Quantification of strain localization in the continental lithosphere is hindered by the lack of reliable deformation rate measurements in the deep crust. We describe the Quartz-strain-rate-metry (QSR) method, a convenient tool for performing such measurements at a cm-scale from the deformation of quartz, the most ubiquitous mineral in the continental crust. We applied the QSR to two major continental strike-slip ductile shear zones, the Ailao Shan -Red River (ASRR, southwest China) and the Karakorum (KSZ, northwest India). The strain rates were determined by measuring the mean recrystallized quartz grain size, and the thermodynamic conditions of this recrystallization event. The deformation regimes were investigated using the Crystallographic Preferred Orientation of quartz. The pressure and temperature conditions were obtained combining the TitaniQ thermo-barometer and the local exhumation path of the host rocks. Both shear zones undergo exhumation during shearing, and we specially relate to the relation between the measured pressure-temperature conditions and the quartz recrystallization events. When applied to majors shear zones, the QSR-metry method highlights across-strike strain rate variations, from 1 × 10-15 s-1 in zones where strain is weak, to >1 × 10-13 s-1 in zones where it is localized. Strain rates integrated across the shear zones imply fast fault slip rates ~ 1.3 cm yr-1 (Karakorum) and ~ 4 cm yr-1 (ASRR), proving strong strain localization in these strike-slip continental shear zones.
In Ductile Shear Zones: From Micro- to Macro-scales, edited by S. Mukherjee and K.F. Mulchrone, John Wiley & Sons, Chichester, 2016.
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