Development of local orthorhombic fabric within a simple-shear dominated sinistral transpression zone: the Arronches sheared gneisses

Abstract

The Coimbra-Cordoba shear zone (Iberian Massif), characterized by simple-shear dominated sinistral transpression, exposes several outcrops of strongly sheared peralkaline gneisses surrounded by mica schists and amphibolites. These gneisses are included in the Arronches Tectonic Unit, a thick unit of mylonitic rocks with a steep foliation and associated gently plunging stretching lineation parallel to fold axes. Strain partitioning is testified by widely spaced anastomosed shear bands around less-strained domains and by the existence of different shearing domains ranging from the relatively “less-strained” and coarse-grained mylonites to highly strained and fine-grained ultramylonites. Three shearing domains defined by textural and structural changes resulted from progressive deformation and increasing strain, which leads to increased mylonitization of gneisses. This is revealed by increased modal percentage of matrix and decreased percentage of porphyroclasts accompanied by an evolution from orthorhombic to monoclinic fabrics: Conjugate Shearing Domain (CSD), Intermediate Sinistral Domain (ISD), and Sinistral Domain (SD). This contribution shows that in a simple-shear sinistral dominated transpression zone with a well-developed and widespread monoclinic fabric, it is possible to find mechanical conditions to produce local orthorhombic fabrics. In the Arronches gneisses a local strain regime exists in apparent contradiction with the bulk deformation regime.