LINKING GRAIN- TO PLATE-SCALE DEFORMATION AND KINEMATICS THROUGH ANALYSIS OF MICRO-ROTATIONS IN POLYPHASE SHEAR ZONES (Invited Presentation)

Calculating shear-induced rotational axes from EBSD data provides a powerful new tool for textural analysis of shear zone rocks that can be combined with more traditional methods of structural and vorticity analysis to unravel the history of deformation. Results from the Burlington mylonite zone (BMZ) – an enigmatic paleotectonic margin formed at the boundary of the Nashoba and Avalon terranes in eastern Massachusetts – demonstrate the utility of crystallographic orientation-dispersion methods for constraining grain- to plate-scale deformation geometries.

The BMZ includes mylonitic gneisses, quartzites, and granites characterized by a NE-SW–trending, NW-dipping mylonitic foliation and a shallowly plunging NE-SW lineation. Structural relationships indicate dominant sinistral top-to-the-south shear-sense kinematics within the BMZ. Vorticity axes, calculated from lattice rotations using the Crystallographic Vorticity Axis method, lie within the plane of mylonitic foliation perpendicular to lineation. The kinematic vorticity number (W_k) is calculated using Rigid Grain Net analysis and ranges from 0.25-0.55, indicating dominant general shear. Quartz textures are most compatible with dominant prism<a> slip during deformation at 450 to 600 ºC. We estimate differential stresses of 44 to 92 MPa in the BMZ mylonites using the recrystallized grain size piezometer for quartz.

For the sake of shear zone analyses, orientation-dispersion methods provide a unique microkinematic link between the behavior of crystals during deformation and the larger-scale deformation geometry of tectonic shear zones. We combine W_k values, vorticity axes and geographic fabric orientations to constrain the angle of convergence between the Nashoba and Avalon terranes to ~56-75º, with a convergence vector trending ~142-160° and plunging ~3-10°. We conclude that crustal strain localization in the BMZ involved a combination of pure and simple shear in a sinistral reverse transpressional shear zone formed at or near the brittle-ductile transition under relatively high stress conditions. We further 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.