LONGEVITY OF DUCTILE SHEAR ZONES FROM DIFFUSION GEOSPEEDOMETRY OF KINEMATICALLY-SENSITIVE MINERAL INCLUSIONS (Invited Presentation)

Constraining the timescales of crustal deformation typically relies on linking isotope ages with overprinting structures involved in the deformation event. We developed an alternative method for determining shear zone longevity that uses geospeedometry to model incomplete diffusion profiles around oriented inclusions in deformed minerals. This approach was tested using acicular inclusions of rutile (TiO₂) in mylonitized quartz (SiO₂) because: (1) there are published experimental data for Ti diffusion in quartz, (2) rutile inclusions can act as either a source or sink for Ti diffusion in the host quartz, and (3) their elongate aspect ratio makes rutile needles sensitive markers of deformation kinematics.

We applied geospeedometry of rutilated quartz mylonites to the Wildhorse Shear Zone (WSZ) of the Pioneer Core Complex, Idaho, USA. Quartzite mylonites in the WSZ are pervasively and densely rutilated with needles strongly oriented into the lineation direction of deformation. Maps of Ti concentration in quartz surrounding oriented rutile inclusions reveal compositional depletion halos, indicating that rutile exsolved from quartz during the deformation event. By applying experimentally-derived diffusion parameters for Ti in quartz, we calculate timescales of 5-10 Myr for deformation in the WSZ, consistent with independent constraints from geochronology. Using structural offset across the shear zone, these results correspond to paleo strain rates of ~1x10^-12 s^-1. This method was also tested in deformation experiments where temperature and time are precisely known. Results from experiments are generally consistent with known experiment duration but reveal important information on the effects of deformation and water fugacity on rates of lattice diffusion. Given the abundance of rutile needle inclusions in quartz and the importance of quartz for controlling the rheological behavior of the continental crust, diffusion geospeedometry applied to rutilated quartz mylonites could provide a useful approach to estimate the longevity of ductile shear zones.