Strain localization in granodiorite mylonites: a microstructural and electron backscatter diffraction (EBSD) study of the South Mountains core complex, Arizona

The quantification 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. We present a microstructural and EBSD study of naturally deformed shear zone rocks from the footwall of the South Mountains, Arizona, core complex to interpret the deformation mechanisms that lead to strain localization along the detachment fault. The footwall of the South Mountains core complex is dominated by a Miocene composite pluton that exhibits Miocene extensional mylonitic fabrics associated with the structural development of the core complex. The composite pluton is composed of granodiorite, granite, and quartzolite, but the granodiorite is the most voluminous of the intrusive units. The detachment fault shear zone is developed within the granodiorite and quartzolite intrusive units. We conducted a microstructural and EBSD study of the Tertiary South Mountains granodiorite and quartzolite mylonites to determine the deformation mechanisms that promote strain localization. We hypothesize that the strength of quartz strongly influences strain localization in the naturally deformed granodiorite mylonites and quartzolite mylonites. The five samples were collected on an up-structure traverse through the ~60 meter thick mylonitic shear zone towards the interpreted detachment fault surface. 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. We interpret these microstructures 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, which we interpret as evidence of Regime 1 dynamic recrystallization. Up the traverse in the quartzolite mylonites, we observe elongate quartz ‘ribbon grains’ with subgrain development indicative of Regime 2 recrystallization. We use EBSD data from a granodiorite mylonite in the middle of the shear zone to interpret a lattice preferred orientation in quartz grains. Pole figures of quartz data exhibit c-axis maxima that we interpret as evidence of lattice preferred orientation. The locations of the maxima are indicative of rhomb <a> slip and prism <a> slip. Based on the interpreted slip systems, we infer that deformation occurred at moderate temperatures of 500-650°C. Based on the microstructural evidence for crystal plasticity and the interpreted lattice preferred orientation, we suggest 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. We will use the results of our microstructural and EBSD study on naturally deformed shear zone rocks to help evaluate experimental flow laws for both mono-and polymineralic aggregates.