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

Abstract

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 (6km 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 1.6GPa at 450–500°C to 1.0GPa 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 0.8GPa at 400°C and 0.4GPa at 500°C, while S2 at the top of the unit developed between 0.6GPa at 300°C and 0.2GPa 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 15mm/year, being exhumed along an S-shaped P-T path after peak metamorphic conditions and its detachment from the subducting slab.