[1] Multichannel seismic reflection images across the transition between the east Alborán and the AlgeroBalearic basins show how crustal thickness decreases from about 5 s two-way traveltime (TWTT, 15 km thick) in the west (east Alborán basin) to 2 s TWTT typical of oceanic crust (6 km thick) in the east (Algero-Balearic basin). We have differentiated three different crustal domains in this transition, mainly on the basis of crustal thickness and seismic signature. Boundaries between the three crustal domains are transitional and lack evidence for major faults. Tilted blocks related to extension are very scarce and all sampled basement outcrops are volcanic, suggesting a strong relationship between magmatism and crustal structure. Stratigraphic correlation of lithoseismic units with sedimentary units of southeastern Betic basins indicates that sediments onlap igneous basement approximately at 12 Ma in the eastern area and at 8 Ma in the western area. Linking seismic crustal structure with magmatic geochemical evidence suggests that the three differentiated crustal domains may represent, from west to east, thin continental crust modified by arc magmatism, magmatic-arc crust, and oceanic crust. Middle to late Miocene arc and oceanic crust formation in the east Alborán and Algero-Balearic basins, respectively, occurred during westward migration of the Gibraltar accretionary wedge and shortening in the Betic-Rif foreland basins. Arc magmatism and associated backarc oceanic crust formation were related to early to middle Miocene subduction and rollback of the Flysch Trough oceanic basement. Subduction of this narrow slab beneath the Alborán basin was coeval with collision of the Alborán domain with the Iberian and African passive margins and subsequent subcontinental-lithosphere edge delamination along the Betic-Rif margins.

The combination of metamorphic petrology tools and in situ laser 40 Ar/ 39 Ar 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°C) from 20 kbar for the Be´dar-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°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.

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International audienceUplift and exhumation of vast exposures of diamond facies, subcontinental mantle peridotite in the Western Mediterranean arc are attributed to tectonic scenarios including pure extension, transpression or subduction followed by delamination-driven or rollback-driven stretching. In the Ronda peridotite (southern Spain) the strong overprint of low-pressure assemblages has precluded accurate determination of the pressure and temperature conditions for the onset of exhumation that formed the spinel tectonite and garnet-spinel mylonite domain in this massif. Here we report unequivocal petrographic evidence for the existence of prekinematic, coarse-grained garnet lherzolite assemblages from the garnet-spinel mylonite domain of the Ronda peridotite. Application of well-calibrated geothermobarometers yields prekinematic minimum equilibration conditions of 2.4-2.7 GPa and 1020-1100 degrees C, demonstrating that the Ronda peridotite equilibrated at similar to 85 km depth before shearing. We also show the existence of synkinematic garnet and spinel assemblages that overprinted garnet lherzolite assemblages at 800-900 degrees C and 1.95-2.00 GPa. The decompressional cooling path and high pressure recorded by garnet-spinel mylonites rule out their formation by near-isobaric cooling above a subduction-collision wedge or during or after the emplacement of the peridotite massif into the crust. Ronda garnet-spinel mylonites represent the vestiges of subcontinental mantle ductile shear zones formed at early stages of lithosphere extension during backarc extension in the western Mediterranean. Southward to westward retreat of the African slab during the Oligocene-Early Miocene accounts for intense backarc lithosphere extension and development of the Ronda extensional shear zone, coeval with extreme thinning of the Alboran domain overlying crust