Field and Microstructural Constraints on Deformation Conditions and Shear Zone Kinematics in the Burlington Mylonite Zone, Massachusetts

Abstract

The Burlington Mylonite Zone (BMZ) is a northeast-trending, greenschist- to amphibolite-facies shear zone located entirely within the Boston Avalon terrane in Eastern Massachusetts along the tectonic boundary with the Nashoba terrane (the trailing marginal terrane of Ganderia). The juxtaposition of these terranes, and the development of the BMZ, is hypothesized to represent the amalgamation of Avalon and Laurentia during the late Silurian-early Devonian Acadian orogeny, but the timing of its formation and its structural evolution remain largely unconstrained. Field observations and microstructural analysis using electron backscatter diffraction (EBSD) of 24 samples from 16 field sites throughout the BMZ provide new constraints on the kinematics and conditions of deformation that facilitated the development of this large-scale crustal shear zone. The BMZ samples comprise a heterogeneous mix of quartzofeldspathic +/- hornblende-bearing gneisses and quartzites with varying microstructures. Nearly all samples contain abundant mixed, but predominantly sinistral, kinematic indicators (e.g., asymmetric porphyroclasts, tiled feldspars) and a strong crystallographic preferred orientation (CPO). Quartz – the dominant mineral by mode in all of the samples analyzed – is known from experimental deformation studies to develop distinct patterns of CPO which vary as a function of deformation kinematics, temperature, and strain geometry. Patterns of CPO in quartz are used to determine the dominant intracrystalline deformation mechanisms that accommodated the formation of the BMZ. Quartz CPO patterns in the BMZ samples are characterized by variably developed c- and a-axis distributions, broadly consistent with patterns expected for mixed to prism slip at intermediate temperatures of deformation. Corresponding intragranular misorientation axis plots are more diagnostic and indicate dominant prism slip in all of the shear zone samples analyzed, consistent with microstructures observed in thin section (e.g., undulose extinction, subgrain development, grain boundary migration, dynamic recrystallization) and metamorphic conditions inferred from shear zone mineral parageneses. Application of the quartz recrystallized grain size piezometer places additional constraints on deformation conditions, indicating that the BMZ rocks record differential stresses ranging from ~44 to 92 MPa. Field and microstructural observations of shear sense indicators are combined with two analytical methods for determining aspects of kinematic vorticity and deformation geometry in the BMZ. This study applies a new analytical method - crystallographic vorticity axis (CVA) analysis - that leverages rotational statistics on crystallographic orientations within the interiors of grains to constrain the dominant axis of material rotation in deformed samples. This dominant axis provides a uniquely objective proxy for the vorticity normal reference frame required for further quantitative kinematic vorticity analyses. The rotational axis of kinematic vorticity, and its relationship to structural fabrics (i.e. foliation and lineation), provides an important constraint on the geometry of the deforming zone (e.g., monoclinic versus triclinic shear zones). The results of the CVA analysis are invariable across the entire length of the BMZ; the kinematic vorticity axis lies within the plane of mylonitic foliation perpendicular to lineation – the pattern expected for monoclinic deformation geometries. The mean kinematic vorticity number (Wm: a measure of the relative contribution of pure and simple shear) is calculated using Rigid Grain Net (RGN) analysis for the BMZ mylonites and ranges from 0.4-0.5, indicating general shear. Combined field, microstructural, and vorticity analyses are interpreted to suggest that crustal strain localization along the Avalon-Nashoba boundary, as recorded in the BMZ mylonites, involved the combined effects of pure and simple shear in a predominantly sinistral, monoclinic transpressional shear zone. Rock microstructures, patterns of crystallographic preferred orientation, and paleostress estimates suggest that mylonitization occurred at or near the brittle-ductile transition under relatively high stress conditions. This study demonstrates the power of new microstructural methods, such as CVA analysis of electron backscatter diffraction data, to augment traditional field-based methods of kinematics and deformation analysis in enigmatic, large-scale crustal shear zones.