Titanium-in-quartz thermometry on synkinematic quartz veins in a retrograde crustal-scale normal fault zone

Abstract

Abstract Previous studies have suggested that estimation of deformation temperatures in quartz mylonites by titanium-in-quartz geothermometry is only possible at temperatures > 500 °C, above which efficient Ti-exchange is achieved via grain boundary migration recrystallization. Based on quartz mylonite samples collected across the Simplon Fault Zone (SFZ) we demonstrate that deformation temperatures of dynamic recrystallization can be obtained down to ~ 350 °C. A prerequisite for such temperature estimates at the low temperature end of ductile deformation of quartz is the formation of synkinematic quartz veins and their immediate overprint either by subgrain rotation (SGR) or bulging recrystallization (BLG). It is the slow growth of the synkinematically precipitating vein quartz that allows for equilibration of Ti in the vein quartz. This Ti-concentration may only slightly be modified during SGR; hence, Ti-in-qtz thermometry provides a close approach to the vein formation temperature. Ti-concentrations are partially reset during BLG, and resulting temperatures are thus maximum temperatures of quartz recrystallization. Importantly, undeformed vein quartz always yield vein formation temperatures. Investigation of the dynamic recrystallization processes overprinting synkinematic quartz veins thus allows for a critical, independent evaluation of the Ti-in-quartz temperatures obtained. For the SFZ, there is a decrease in recrystallized grain sizes towards the fault plane and a change in the dominant recrystallization process associated with a narrowing of the shear zone. As indicated by the Ti-in-quartz temperature estimates, this strain localization correlates with cooling from ~ 560 °C in the oldest microstructures at the periphery of the shear zone down to ~ 350 °C in the youngest microstructures of the footwall near the hanging wall contact. A great benefit of the approach presented here is that intermediate to low temperature plastic deformation in quartz can now also be assessed. Such novel temperature constraints on quartz crystallization are essential for better constraining deformation and rheology in the upper Earth's crust.