Basement-cover evolution and deformation sequencing in the Badajoz-Córdoba shear zone, south west Spain

Variscan deformation in the Badajoz-Códoba Shear Zone, a long-lived tectonic lineament in SW Iberia, has the overall appearance of being transpressive in nature. Metamorphic basement and sedimentary cover sequences show, however, this is the result of deformation sequencing involving the repeated superposition of pure and simple shear deformation couplets. Mylonitic basement gneisses in the Badajoz-Códoba Shear Zone record a sequence of temporal deformation partitioning. Limbs of early open to tight, gently plunging folds were exploited during an overprinting simple shear event, which developed the pervasive fabric now evident in outcrop. Asymmetric structures and a sub-horizontal mineral lineation indicate this was of left-lateral wrench nature. Rb-Sr dating on muscovite from mylonites in this sequence gives an age of late Tournaisian-early Visén for the mylonitisation and associated metamorphism (Garcia Casquero et al., 1988). Igneous sheets were emplaced in the basement gneisses during the latter part of this left-lateral shearing episode. They record a metamorphic path from greenschist to amphibolites facies and then retrogressive deformation in greenschist facies. They were emplaced by exploitation of the subvertical foliation in the gneisses. Post-emplacement deformation resulted in both left-lateral shearing and also a further compressive event. Highly retrogressed metasediments demonstrate comparable structures to those in the gneisses. Early open to tight folds were modified and exploited by later sub-vertical strike-slip shear zones. These features are cross-cut by a sequence of later thrusts with a top-to-the northeast sense of movement. Pre-Carboniferous brittle faulting in the basement sequences resulted in the formation of numerous secondary shear zones. These features rotated the earlier structures away from their original trend parallel to the Badajoz-Códoba Shear Zone. Lower Carboniferous cover sequences (turbidite successions) show a single slaty cleavage S1 which is sub-parallel to bedding S0, both being subsequently folded by F2 folds with very limited cleavage development.