Pressure, temperature, and stress conditions of Southern Alpine Fault (New Zealand) mylonites

Author

Jake T. Reitman, CUNY City CollegeFollow

Date of Award

2019

Document Type

Thesis

Department

Earth and Atmospheric Sciences

First Advisor

Steven Kidder

Keywords

Ti-in-quartz, quartz, New Zealand, Alpine Fault

Abstract

The Alpine Fault, New Zealand, provides a unique window into the rheologic history of a seismically-active continental plate boundary. Due to lower exhumation rates along southern portions of the fault relative to the well-studied central Alpine Fault, deformation ages vary along strike by ~15 m.y.. Mylonites collected from two southern creeks near Haast, NZ, display a uniform recrystallized grain size of ~11 𝜇m in mylonitic quartz indicating differential stress values in the south of ~100 MPa, roughly twice the stress values observed in the central Alpine Fault. Constraints from Ti-in-quartz thermobarometry and Raman Spectroscopy of Carbonaceous Material (RSCM) indicate mylonitization occurred at 390 – 495 °C, somewhat cooler than observed along the central Alpine Fault. Given similar bulk compositions, and no known variation in strain rate along strike, it is likely that these differences in stress and temperature are related to variation in geothermal gradient between the two areas, with the southern Alpine Fault escaping rapid exhumation that advected considerable heat into the upper crust of the central Alpine Fault over the past ~6 m.y.. The observed magnitudes of peak stress in the two areas are consistent with those predicted by quartz flow laws using previously estimated geothermal gradients for the two areas, and suggest that integrated crustal strength has decreased by a factor of two in the central Alpine Fault during the Neogene.

Recommended Citation

Reitman, Jake T., "Pressure, temperature, and stress conditions of Southern Alpine Fault (New Zealand) mylonites" (2019). CUNY Academic Works.
https://academicworks.cuny.edu/cc_etds_theses/767