Continental subduction, intracrustal shortening, and coeval upper-crustal extension: P-T evolution of subducted south Iberian paleomargin metapelites (Betics, SE Spain)

- The Nevado-Filabride rocks were exhumed by sequential extension in the upper crust.
- Extension in the upper crust was coeval to mid-crustal shortening.
- New P-T paths show isobaric cooling by 250° after peak metamorphic conditions.
- Cooling occurred during underplating of the Ragua unit at lower-crustal depths.
- Mylonites and S2 cleavage register decompression and heating during mid-crustal shortening.

The Nevado-Filabride complex represents the basement and Permo-Triassic cover of the south Iberian passive margin, metamorphosed during Miocene continental subduction under the Alboran crustal domain. It forms a nappe stack with the thick Calar-Alto unit (6 km thick) sandwiched between the Ragua unit below and the Bédar-Macael unit above. The nappe stacking occurred by ductile flow along basal syn-metamorphic shear zones. Structural analysis of the Calar-Alto unit together with thermobarometric data of samples from both the bottom and top of the unit served us to understand the tectonic significance of this episode of intracrustal shortening in the context of a strongly extended orogen. Multiequilibrium thermobarometry shows that the Calar-Alto schist underwent a prograde P-T path at mantle depths from 1.6 GPa at 450-500 °C to 1.0 GPa at 550-570 °C during the growth of the schistosity (S1), parallel to the compositional layering. After detachment from the downgoing slab and underplating, the unit underwent nearly isobaric cooling by up to 250 °C from peak metamorphic conditions. The main crenulation cleavage (S2) and the mylonitic foliation (Sm) developed within the crustal nappe stack during decompression and heating. Sm at the base of the unit grew between 0.8 GPa at 400 °C and 0.4 GPa at 500 °C, while S2 at the top of the unit developed between 0.6 GPa at 300 °C and 0.2 GPa at 400 °C. These shortening structures developed simultaneously to the activity of overlying middle Miocene extensional detachments. Intracrustal shortening and coeval upper-crustal extension resulted in a mode of exhumation where the material rise entailed an important horizontal component of displacement at rates of approximately 15 mm/year, being exhumed along an S-shaped P-T path after peak metamorphic conditions and its detachment from the subducting slab.