Previous studies have suggested that esti
mation of defor
mation te
mperatures in quartz
mylonites by titaniu
m-in-quartz geother
mo
metry is only possible at te
mperatures > 500 掳C, above which efficient Ti-exchange is achieved via grain boundary
migration recrystallization. Based on quartz
mylonite sa
mples collected across the Si
mplon Fault Zone (SFZ) we de
monstrate that defor
mation te
mperatures of dyna
mic recrystallization can be obtained down to ~ 350 掳C. A prerequisite for such te
mperature esti
mates at the low te
mperature end of ductile defor
mation of quartz is the for
mation of synkine
matic quartz veins and their i
mmediate overprint either by subgrain rotation (SGR) or bulging recrystallization (BLG). It is the slow growth of the synkine
matically 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 ther
mo
metry provides a close approach to the vein for
mation te
mperature. Ti-concentrations are partially reset during BLG, and resulting te
mperatures are thus
maxi
mu
m te
mperatures of quartz recrystallization. I
mportantly, undefor
med vein quartz always yield vein for
mation te
mperatures. Investigation of the dyna
mic recrystallization processes overprinting synkine
matic quartz veins thus allows for a critical, independent evaluation of the Ti-in-quartz te
mperatures 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 掳C in the oldest microstructures at the periphery of the shear zone down to ~ 350 掳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.