Thermal structure of shear zones from Ti-in-quartz thermometry of mylonites: Methods and example from the basal shear zone, northern Scandinavian Caledonides

The titanium-in-quartz (TitaniQ) thermobarometer was applied to ductilely sheared rocks (mylonites) from the Narvik region of the northern Scandinavian Caledonides, c. 68°N latitude, to evaluate TitanitQ's potential to re-equilibrate during shearing and discriminate among competing models of orogenesis. Sheared metagranite, quartzite, and cross-cutting quartz veins were analyzed across the orogen along the basal thrust zone, a long-lived structure that separates allochthonous from para-autochthonous units. Structural positions ranged from the currently exposed, easternmost thrust front at the brittle-ductile transition to upper amphibolite-facies rocks in the western hinterland. Quartz in polished thin sections was imaged using cathodoluminescence (CL), and domains exhibiting different CL behavior were analyzed for Ti concentrations using ion microprobe. At each position along the thrust, TitaniQ temperatures show temperature ranges of a few tens of °C up to 200°C, but minimum temperatures both from mylonites and cross-cutting quartz veins systematically decrease SE towards the thrust front. These lowest temperatures are interpreted to record final movement of the basal shear zone, and exhibit a low thermal gradient along the thrust of 1.5±0.1°C/km. Such a low gradient is consistent with the critical taper model for the evolution of orogens, but contrasts markedly with channel flow models. Comparisons with quartz microstructure show major differences between TitaniQ temperature and temperatures commonly assumed in microstructural studies. Our work cautions against using microstructures for thermometry, but provides a new method for investigating crustal thermal structure during orogenesis.