Description: The Eclogite Zone, of the Tauern Window is an exhumed subduction channel comprising eclogites with different grades of retrogression in a matrix of high-pressure metasediments. The rocks were exposed to 600 °C and 20–25 kbars, and then retrogressed during their exhumation, first under blueschist facies and later under amphibolite facies metamorphism. To gain insights into the deformation within the subduction channel during subduction and exhumation, both fresh and retrogressed eclogites, as well as the surrounding metasediments were investigated with respect to their deformation microstructures and crystallographic preferred orientations (CPOs). Pristine and retrogressed eclogites show grain boundary migration and subgrain rotation recrystallization microstructures in omphacite. A misorientation axes analysis reveals the activity of complementary deformation mechanisms including grain boundary sliding and dislocation creep. The omphacite CPOs of the eclogites correspond to dominant SL-fabrics characteristic of plane strain deformation, though there are local variations towards flattening or constriction within the paleosubduction channel. The glaucophane CPOs in retrogressed eclogites match those of omphacite, suggesting that a constant strain geometry persisted during exhumation at blueschist facies conditions. Plastic deformation of the host high-pressure metasediments outlasted that of the eclogites, as indicated by white mica fabrics and quartz CPO. The latter is consistently asymmetric, pointing to the operation of non-coaxial deformation. The microstructures and CPO data indicate a continuous plastic deformation cycle with eclogite and blueschist facies metamorphism related to subduction and exhumation of the different rock units.

Description: Quartz veins in the Eastern Tonale mylonite zone (Italian Alps) were deformed in strike-slip shear. Due to the synkinematic emplacement of the Adamello Pluton, a temperature gradient between 280°C and 700°C was effected across this fault zone. The resulting dynamic recrystallization microstructures are characteristic of bulging recrystallization, subgrain rotation recrystallization and grain boundary migration recrystallization. The transitions in recrystallization mechanisms are marked by discrete changes of grain size dependence on temperature. Differential stresses are calculated from the recrystallized grain size data using paleopiezometric relationships. Deformation temperatures are obtained from metamorphic reactions in the deformed host rock. Flow stresses and deformation temperatures are used to determine the strain rate of the Tonale mylonites through integration with several published flow laws yielding an average rate of approximately 10−14s−1 to 10−12s−1. The deformation conditions of the natural fault rocks are compared and correlated with three experimental dislocation creep regimes of quartz of Hirth & Tullis. Linking the microstructures of the naturally and experimentally deformed quartz rocks, a recrystallization mechanism map is presented. This map permits the derivation of temperature and strain rate for mylonitic fault rocks once the recrystallization mechanism is known.

Description: This study discusses the fabric development in naturally sheared quartz aggregates in comparison to results from texture modeling according to the polycrystalline plasticity theory with particular emphasis on the formation of a single c-axis maximum. The investigated natural shear zone samples were deformed at about 650 ± 50 °C with increasing strain up to γ ≈ 14 and show dynamic recrystallization microstructures of grain boundary migration recrystallization. Neutron diffraction texture analysis results in c-axis pole figures with a single maximum at the periphery of the pole figure. This maximum does not align with the shear plane normal towards higher strain, but rotates towards an inclined orientation in accordance with the sense of shear. Such a rotation is inconsistent with the single-slip hypothesis and suggests that the formation of this c-axis pattern is controlled by multi-slip on several slip systems. Based on the polycrystalline plasticity theory, this quartz fabric can develop if combined View the MathML source {r}〈a〉, View the MathML source {z}〈a〉 and View the MathML source prism〈a〉 slip dominates and must not be related to the commonly proposed View the MathML source basal〈a〉 slip. The multi-slip texture development is in agreement with the shear sense interpretation from the asymmetry between well-defined quartz fabrics and the foliation. For dominant View the MathML source basal〈a〉 slip in quartz and γ 〉 2, numerical simulations predict a single peripheral maximum perpendicular to the shear plane and two a-maxima with a ∼30°-inward position parallel to the shear plane. This simulation corresponds to naturally observed CPO patterns of quartz formed at different deformation conditions and it is in agreement with the single-slip hypothesis. Hence, our combined natural and numerical data suggest that the single-slip hypothesis is a possible explanation for a single c(0001)-maximum but not universally true.