Microfabric memory of vein quartz for strain localization in detachment faults: A case study on the Simplon fault zone

This manuscript deals with the adaptation of quartz-microfabrics to changing physical deformation conditions, and discusses their preservation potential during subsequent retrograde deformation. Using microstructural analysis, a sequence of recrystallization processes in quartz, ranging from Grain-Boundary Migration Recrystallization (GBM) over Subgrain-Rotation Recrystallization (SGR) to Bulging Nucleation (BLG) is detected for the Simplon fault zone (SFZ) from the low strain rim towards the internal high strain part of the large-scale shear zone. Based on: (i) the retrograde cooling path; (ii) estimates of deformation temperatures; and (iii) spatial variation of dynamic recrystallization processes and different microstructural characteristics, continuous strain localization with decreasing temperature is inferred. In contrast to the recrystallization microstructures, crystallographic preferred orientations (CPO) have a longer memory. CPO patterns indicative of prism and rhomb glide systems in mylonitic quartz veins, overprinted at low temperatures (�400 �C), suggest inheritance of a high-temperature deformation. In this way, microstructural, textural and geochemical analyses provide information for several million years of the deformation history. The reasons for such incomplete resetting of the rock texture is that strain localization is caused by change in effective viscosity contrasts related to temporal large- and small-scale temperature changes during the evolution of such a long-lived shear zone. The spatially resolved, quantitative investigation of quartz microfabrics and associated recrystallization processes therefore provide great potential for an improved understanding of the geodynamics of large-scale shear zones.

The evolution of high-temperature mylonitic microfabrics: evidence from simple shearing of a quartz analogue (norcamphor)

Plane strain simple shearing of norcamphor (C7H10O) in a see-through deformation rig to a shear strain of γ = 10.5 at a homologous temperature of Th = 0.81 yields a microfabric similar to that of quartz in amphibolite facies mylonite. Synkinematic analysis of the norcamphor microfabric reveals that the development of a steady-state texture is linked to changes in the relative activities of several grain-scale mechanisms. Three stages of textural and microstructural evolution are distinguished: (1) rotation and shearing of the intracrystalline glide planes are accommodated by localized deformation along three sets of anastomozing microshears. A symmetrical c-axis girdle reflects localized pure shear extension along the main microshear set (Sa) oblique to the bulk shear zone boundary (abbreviated as SZB); (2) progressive rotation of the microshears into parallelism with the SZB increases the component of simple shear on the Sa microshears. Grain-boundary migration recrystallization favours the survival of grains with slip systems oriented for easy glide. This is associated with a textural transition towards two stable c-axis point maxima whose skeletal outline is oblique with respect to the Sa microshears and the SZB; and (3) at high shear strains (γ > 8), the microstructure, texture and mechanism assemblage are strain invariant, but strain continues to partition into rotating sets of microshears. Steady state is therefore a dynamic, heterogeneous condition involving the cyclic nucleation, growth and consumption of grains.