Structures and fabrics in a crustal-scale shear zone, Betic Cordillera, SE Spain

Abstract

A broad zone of dominantly ductile high-strain deformation lies beneath the Aguilón nappe in the Sierra Alhamilla, southern Spain. It forms part of a crustal-scale movement zone, traceable through much of the Betic Cordillera, which separates the Higher Betic Nappes from the underlying Nevado-Filabride Complex. The zone is characterized in outcrop by a distinctive platy foliation and a strong NNE-trending stretching lineation. Microstructural characteristics include quartz ribbons, mica fish, augen of feldspar and other minerals in a matrix of dynamically recrystallized quartz, and extensional crenulation cleavages. Narrow bands of ultramylonite and cataclasite occur within and on the margins of the movement zone. Deformation occurred under lower greenschist-facies conditions and was accompanied by retrogression of earlier higher-grade mineral assemblages. Structures in the movement zone developed in a temporal sequence, beginning with isoclinal folding and transposition of older foliations. This was followed by the formation of extensional crenulation cleavages, and the progressive localization of strain into the ultramylonite bands. Mylonitic foliation in these bands is deformed by syn-mylonite folds restricted to the bands. All these structures were then deformed by S- to SE-vergent small-scale folds restricted to the movement zone as a whole. Cataclasis, associated with alteration, is localized along the ultramylonite bands and indicates a transition to late-stage brittle deformation. The lower boundary of the movement zone is gradational: strain decreases, recrystallized grainsize and the degree of recrystallization of quartz increases, and pressure solution becomes the dominant deformation mechanism in mica-schist. Asymmetric quartz fabrics in the movement zone indicate a NNE sense of shear; but variations in the degree of asymmetry suggest that flow was partitioned, with the ultramylonite bands taking up much of the shear strain, and the intervening rocks deforming more slowly and with a lower degree of non-coaxiality. Diffuse fabrics in the fine-grained ultramylonite bands may indicate a switch to a grainsize-sensitive deformation mechanism, and an overall downward increase in the opening angle of crossed-girdle fabrics may reflect increased water activity at depth.