Multi-scale structural and kinematic analysis of a Neoarchean shear zone in northeastern Minnesota: Implications for assembly of the southern Superior Province

This dissertation is a multi-scale structural and kinematic analysis of the Shagawa Lake shear zone in northeastern Minnesota (USA). The Neoarchean Shagawa Lake shear zone is an ~70 km long ~7 km wide subvertical package of L-S tectonites located within the Wawa Subprovince of the Archean Superior Province. In this dissertation, I (1) discuss a new method for mapping regional tectonic fabrics using high-resolution LiDAR altimetry data; (2) examine the geometric relationships between metamorphic foliation, elongation lineation, vorticity, and non-coaxial shear direction within individual L-S tectonites; and (3) incorporate LiDAR, field, and microstructural data sets into a comprehensive structural and kinematic analysis of the Western Shagawa Lake shear zone. Lastly, I discuss implications for assembly of the southern Superior Province.;In Chapter one I examine an Archean granite-greenstone terrane in NE Minnesota to illustrate the application of high-resolution LiDAR altimetry to mapping regional tectonic fabrics in forested, glaciated areas. I describe the recognition of lineaments and distinguishing between tectonic and glacial lineament fabrics. I use a 1-m posted LiDAR derived bare-earth digital elevation model (DEM) to construct multiple shaded-relief images for lineament mapping with sun elevation of 45˚ and varying sun azimuth in 45˚ intervals. Two suites of lineaments are apparent. Suite A has a unimodal orientation, mean trend of 035, and consists of short (> 2 km long) lineaments within sediment deposits and bedrock. Suite B lineaments, which are longer (1-30 km) than those of suite A, have a quasi-bimodal orientation distribution, with maximum trends of 065 and 090. Only one lineament suite is visible in areas where suites A and B are parallel. I interpret suite A as a surficial geomorphologic fabric related to recent glaciation, and suite B as a proxy for the regional tectonic fabric.;In Chapter two I present a detailed kinematic study of seven Neoarchean L-S tectonite samples in order to determine vorticity and non-coaxial shear direction relative to foliation and elongation lineation. I compare data from thin-sections, x-ray computed tomography, and quartz crystallographic fabric analysis to monoclinic shear models to determine vorticity and better understand geometric relationships between vorticity, non-coaxial shear direction, foliation, and elongation lineation. Kinematic indicators in thin-section and image slices from X-ray computed tomography consistently record asymmetric microstructural fabrics in foliation-normal lineation-parallel planes, whereas planes normal to foliation and elongation lineation display dominantly symmetric microstructural fabrics. Data indicate that tectonites record non-coaxial shear broadly parallel to elongation lineation regardless of lineation geographic orientation.;In Chapter three I present a detailed structural and kinematic study of the eastern Shagawa Lake shear zone. I recognize no strain partitioning, crosscutting relationships between multiple shear events, or metamorphic overprinting within the study area. L-S tectonites record roughly isobaric/isothermal greenschist facies metamorphic conditions across the Shagawa Lake shear zone. The Shagawa Lake shear zone records a broad deformation event characterized by dominantly dip-slip shear in varying directions with multiple shear-senses. Structural and kinematic data indicate that the Shagawa Lake shear zone records deformation within a rheologically weak crust. These data are inconsistent with existing sagduction-diapirism/crustal overturn models and with plate-tectonic/terrane accretion scenarios for assembly of the southern Superior Province. Channel flow induced collapse and exhumation of high-grade crustal material during regional shortening provides a plausible mechanism for assembly of the southern Superior Province and is consistent with the rheological implications of this study. (Abstract shortened by UMI.).