Kinematics and Vorticity of High-Strain Zones
Christopher M. Bailey, Geology, College of William & Mary, Williamsburg, VA 23187
Andy Bobyarchick, Geography and Earth Sciences, University of North Carolina, Charlotte, NC 28223
Dazhi Jiang, Geology, University of Maryland, College Park, MD 20742
This GSA Field Forum brought together 20 geologists to examine high-strain zones and penetratively deformed rocks in the Virginia Piedmont and Blue Ridge. Recent years have brought to the fore many complex deformation models and the time was right to assemble geologists in the field to discuss what information can and cannot be gained about the kinematics and vorticity of naturally deformed rocks. Some key questions included: What structures in high-strain zones can be used to characterize mean vorticity and progressive vorticity changes during deformation? How are triclinic high-strain zones recognized? How does the recognition of triclinic symmetries influence kinematic and tectonic interpretations? Does the specific tectonic environment in which material is deformed influence the deformation path?
The forum was convened at the Graves Mountain Lodge, near Madison, Virginia, April 16-21, 2002. The first evening's presentation focused on the goals of the forum and provided a regional geologic overview. The first day in the field involved a traverse across the Piedmont. Stops included two quarries in the transpressional Hylas Zone, mylonitic biotite-rich gneiss in the Proterozoic Goochland terrane, and L-tectonites from the Columbia granodioritic gneiss. Discussions centered on how best to decipher rocks that have experienced multiple deformations, yet display only one penetrative fabric. The evening discussion reviewed the history of high-strain zones studies from simple shear in the classic "Ramsay & Graham" model to general shear models with triclinic symmetries.
The second day in the field involved a steep hike to debris flow scars on the edge of Shenandoah National Park in the Blue Ridge Mountains to examine a number of contractional high-strain zones. At these localities, undeformed charnockites are transformed into mylonites and the high-strain zone boundaries are well exposed. Discussions concerned how best to quantify strain in these mylonitic rocks; although there was a difference of opinion as to the total finite strain, all agreed that these rocks had experienced bulk general shear. At lunch, an energetic thunderstorm drove the group from the outcrops. The afternoon was spent drying off and examining amphibolite and greenschist facies high-strain zones in the eastern Blue Ridge. Discussions focused on the determination of strain symmetry and the pitfalls associated with mylonite geochronology. The evening session considered the implications of triclinic deformation models and how best to apply these models to natural examples. Although deformation models have proliferated in the past ten years, participants agreed that a robust kinematic understanding of real high-strain zones has been documented in only a few examples.
Stops on the third day included outcrops with high-temperature Grenville-age fabrics in Blue Ridge gneisses, penetratively deformed Neoproterozoic metasedimentary rocks, and anastomosing mm-scale to m-scale high-strain zones cutting massive granites. The highlight of day three was the Garth Run high-strain zone. This contractional high-strain zone is characterized by flattening strains and strong fabric asymmetries. On faces parallel to the elongation lineation and normal to the foliation, a top-to-the-northwest (reverse movement) sense of shear is present, but on faces normal to both foliation and lineation, a sinistral sense of shear predominates. The geometry of Garth Run mylonites appears triclinic, however, some argued that strain could be partitioned into complimentary triclinic domains that yield an overall monoclinic deformation symmetry. Folded pegmatite boudins at Garth Run were discussed as possible indicators of incremental vorticity changes during deformation.
The final field trip day included stops in the regionally significant Rockfish Valley high-strain zone. This zone is up to 2 km thick and generally poorly exposed. Discussions concerned how best to estimate total displacement in regional-scale zones. The final stop on the forum was at the Lawhorne Mill high-strain zone, a 300 m thick zone of mylonite derived from Blue Ridge granitoids. These mylonitic rocks have a well-developed down-dip lineation, but display only weakly asymmetric structures. Possible kinematic interpretations for this mylonite zone include pure shear dominated deformation or transpression with the mineral elongation lineation forming a rolling lineation at right angles to the transport direction.
The final evening session included informal presentations about ongoing kinematic research in the Himalayas, Basin and Range, and the Canadian Shield. The forum closed with a consensus that quantitative estimates of strain and vorticity in deformed rocks are valuable, even though these results generally provide non-unique solutions as to deformation path. Challenges for the future include testing existing vorticity indicators in three dimensional and triclinic deformations, developing new vorticity indicators, and elucidating deformation paths, not just finite strains and mean vorticities. It is clear that the kinematics of deformation is rarely simple, and in spite of space and compatibility "problems" with non-simple shear deformations, the evidence from Virginia and many other regions indicates that in the crust, general shear may be the rule rather than the exception.