VORTICITY ANALYSIS OF EXTENSIONAL AND TRANSPRESSIONAL HIGH-STRAIN ZONES: EXAMPLES FROM THE ARIZONA BASIN & RANGE AND VIRGINIA PIEDMONT

The bulk deformation path in high-strain zones may vary as a function of the tectonic setting in which the rocks are deformed. The purpose of this research was to quantify deformation and vorticity in two well-studied high-strain zones of contrasting tectonic style. We sampled the Pinaleño Mountain high-strain zone (PMHSZ) in southeastern Arizona, a metamorphic core complex formed during Basin & Range extension, and the Brookneal high-strain zone (BHSZ), a Paleozoic transpressional zone in the central Piedmont of Virginia. Previous studies of each zone assumed simple shear deformation.

The finite kinematic vorticity number (W_m) of a deformation can be determined if the strain ratio (R_s) and the angle (q) between the long axis of the strain ellipse and the high-strain zone boundary are known. Undeformed granodioritic protoliths outside each zone contain quartz blebs with no preferred orientation. In the high-strain zones quartz grains are elongate and have undergone crystal-plastic deformation. Although many quartz grains have dynamically recrystallized, grain boundaries between quartz aggregates and other phases remain distinct. We estimated strain using standard R_f-phi techniques on quartz grain shapes. Strain analysis indicates that X-axes are subparallel to the macroscopic fabric elements. Theta was determined by calculating: 1) the angle between the macroscopic foliation and the published orientation of the high-strain zone boundary, and/or 2) the angle between the S and C planes (where present) as measured in the field or from thin sections.

In the PMHSZ, strain ratios from XZ sections (6 samples) ranged from 2.6 - 8.3, with a W_m of 0.3 - 0.9. Porphyroclast hyperbolic distribution analysis of ultramylonites yielded a W_m of 0.3 - 0.4. Mylonites from the BHSZ yielded a mean strain ratio (composite analysis of 5 samples) of 7.8 and a W_m of 0.9. Mylonitic rocks in both the extensional Pinaleño Mountain high-strain zone and transpressional Brookneal high-strain zone experienced general shear deformation.