Kruckenberg, S.C., Michels, Z.D., and Parsons, M.M.
Geosphere; doi: 10.1130/GES01585.1
Publication year: 2019


We present results from integrated field, microstructural, and textural analysis of the Burlington mylonite zone (BMZ) in eastern Massachusetts (northeastern USA) to establish a unified microkinematic framework for vorticity analysis in heterogeneous shear zones. Specifically, we develop a methodology for the structural analysis of polyphase lithologies that defines the vorticity-normal surface based on lattice-scale rotation axes calculated from electron backscatter diffraction data using orientation statistics. In doing so, we objectively identify a suitable reference frame for rigid grain methods of vorticity analysis that can be used in concert with field and microstructural methods of strain analysis and textural studies to constrain field- to plate-scale kinematics and deformation geometries without assumptions that may bias tectonic interpretations, such as relationships between kinematic axes and fabric-forming elements or the nature of the deforming zone (e.g., monoclinic versus triclinic shear zones).

Rocks within the BMZ comprise a heterogeneous mix of quartzo­feld­spathic ± hornblende-bearing mylonitic gneisses and quartzites. Vorticity axes inferred from lattice rotations lie within the plane of mylonitic foliation perpendicular to lineation—a pattern consistent with monoclinic deformation geometries involving simple shear and/or wrench-dominated transpression. The mean kinematic vorticity number (Wm) is calculated using rigid grain net analysis and ranges from 0.25 to 0.55, indicating dominant general shear. Using the calculated vorticity values and the dominant geographic fabric orientation, we constrain the angle of paleotectonic convergence between the Nashoba and Avalon terranes to ~56°–75° with the convergence vector trending ~142°–160° and plunging ~3°–10°. Application of the quartz recrystallized grain size piezometer suggests differential stresses in the BMZ mylonites ranging from ~44 to 92 MPa; patterns of quartz crystallographic preferred orientation are consistent with deformation at greenschist- to amphibolite-facies conditions. We conclude that crustal strain localization in the BMZ involved a combination of pure and simple shear in a sinistral reverse transpressional shear zone that was active at or near the brittle-ductile transition under relatively high stress conditions. Moreover, we demonstrate the utility of combined crystallographic and rigid grain methods of vorticity analy­sis for deducing deformation geometries, kinematics, and tectonic histories in polyphase shear zones.