Rheological constraints on quartz derived from scaling relationships and numerical models of sheared brittle-ductile quartz veins, central Southern Alps, New Zealand

The mechanical properties of quartz strongly influence the strength of continental crust, but natural examples to constrain quartz rheology are rare. Here, brittle-ductile fault arrays in the Southern Alps, New Zealand, provide a natural laboratory into the rheology of quartz rocks. The faults formed in the hanging wall of the Alpine Fault during the late Cenozoic at ≥21 km depth. They are near-vertical, extend over tens of metres, strike sub-parallel to the Alpine Fault, and displace quartzofeldspathic metagreywacke through predominantly brittle processes. They also displace centimetre-thick quartz veins that are discordant to the dominant schist foliation with variably ductile to brittle slip. We use field-observed geometrical scaling relationships related to the sheared quartz veins and interactions between brittle faults and ductilely deforming quartz veins that intersect them to produce a set of viable numerical models. Quartz rheology is modelled by linear or power law creep, and the material parameters extracted for the quartz veins, together with viscous and brittle strength ratios between vein quartz and schist. The results indicate that under the prevailing deformation conditions, the dominant deformation mechanism in the quartz veins was dislocation creep resulting in a non-linear quartz rheology.