P-T paths and field metamorphic gradients in metapelites of the Calar-Alto unit, key to understand middle to late Miocene syn-orogenic exhumation processes in the Betics

• Autores: Guillermo Booth Rea, José Miguel Martínez Martínez, José Miguel Azañón Hernández
• Localización: Geotemas (Madrid), ISSN 1576-5172, Nº. 10, 2008 (Ejemplar dedicado a: VII Congreso Geológico de España), págs. 325-328
• Idioma: español
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• Resumen

  • The architecture of the Nevado-Filabride complex consists in a nappe-stack formed by the thick Calar Alto unit (7 km thick) sandwiched between the lower less-metamorphic Ragua unit and the upper more-metamorphic Bédar-Macael unit. The stacking of the units was accomplished by ductile flow along basal syn-metamorphic shear zones (several hundred meters thick). Structural analysis of the Calar Alto unit together with the P-T path of samples from both the bottom and top of the unit served us to understand the tectonic significance of this episode of mid-crustal shortening in the context of a strongly extended orogen. Multiequilibrium thermobarometric results indicate that these crustal-shortening structures developed during decompression and heating under greenschist conditions after cooling from the main HP/LT metamorphic conditions registered in the Calar Alto unit by eclogites and by metapelites with kyanite + Mg-rich chloritoid assemblages. Garnets in the eclogites (Lu-Hf) and zircons (U-Pb SHRIMP) have recently been dated as early to middle Miocene. Thus, the later ductile flow and contractive structures observed in the Calar Alto unit formed coeval to the overlying extensional detachments. This combination of mid-crustal shortening and upper-crustal extension resulted in a mode of exhumation where the material rise entails an important horizontal component of displacement, following a complex S-shaped P-T path after the HP metamorphic peak.