General shear deformation in the Pinaleٌo Mountains metamorphic core complex, Arizona

Granitic mylonites from the Pinaleño Mountains metamorphic core complex in southeastern Arizona record general shear deformation. The mean kinematic vorticity number (Wm) was estimated using (1) the strain ratio (Rs) and the angle (Θ) between the long axis of the strain ellipsoid with respect to the high-strain zone boundary and (2) the porphyroclast hyperbolic distribution method. Wm for protomylonites and mylonites ranged from 0.6 to 0.9. Ultramylonites record lower Wm values (0.1–0.3) suggesting that the incremental vorticity changed during deformation. Three-dimensional strain analysis indicates that deformation in protomylonites and mylonites approximates plane strain and has a monoclinic symmetry. The vorticity path followed by rocks tectonically exhumed in the footwall of major extensional fault systems may evolve from a significant pure shear component early in the deformation towards simple shear as overburden load decreases during uplift.