Exhumation, doming and slab retreat in the Betic Cordillera (SE Spain):: in situ 40Ar/39Ar ages and P-T-d-t paths for the Nevado-Filabride complex

The combination of metamorphic petrology tools and in situ laser 40Ar/39Ar dating on phengite (linking time of growth, compositions and P-T conditions) enables us to identify a detailed P-T-d-t path for the still debated tectonometamorphic evolution of the Nevado-Filabride complex and infer new geodynamic-scale constraints. Our data show an isothermal decompression (at 550 degrees C) from 20 kbar for the Bedar-Macael unit and 14 kbar for the Calar Alto unit down to c. 3-4 kbar for both units at 2.8 mm year(-1). At 22-18 Ma, this first part of the exhumation is followed by a final exhumation at 0.6 mm year(-1) along a high-temperature low-pressure (HTLP) gradient of c. 60 degrees C km(-1). The age of the peak of pressure is not precisely known but it is shown that it is around 30 Ma and possibly older, which is at variance with recent models suggesting a younger age for high-pressure (HP) metamorphism. Most of the exhumation is related to late-orogenic extension from c. 30 to 22-18 Ma. Thus the formation of the main ductile extensional shear zone, the Filabres Shear Zone (FSZ), occurred at 22-18 Ma and is clearly associated with a top-to-the-west shear sense once the FSZ is well localized. The transition from ductile to brittle then occurred at c. 14 Ma. The final exhumation, accommodated by brittle deformation, occurred from c. 14 to 9 Ma and was accompanied, from 12 to 8 Ma, by the formation of nearby extensional basins. The duration of the extensional process is c. 20 Myr which argues in favour of a progressive slab retreat from c. 30 to 9 Ma. The change in the shape of the P-T path at 22-18 Ma together with strain localization along the main top-to-the-west shear zone suggests that this date corresponds to a change in the direction of slab retreat from southwards to westwards.