COOLING AND EXHUMATION OF THE WESTERN BETIC CORDILLERAS, 40AR 39AR THERMOCHRONOLOGICAL CONSTRAINTS ON A COLLAPSED TERRANE

New Ar-40/Ar-39 data on amphiboles, muscovites, biotites and potassium-feldspars from different tectono-metamorphic units of the Western Alpujarrides (Betic Cordilleras, southern Spain) help to constrain the P-T-t evolution of this Alpine collisional belt. During an initial stage of plate convergence between Africa and Eurasia, the Alpujarride metamorphic rocks evolved along increasing pressure-temperature paths, locally reaching eclogitic conditions, but the timing of peak metamorphism is only constrained to be earlier than 25 Ma. In the interval of 25-22 Ma, the Alpujarride rocks underwent strong adiabatic decompression related to the collapse of the previously thickened crust. We propose that the main phase of synmetamorphic ductile deformation and thinning of the metamorphic pile was related to this extensional event. The last step is marked by fast cooling of the hot Alpujarride rocks below 600 degrees C, resulting in a striking convergence of our Ar-40/Ar-39 determinations in the range 19-20 Ma. Cooling rates in the range 100-350 degrees C/m.y. are indicated for this period, associated with exhumation rates of less than 3 km/m.y. We suggest that fast cooling took place primarily as a result of thermal relaxation of the abnormally steep geotherm resulting from extensional tectonics which has the effect of juxtaposing thinned rock bodies with contrasting temperatures along shear zones and faults. The main implication of this data set is that the Western Alpujarrides present a structural and metamorphic development which should be regarded as characteristic of ''collapsed terranes''.