Abstract
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.
Highlights
The Alpine Fault is the major structure of the Pacific– Australian plate boundary through New Zealand’s South Island
While quantifying temperatures using Ti-in-quartz thermobarometry is complicated by uncertainty in activity of TiO2 (Grujic et al, 2011; Bestmann and Pennacchioni, 2015; Nevitt et al, 2017) and differing calibrations (Huang and Audétat, 2012; Thomas et al, 2015), we find that variations in Ti-in-quartz concentrations in Alpine Fault rocks span a range large enough that these issues can be largely bypassed using independent constraints on maximum and minimum temperatures associated with quartz recrystallization
7.1.1 Initial Ti-in-quartz concentrations were 2-4 ppm Coarse polygonal quartz grains in non-mylonitic schist outside the present study area are inferred to have formed in the Mesozoic during an extended, non-orogenic period when the rocks resided at high temperature in the middle to lower crust (Little, 2004), i.e., prior to their Neogene exhumation resulting from shearing on the Alpine Fault
Summary
The Alpine Fault is the major structure of the Pacific– Australian plate boundary through New Zealand’s South Island. Modern-day Alpine Fault systematics began with the onset of a transpressive phase at ∼ 5–8 Ma along the Australia–Pacific plate boundary (e.g., Walcott, 1998; Batt et al, 2004; Cande and Stock, 2004). Mylonites formed during this period have been exhumed in the hanging wall of the Alpine Fault (Norris and Cooper, 2007). One sample was analyzed from Stony Creek (Fig. 1), 11 km southwest of Gaunt Creek
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
