CRUSTAL-SCALE DUCTILE THINNING OF THE ALBORAN DOMAIN - STRUCTURAL AND PETROLOGICAL ANALYSIS FROM THE ALPUJARRIDE COMPLEX, BETIC CORDILLERA, S. SPAIN
Stretching lineations approximate the finite extensional strain axis, and in previous studies of the Alpujárride complex are shown to change orientation near Motril, in the central Betics. We conducted a km to mm-scale structural survey which strategically focused on this area. We define a gradual swing in stretching lineation trend and also two regionally identifiable generations of stretching lineation. Syn-sillimanite stretching lineations associated with ENE-directed shear, and syn-andalusite stretching lineations associated with N-directed shear track counterclockwise rotation of the X-strain axis during rapid decompression.
A model of progressive extensional exhumation is proposed for the Alpujárride Complex. Paleogene contractional structures and HP-LT assemblages are overprinted by early Miocene high strain top-to-the ENE extensional shear and LP-HT metamorphism. This produces sheath folds, mylonitic bands and a pervasive S-L fabric; structural features prominent in the western section of the orogen. During continued exhumation and decreasing strain top-to-the N extensional shear is partially controlled by and reworks the previous anisotropic fabric. This produces km-scale recumbent folds and localized zones of new axial planar S-L fabrics in the eastern part of the Alpujárride Complex. As deformation transitions into the brittle field ~N-S orientated extension continues to form conjugate normal fault sets that transect ductile fabrics and exploit fold axial planes late in the tectonic history.
Our findings are inconsistent with tectonic models of syn-contractional or brittle-field dominated exhumation and demonstrate the value of coupling microstructure with petrology in metamorphic terranes.