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