Intrusion age of Pan-African augen gneisses in the southern Menderes Massif and the age of cooling after Alpine ductile extensional deformation

Pb–Pb single zircon ages of 546.2 ± 1.2 Ma demonstrate that the augen gneisses in the southern Menderes Massif were generated from Pan-African intrusions. During the Alpine orogenic evolution of the Menderes Massif these granites were metamorphosed and transformed into augen gneiss in an extensional top-to-the-south shear zone, located between augen gneisses and overlying schists. Quartz fabrics suggest a pronounced static recrystallization that post-dates the ductile deformation in the shear zone. Ar–Ar muscovite ages of 43–37 Ma from augen gneisses and schists suggest that ductile deformation and subsequent cooling occurred in the Eocene. These results contradict previous models that (1) interpret the extensional shear zone as late Oligocene in age and (2) regard the augen gneisses as syntectonic with respect to ductile extensional deformation.

Quartz deformation in the Marquette and Republic troughs, Upper Peninsula of Michigan

Quartz fabrics and microstructures in quartzites of the Mesnard and Goodrich formations were examined to better define the deformation and metamorphic history of the Marquette Synclinorium, northern Michigan. Fabric development is very weak in the whole-rock samples and indicates that mica contamination, extensive annealing, and (or) low imposed strains prevented the formation of distinct c-axis fabrics. However, well defined fabrics are preserved in five quartzite pebbles from the Goodrich conglomerate. Double maxima of varying intensities among these pebbles suggest that the northwest section of the district experienced inhomogeneous shear strain. Measured grain sizes yielded differential stresses ranging from 44 to 548 bar (4.4 to 54.8 MPa). Subgrain size data indicate stresses ranging from 151 to 248 bar (15.1 to 24.8 MPa). Dislocation densities determined by observed etch-pit densities using scanning electron microscopy indicate a range of stresses from 330 to 730 bar (33 to 73 MPa). Stress values from dislocation density measurements vary inversely with metamorphic intensity. Cumulatively, these microstructural data indicate that a low-stress regime of deformation preceded a cooler, higher stress pulse. The data also imply that deformation of the Marquette Synclinorium continued after peak metamorphism, contrary to early hypotheses.

Ductile Shear Bands in a Naturally Deformed Quartzite

Microscale shear bands are features that often occur oblique to the mylonitic foliation in mylonites. This paper is concerned with such structures within a quartz-mylonite. Geometrical features, microstructures and fabrics associated with shear bands are described. Both optical and transmission electron microscopy have been used. It was observed that the development of shear bands is closely related to (i) the onset of dynamic recrystallisation during deformation, (ii) a change of bulk deformation within the mylonites from relatively homogeneous to inhomogeneous and (iii) a marked softening of the mylonite. Across shear bands, dominant deformation mechanisms change from a dislocation creep type to grain boundary sliding. This induces strong modification of quartz lattice preferred orientations. The asymmetry of quartz fabrics due to shear should generally be favoured by the development of shear band structures. Our results indicate that the production of ductile shear band structures helps to accommodate large strain deformations at low temperatures. Results also indicate that grain and sub-grain sizes are not affected by variations in strain rate.