Constraints on Alpine Fault (New Zealand) mylonitization temperatures and the geothermal gradient from Ti-in-quartz thermobarometry

Abstract

We constrain the thermal state of the central Alpine Fault using approximately 750 Ti-in-quartz secondary ion mass spectrometer (SIMS) analyses from a suite of variably deformed mylonites. Ti-in-quartz concentrations span more than 1 order of magnitude from 0.24 to ∼ 5 ppm, suggesting recrystallization of quartz over a 300 °C range in temperature. Most Ti-in-quartz concentrations in mylonites, protomylonites, and the Alpine Schist protolith are between 2 and 4 ppm and do not vary as a function of grain size or bulk rock composition. Analyses of 30 large, inferred-remnant quartz grains ( > 250 µm) as well as late, crosscutting, chlorite-bearing quartz veins also reveal restricted Ti concentrations of 2–4 ppm. These results indicate that the vast majority of Alpine Fault mylonitization occurred within a restricted zone of pressure–temperature conditions where 2–4 ppm Ti-in-quartz concentrations are stable. This constrains the deep geothermal gradient from the Moho to about 8 km to a slope of 5 °C km−1. In contrast, the small grains (10–40 µm) in ultramylonites have lower Ti concentrations of 1–2 ppm, indicating a deviation from the deeper pressure–temperature trajectory during the latest phase of ductile deformation. These constraints suggest an abrupt, order of magnitude change in the geothermal gradient to an average of about 60 °C km−1 at depths shallower than about 8 km, i.e., within the seismogenic zone. Anomalously, the lowest-Ti quartz (0.24–0.7 ppm) occurs away from the fault in protomylonites, suggesting that the outer fault zone experienced minor plastic deformation late in the exhumation history when more fault-proximal parts of the fault were deforming exclusively by brittle processes.