DEFORMATION PATHS IN METAMORPHIC CORE COMPLEXES, ARIZONA

Understanding the deformation path followed by mid-crustal rocks from the lower plate in metamorphic core complexes is important for reconstructing the kinematics of flow during extension and exhumation. Granitic mylonites of Tertiary age from the Coyote Mountains, Rincon, Pinalenos, Santa Catalina, Suizo, and South Mountain core complexes in southern Arizona record general shear (W_m = 0.6 – 0.9), modest to high strain (XZ = 2 to >15), and plane to moderately constrictional strain (K = 1 – 3). Fabric asymmetries in protomylonites and mylonites are consistent with monoclinic flow. Possible deformation paths, elucidated from D-values vs. W_m-values or D-values vs. K-values plots are approximately linear. Narrow ultramylonite bands, which cut the main penetrative fabrics in these high-strain zones, typically record pure shear dominated deformation and triclinic asymmetries. Strain softening in ultramylonite zones is important and may causes these zones to follow significantly different deformation paths from the main mylonite zone. Quartzite screens in the Coyote Mountains preserve strong constrictional fabrics with a maximum elongation direction parallel to fabrics in the granitic mylonites. In many core complexes, foliation orientation is spatially variable or, in some cases, corrugated but the maximum elongation direction remains consistent. The deformation path followed footwall mylonites may influence the geometry of continued brittle deformation during detachment faulting and upper crustal extension in core complexes. Penetrative general shear with constrictional strain is geometrically compatible with the arched and corrugated architecture of many core complexes.