Quartz Fabric Analysis of the Kawishiwi Shear Zone, NE Minnesota

Structural fabrics within the Vermilion district, northeastern Minnesota, including metamorphic foliation and elongation lineation (Le), are well established and can be summarized as near-vertical foliation striking northeast containing oblique to vertical Le. The oblique to vertical lineations are present throughout the Vermilion district, yet areas of subhorizontal lineation orientations are also present locally. Despite broad agreement in structural data, first-order interpretations including vorticity axis orientation and shear sense have yet to be agreed upon. Two different interpretations emerge from previous studies: 1) dextral transpression associated with terrane accretion and 2) Le-parallel shearing consisting of regional dip-slip shearing and later, more focused, strike-slip shearing. The Kawishwi Shear Zone (KSZ), one of several Vermilion district shear zones, shows structural relationship particularly well. Foliation consistently strikes east-northeast and dips vertically. Le is broadly down-dip with discrete areas of subhorizontal orientation. Goodman (2008) performed a structural and kinematic analysis of the KSZ and interpreted lineation-parallel shearing with dip-slip shearing followed by strike-slip shearing. However, previous studies do not specifically constrain flow within L-S tectonites relative to Le.;This study aims to characterize the kinematic pattern of flow through use of quartz fabric analysis of c- and a-axis petrofabrics. The data acquired reveal the dominant slip planes and direction of flow during deformation, and also provide information about deformation temperature and strain geometry. Oriented samples from the KSZ were analyzed by a scanning electron microscope and electron backscatter diffraction (SEM/EBSD) system. Sample KS7J contains a vertical Le and quartz petrofabric data indicate flow nearly parallel to Le, thus dominantly dip-slip displacement. Quartz microstructures are consistent with greenschist-facies deformation. Sample KS6U1, collected from a localized zone with strike-parallel lineation, displays quartz petrofabric data indicative of slip along the prism plane with flow parallel to Le. Activation of the higher temperature prism plane is attributed to conditions of the quartz vein emplacement. The results of this study support the lineation-parallel shearing hypothesis for deformation within the KSZ. Similar to the results of Goodman (2008), data of this study support at least two shearing events in the Kawishiwi Shear Zone. Sample KS7J is interpreted to record the pervasive, broad dip-parallel deformation, whereas sample KS6U1 is interpreted to record a late-stage localized strike-parallel deformation. The general shear flow geometry of sample KS7J indicates displacement near-parallel to Le and does not contain a significant strike-parallel component. Data of this study are in line with regional displacement parallel to Le resulting in dominantly vertical movements.