title: Kink band development in the Darrington Phyllite on Samish Island, northwestern Washington
Creators: name: Dunham, Rachel Eliades
Publisher: name: Western Washington University
Year: 2010

description: <p>Kink bands are sharp-hinged monoclinal folds that are common contractional deformation features in fine-grained, foliated rocks. Two competing geometric models of kink band formation are mobile-hinge kinking, where a kink band grows by lateral expansion of kink band hinges, and fixed-hinge kinking, where a kink band initiates at a given width and rotates to accommodate shortening. This study investigates previously identified but poorly characterized kink bands in the Darrington Phyllite on Samish Island, northwestern Washington, in order to evaluate the applicability of each model and to characterize the complex geometries of kink bands in plan view. Two sets of kink bands are present on Samish Island, kinking a steeply southdipping foliation. The majority of bands have axial surfaces that dip moderately NE, and a small number of bands have axial surfaces that dip steeply SW; true conjugate bands are rare. Mapping and measurements of more than 500 kink bands in the field indicate that bands are generally narrow (cm) and closely spaced (cm), with small populations of wide and widely spaced bands. Kink band angles measured in the field indicate that many bands conform to an ideal kink band geometry with equal internal and external angles between the kinked and unkinked foliations; however, a significant number of bands deviate from the ideal case. Veins are closely associated with many kink bands; sigmoidal veins crossing kink bands and triangular voids along kink band boundaries suggest dilation during kinking. This is the first study to apply geometric curvature analysis to kink bands in order to quantitatively describe kink band morphologies in plan view. 3D scans of hand samples produced digital elevation maps of complicated kink band patterns and intersections, and geometric curvature calculations based on these scans provide detailed maps of kink band hinges, intersections, and locations of elevated strain. Curvature maps clearly outline the behavior of intersecting bands in crossing (X), bifurcating or merging (Y), and previously unrecognized oblique (λ) intersections. Statistical analyses on values extracted from the curvature maps show relationships between relief, gradient, and curvature of hinges. Shortening accommodated by kink bands is less than 5%, comparable to shortening of less than 10% calculated from field data. Thin section and magnetic fabric analyses indicate that interlayer slip occurred during kinking. Interlayer slip during kinking may have sheared the magnetic fabric within the band, producing less anisotropic and more shallowly rotated fabric than predicted from the kink band geometry. Dilation spaces are also visible in thin section, where unrecrystallized quartz and/or calcite fill dilation spaces, and mica layers protrude into the dilation spaces from inside the kink bands due to interlayer slip. No broken grains or other evidence for boundary migration were observed in thin section. The variability of kink band angles, prominence of dilation spaces along kink band hinges and inside bands, and the lack of migration structures in the field or in thin section all suggest fixed-hinge, rotational kinking was the dominant mechanism of kink band development for these bands in the Darrington Phyllite. The orientation of the monoclinal kink band sets, the small amount of shortening, and the interpretation of fixed-hinge kinking are all consistent with kink band development during late-stage unroofing at low confining pressure under northeast/southwest compression.</p>