Attenuation and excision of a crustal section during extensional exhumation: the Carratraca Massif, Betic Cordillera, southern Spain

Abstract

Extensional dismemberment of the Betic Cordillera during Late Oligocene to Early Miocene time caused attenuation and excision of a crustal section overlying mantle peridotite. An integrated study of metapelites overlying the peridotite sheet in the Carratraca area shows truncation of a typical orogenic cycle of burial and heating by rapid exhumation and late- to post-tectonic, low-pressure metamorphism. Attenuation by a factor of five or more condensed a greenschist to granulite section to 4 km structural thickness. Initially coaxial attenuation of the sequence, involving formation of a flat-lying foliation and conjugate shear bands, was superseded by non-coaxial deformation concentrated along low-angle, top-to-the-NE, extensional fault zones. These excised parts of the metamorphic section; a 2 km loss was calculated across one fault zone. Thermobarometry throughout this section demonstrates that the structural attenuation was related to isothermal decompression to low pressures (2-4 kbar) even at the base of the crustal section. A late thermal event largely postdates the ductile stage of attenuation, particularly in the upper part of the section. Thermal modelling implies that this thermal event could not have been produced by the hot peridotite sheet alone. Rapid tectonic exhumation and asthenospheric upwelling are the probable causes, following detachment of material from the base of the lithosphere.