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Discussion papers | Copyright
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

Research article | 07 Mar 2018

Review status
This discussion paper is a preprint. A revision of this manuscript was accepted for the journal Solid Earth (SE) and is expected to appear here in due course.

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 Steven Kidder et al.
  • 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 ~5ppm, 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 4ppm and do not vary as a function of grain size or bulk rock composition. Analyses of 30 large, inferred-remnant quartz grains (>250µm), as well as late, cross-cutting, chlorite-bearing quartz veins also reveal restricted Ti concentrations of 2–4ppm. These results indicate that the vast majority of Alpine Fault mylonitization occurred within a restricted zone of pressure-temperature conditions where 2–4ppm Ti-in-quartz concentrations are stable. This constrains the deep geothermal gradient from the moho to about 8km to a slope of 5 °/km. In contrast, the small grains (10–40µm) in ultramylonites have lower Ti concentrations of 1–2ppm, 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 8km, i.e. within the seismogenic zone. Anomalously, the lowest-Ti quartz (0.24–0.7ppm) 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.

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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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