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Tectonics of the Western Betics: From mantle extensional exhumation to westward thrusting

Abstract : The thesis focuses on the Western Betics, which is characterized by two major thrusts: 1/ the Internal/External Zone Boundary limits the internal metamorphic domain (Alboran Domain) from the fold-and-thrust belts inthe External Zone, and 2/ the Ronda Peridotites Thrust allows the juxtaposition of a hyperstretched lithosphere with large bodies of sub-continental mantle rocks on top of upper crustal rocks. First part: New structural data are presented and used to argue for two Lower Miocene E-W-trending strike-slip corridors played a major role in the deformation pattern of the Alboran Domain, in which E-W dextral strike-slip faults, N60°-trending thrusts and N140°-trending normal faults developed simultaneously during dextral strike-slip simple shear. The inferred continuous westward translation of the Alboran Domain is accommodated by a major E-W-trending lateral ramp (strike-slip) and a N60°-trending frontal thrust. At lithosphere-scale, we interpret the observed deformation pattern as the upper-plate expression of a lateral slab tear and of its westward propagation since Lower Miocene. The crustal emplacement of the Ronda Peridotites occurred at the onset of this westward motion. Second part: New structural data together with Ar-Ar ages serve to document the changes in deformation processes that accommodate the progressive necking of a continental lithosphere. We identify three main successive steps. First, a mid-crustal shear zone and a crust-mantle shear zone accommodate ductile crust thinning and ascent of the sub-continental mantle. The shear zones act synchronously but with opposite senses of shear, top-to-W and top-to-E respectively in the crust-mantle extensional shear zone, and at the brittle-ductile transition in the crust. Second, hyper-stretching localizes in the neck, leading to an almost disappearance of the ductile crust and to crustal stretching values larger than 2000%, and bringing the upper crust into contact with the subcontinental mantle, each of them with theiralready acquired opposite senses of shear. Finally, high-angle normal faulting, dated by 40Ar-39Ar step-heating method on muscovite at ca. 21 Ma, cut through the Moho, where the ductile crust almost disappear and related block tilting ends the full exhumation of mantle in the zone of localized stretching. Third part: New geochronological data precisely constrain the transition from rifting to thrusting. Using U-Pb LA-ICP-MS dating, we identify two distinct episodes of crustal melting associated with two large-scale tectonic contacts that bound the Ronda Peridotites. The first episode of partial melting within the HT foliation at ca. 22.5 Ma is related to the extreme thinning of the continental crust and to mantle exhumation. The second episode of crustal melting at ca. 20 Ma, marked by leucocratic granite dikes, is related to the thrust emplacement of the section of thinned and hot continental lithosphere on top of crustal rocks.
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Gianluca Frasca. Tectonics of the Western Betics: From mantle extensional exhumation to westward thrusting. Tectonics. Université de Rennes 1, 2015. English. ⟨tel-02404302⟩

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