Type of Culminating Activity
Master of Science in Geosciences
Matthew J. Kohn, Ph.D.
The Titanium-in-quartz (TitaniQ) thermobarometer was evaluated in ductilely sheared rocks (mylonites) from the Scandinavian Caledonides in comparison with several other thermometric methods, including: (1) TitaniQ thermometry in cross-cutting quartz veins, (2) garnet-biotite Fe-Mg exchange thermometry, (3) metamorphic phase equilibria, and (4) quartz microstructures as calibrated experimentally and empirically. In all instances, quartz vein temperatures mimic TitaniQ temperatures of the host rocks. Similarly, TitaniQ temperatures of dynamically recrystallized quartz, ranging from ~210°C at the thrust front to 475°C at the deepest structural levels, reflect the best estimate of the final temperature of deformation. Higher temperatures are also preserved locally and more closely reflect peak metamorphic temperatures derived from metamorphic phase equilibria. Of the two samples analyzed for garnet-biotite Fe-Mg exchange thermometry, one gives results consistent with TitaniQ temperatures and the other gives a higher temperature. A larger sample size would be necessary for more confident comparisons. Lastly, TitaniQ temperatures are mostly consistent with temperatures expected from quartz microstructures. However, TitaniQ temperatures reveal grain boundary migration recrystallization (GBM) occurred at temperatures as low as 340°C, which is much lower than previously published temperatures for GBM (Hirth and Tullis, 1992; Stipp et al., 2002).
Two competing end-member models for the steady state thermal and kinematic evolution of orogens exist: The critical wedge model and the channel flow-extrusion model. Due to the unusual and consistent exposure of thrust surfaces for ~140 km across the orogen, the Caledonides provide insights into the down-dip behavior of thrusts not determinable in other collisional orogens. Using TitaniQ, in combination with other thermometers and barometers, to obtain precise and accurate pressure-temperature estimates of quartz recrystallization in the Northern Scandinavian Caledonides, we resolve a consistent down-dip thermal gradient along the basal thrust shear zone of 1.43 ± 2°C/km. This low thermal gradient supports the critical wedge model for the evolution of the Caledonides. We suggest that the Caledonides formed from critical wedge mechanics together with general non-coaxial flow and gravitational spreading of the nappes. In addition to resolving a down-dip thermal gradient across the Caledonides basal shear zone, we calculated strain rates for quartz deformation ranging from 1.00E-22 to 1.00E-13 s-1.
Wolfowicz, Andrea Marilina, "Determining Temperatures of Deformation in Mylonites from the Scandinavian Caledonides, with Implications for the Thermal and Kinematic Evolution of Orogens" (2012). Boise State University Theses and Dissertations. 607.