-Filabride Complex give garnet Lu-Hf ages of 16.3±1.2, 16.9±0.3, and 18.1±0.9 Ma. Garnet growth therefore immediately postdates the main phase of extension and exhumation of the overlying Alpujarride allochthon. Subduction of the Nevado-Filabride beneath the Alpujarride Complex preceded garnet growth, and hence overlapped extension in the Alpujarride Complex. The two events may have been kinematically linked: extension in the Alpujarride Complex caused or was accommodated by subduction of the Nevado-Filabride Complex. The timing and palaeogeographic setting suggest that the Nevado-Filabride Complex represents the leading edge of the rifted South Iberian margin - its sedimentary cover may now be preserved in the Miocene non-metamorphic NW-vergent thrust belt in the external Betics. Ar-Ar and fission-track cooling ages show that the Nevado-Filabride Complex was rapidly exhumed between 15 and 10 Ma, creating the detachment separating it from the overlying Alpujarride Complex.