Lateral variations of pressure-temperature evolution in non-cylindrical orogens and 3-D subduction dynamics: the Betic-Rif Cordillera example

Abstract

The long-term Pressure-Temperature-time-deformation (P-T-t-d) evolution of the internal zones of orogens results from complex interactions between the subducting lithosphere, the overriding plate and the intervening asthenosphere. 2-D numerical models successfully reproduce natural P-T-t-d paths, but most orogens are non-cylindrical and the situation is far more complex due to 3-D pre-orogenic inheritance and 3-D subduction dynamics. The Mediterranean orogens are intrinsically non-cylindrical. Their 3-D geometry results from the complex shape of the Eurasian and African margins before convergence and from the dynamics of slab retreat and tearing leading to strongly arcuate belts. More than many other segments, the Betic-Rif belt is archetypal of this behavior. A synthesis of the tectonometamorphic evolution of the Internal Zones, also based on recent findings by our group in the framework of the Orogen Project (Alboran domain, including the Alpujarride-Sebtide and Nevado-Filabride complexes) shows the relations in space and time between tectonic and P-T evolutions. The reinterpretation of the contact between peridotite massifs and Mesozoic sediments as an extensional detachment leads to a discussion of the geodynamic setting and timing of mantle exhumation. Based on new Ar-40/Ar-39 ages in the Alpujarride-Sebtide complex and a discussion of published ages in the Nevado-Filabride complex, we conclude that the age of the HP-LT metamorphism is Eocene in all complexes. A first-order observation is the contrast between the well-preserved Eocene HP-LT blueschists-facies rocks of the eastern Alpujarride-Sebtide Complex and the younger HT-LP conditions reaching partial melting recorded in the Western Alpujarride. We propose a model where the large longitudinal variations in the P-T evolution are mainly due to (i) differences in the timing of subduction and exhumation, (ii) the nature of the subducting lithosphere and (iii) a major change in subduction dynamics at similar to 20 Ma associated with a slab-tearing event. The clustering of radiometric ages obtained with different methods around 20 Ma results from a regional exhumation episode coeval with slab tearing, westward migration of the trench, back-arc extension and thrusting of the whole orogen onto the African and Iberian margins.