DETERMINING DEFORMATION TEMPERATURES OF A NATURAL SHEAR ZONE

Estimated strain rates even within a single natural shear zone can vary by several orders of magnitude because the temperatures over which deformation take place are difficult to determine. The Titanium-in-quartz thermometer (TitaniQ of Wark and Watson, 2006) offers a new opportunity to quantify the temperature during dynamic recrystallization of quartz in mylonitic rocks from continental crust. We apply TitaniQ and two-feldspar thermometry to assess deformation temperatures for a shear zone in the Sierra Nevada, CA.

We sampled several granitic plutons that were ductilely deformed to varying degrees along the Proto-Kern Canyon fault (PKCF). Plutons from the area we concentrated on range in age from 104–85 Ma. We measured the titanium content of quartz for four samples that were assigned aTiO₂ = 0.8 due to the presence of titanite. We applied Al-in-hbl barometry, and plag-hbl and two-feldspar thermometry, where mineral assemblages allowed. We also performed EBSD analyses and optical microscopy to assess deformation mechanisms.

All samples yield crystallization depths of 11–13 km and temperatures of 700–725˚ C. All recrystallized quartz temperatures from TitaniQ are >500˚ C. Such high recrystallization temperatures suggest shear zone activity took place under hot subsolidus conditions. The youngest igneous member that is deformed along the PKCF, the 85 Ma Gold Ledge Granite, contains quartz that completely recrystallized at T = 575 ± 63˚ C. It also contains ductilely deformed feldspar. Two-feldspar thermometry on plagioclase with stretched orthoclase overgrowths yields T = 446 ± 26˚ C.

These deformation temperatures help refine our ability to determine strain rate along the PKCF. Quartz piezometry-derived stresses for igneous mylonites and deformed quartz veins of our samples are 50–95 MPa. Previous deformation temperature estimates for this part of the PKCF, based largely on microstructural observations, were 350–450˚ C and yielded strain rates of 10^-15–10^-12 s^-1. Assigning higher deformation temperatures of 450–575˚ C places natural strain rates significantly higher, at 10^-12–10^-10 s^-1. Better constraints on fugacity, as well as more precise grain-size measurements, will provide a higher-certainty strain rate.