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Solid Earth An interactive open-access journal of the European Geosciences Union

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https://doi.org/10.5194/se-2018-12
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
07 Mar 2018
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Solid Earth (SE).
Constraints on Alpine Fault (New Zealand) Mylonitization Temperatures and Geothermal Gradient from Ti-in-quartz Thermobarometry
Steven Kidder1, Virginia Toy2, Dave Prior2, Tim Little3, and Colin MacRae4 1Department of Earth and Atmospheric Science, City College New York, New York, 10031, USA
2Department of Geology, University of Otago, Dunedin, New Zealand
3School of Geography, Environment and Earth Sciences, Victoria University of Wellington, Wellington, New Zealand
4CSIRO Mineral Resources, Microbeam Laboratory, Private Bag 10, 3169 Clayton South, Victoria, Australia
Abstract. We constrain the thermal state of the central Alpine Fault using approximately 750 Ti-in-quartz SIMS analyses from a suite of variably deformed mylonites. Ti-in-quartz concentrations span more than an order of magnitude from 0.24 to ~5 ppm, suggesting recrystallization of quartz over a 300° 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, cross-cutting, 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 °/km. 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 °/km 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.
Citation: Kidder, S., Toy, V., Prior, D., Little, T., and MacRae, C.: Constraints on Alpine Fault (New Zealand) Mylonitization Temperatures and Geothermal Gradient from Ti-in-quartz Thermobarometry, Solid Earth Discuss., https://doi.org/10.5194/se-2018-12, in review, 2018.
Steven Kidder et al.
Steven Kidder et al.
Steven Kidder et al.

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Short summary
By quantifying trace concentrations of titanium in quartz (a known geologic thermometer), we constrain the temperature profile for the deep crust along the Alpine Fault. We show there is a sharp change from fairly uniform temperatures at deep levels, to an extremely steep gradient in temperature in the upper kilometers of the crust. Presumably this gradient in temperature at shallow levels is due to the flux of fluids through cracks caused by earthquakes.
By quantifying trace concentrations of titanium in quartz (a known geologic thermometer), we...
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