Strain localization in quartzofeldspathic mylonites: a microstructural and electron backscatter diffraction (EBSD) study of the South Mountains Core Complex, Arizona

The evaluation of strain localization in detachment fault shear zones is essential to the study of continental core complex development and permits insight into continental crust rheology during extension. A microstructural and EBSD study of the naturally defonned Tertiary South Mountains granodiorite and granite mylonitic shear zone rocks was conducted to determine the deformation mechanisms that promote strain localization during metamorphic core complex development. It was hypothesized that the strength of quartz strongly influences strain localization in the naturally deformed granodiorite mylonites and granite mylonites. Microstructural observation of quartz grains reveals the presence of elongate 'ribbon grains' with subgrain development along the rims of these grains, and irregular and sinuous sutured grain boundaries; these microstructures are interpreted as evidence of Regimes 2 and 3 dynamic recrystallization. In contrast, the plagioclase feldspar and potassium feldspar crystals are microfractured with limited development of bulging grain boundaries on the rims of the feldspar grains; this is interpreted as evidence of Regime 1 dynamic recrystallization. Pole figures of quartz EBSD data from granodiorite mylonites exhibit c-axis maxima that are interpreted as evidence of lattice preferred orientation. The locations of the maxima are indicative of rhomb slip and prism slip. Based on the interpreted slip systems, it is inferred that deformation occurred at moderate temperatures of 500-650�C. Based on the microstructural evidence for crystal plasticity and the interpreted lattice preferred orientation, it is suggested that quartz deforms by dislocation creep. In contrast, microstructural observations of the feldspar grains indicate limited crystal plasticity, suggesting that feldspar was more rheologically competent during this period of fabric development, and deformed dominantly by microfracturing. The results ofthis microstructural and EBSD study on naturally deformed shear zone rocks will be used to help evaluate strain localization and the rheology of continental crust during the development of metamorphic core complexes in zones of continental extension along low-angle detachment faults.