Multi-scale structural and kinematic analysis of a Neoarchean shear zone in northeastern Minnesota: Implications for assembly of the southern Superior Province
2014-08
Title
Multi-scale structural and kinematic analysis of a Neoarchean shear zone in northeastern Minnesota: Implications for assembly of the southern Superior Province
Authors
Published Date
2014-08
Publisher
Type
Thesis or Dissertation
Abstract
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. Field measurements of regional tectonic foliation trajectories are largely consistent with suite B lineaments across the study area. Although not all suite B lineaments correlate to mapped structures, our analysis demonstrates that high-resolution LiDAR altimetry can be useful in mapping regional tectonic fabrics in glaciated terrane. 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. Samples are from L-S tectonites of the Wawa Subprovince, more specifically the Vermilion District of NE Minnesota, a NE-trending belt of greenschist grade supracrustal rocks and granitoid bodies. Supracrustal rocks host multiple L-S tectonite packages with a well-developed sub-vertical metamorphic foliation and elongation lineation; elongation lineation generally plunges steeply to obliquely although rare zones of shallow plunge occur locally. The Wawa Subprovince is widely interpreted as a transpressional margin with shear zones recording unidirectional dextral strike-slip, an interpretation held up as fundamental evidence for Archean plate-tectonic processes. However vorticity and shear direction within the Vermilion District L-S tectonites remain unconstrained. 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. Mantled porphyroclast 3D-shapes and star-volume distribution analyses indicate that porphyroclast short-axes are normal to foliation and long-axes parallel elongation lineation. Quartz crystallographic preferred orientation data show a-axes maxima sub-parallel to foliation-normal lineation-parallel planes. Kinematic data are consistent with a vorticity axis within the foliation plane and normal to elongation lineation; thus non-coaxial shear direction is sub-parallel to elongation lineation. Data are inconsistent with shear models in which non-coaxial shear direction is normal to lineation, or vorticity axis is normal to foliation. 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. A subvertical metamorphic foliation strikes NE; elongation lineation forms a splayed orientation distribution; however 70% of elongation lineations pitch ≥ 60° to the NE or SW. Strike-parallel elongation lineations occur within localized zones. Non-coaxial shear direction is sub-parallel to elongation lineation. Kinematic indicators record N-side-up and S-side-up shear, and local strike-slip shear--with both right-lateral and left-lateral shear sense recorded. Elongation lineation and kinematic indicators appear consistent within individual outcrops, but can vary significantly between outcrops. 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.
Description
University of Minnesota Ph.D. dissertation. August 2014. Major: Earth Sciences. Advisor: Vicki L. Hansen. 1 PDF (xi, 117 pages) + 1 PDF containing 14 oversize images and zipped GIS data.
Related to
Replaces
License
Collections
Series/Report Number
Funding information
Isbn identifier
Doi identifier
Previously Published Citation
Suggested citation
Content distributed via the University Digital Conservancy may be subject to additional license and use restrictions applied by the depositor. By using these files, users agree to the Terms of Use. Materials in the UDC may contain content that is disturbing and/or harmful. For more information, please see our statement on harmful content in digital repositories.