GENERAL SHEAR DEFORMATION IN THE PINALEÑO MOUNTAINS METAMORPHIC CORE COMPLEX, ARIZONA

Granitic mylonites formed during extensional deformation in the Pinaleño Mountains metamorphic core complex of southeastern Arizona record bulk general shear. On the northern flank of the range Tertiary granitic rocks are transformed into a discrete 0.5 to 1 km-thick zone of mylonitic rocks. The lower boundary of the high-strain zone is approximately planar over km-scale domains, strikes ~300° and dips 35° ± 3° to the northeast. Mylonitic foliation in the high-strain zone consistently dips less steeply than the high-strain zone boundary and mineral elongation lineations plunge down dip. Sectional strains were estimated from quartz grain shapes. In granitic rocks beneath the high-strain zone boundary quartz grain shapes yield Rs-values of ~1.1. Rs-values range from 2-8 in XZ sections of mylonitic rocks. The mean kinematic vorticity number (Wm) was estimated using the strain ratio (Rs)/theta angle (q) method and ranges from 0.6–0.9 in protomylonites and mylonites. The porphyroclast hyperbolic distribution (PHD) method of vorticity analysis records Wn-values of 0.1–0.3 in ultramylonites. Three-dimensional strains are consistent with plane strain deformation. The overall three-dimensional geometry of the Pinaleño mylonites is that of lengthening/thinning general shear. Fabric elements and asymmetric kinematic indicators are compatible with a monoclinic deformation symmetry. Shear strain integration indicates a minimum displacement of 2.2 km across the high-strain zone.

The lower vorticity number recorded in the ultramylonites may reflect 1) a difference in the respective vorticity gauges (Rs/q vs. PHD method), 2) strain softening in the ultramylonites, or 3) a non-steady vorticity path. The vorticity path followed by tectonically exhumed rocks 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.