Document Type
Thesis
Abstract
Kinematic vorticity is a dimensionless measure of rotation relative to finite stretching and is essential for a complete understanding of displacement and flow in ductile shear zones, common mid-crustal deformational features. The porphyroclast hyperbolic distribution (PHD) method is a widely used technique for measuring the kinematic vorticity. My research tested the effectiveness of the PHD method and explored the ability of a modified form of the PHD method to delineate between monoclinic and triclinic deformations. Ultramylonites from two high-strain zones, the Cabin Canyon area of the Virgin Mountains, Nevada and the Pusch Peak area of the Santa Catalina Mountains, Arizona were analyzed. Pseudo-three-dimensional kinematic vorticity analysis was facilitated by the analysis of two planes, both normal to foliation with one parallel to lineation and one normal to lineation. The analysis identified Cabin Canyon as a zone of general shear with a rolling sub-horizontal lineation with displacement across steep dip slip zones, not strike-slip transpression as hypothesized by Quigley et al. (2002). The PHD suggested that Pusch Peak is a zone of general shear, the result of a single triclinic general shear deformation in disagreement with previous interpretations of monoclinic simple shear (Naruk, 1987). Bootstrapping statistics and a new process called sieving identified several problems with the PHD method. Bootstrapping produced standard errors of +0.24 for kinematic vorticity values yielded by the PHD method, identifying significant uncertainties in the PHD method. The sieving results indicated that feldspar porphyroclasts in ultramylonites may not be rigid and may deform during progressive deformation. Deformation lamellae, undulose extinction, and subgrains in the feldspar porphyroclasts further suggest that the porphyroclasts are deforming crystal-plastically and therefore non-rigid. The PHD method is useful in discerning between zones of pure shear dominated, simple shear dominated, and general shear, but may not be accurate enough to provide specific kinematic vorticity values. The initial results indicate that the PHD method, as modified for this study, can be used to identify zones of monoclinic versus triclinic shear.
Date Awarded
2005
Department
Geology
Advisor 1
Christopher M. Bailey
Recommended Citation
Forte, Adam Matthew, "The Porphytoclast Hyperbolic Distrivution Method of Kinematic Vorticity Analysis" (2005). Geology Senior Theses. William & Mary. Paper 151.
https://scholarworks.wm.edu/geologyseniors/151
