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Abstract: We describe a complex set of monoclinal contractional kink bands, exposed in outcrops of the Darrington Phyllite on Samish Island, northwestern Washington, using traditional field measurements and differential geometry. This study is the first to apply laser scanning and geometric curvature analysis to kink bands to obtain a quantitative description of band geometry on the foliation surface. Kink bands in cross section have straight, parallel boundaries that deform a well-defined foliation; in plan view, however, kink band hinges curve and anastomose across the foliation surface, and adjacent bands commonly intersect. Three types of intersections are common: crossing (X), bifurcating (Y), and obliquely diverging (λ); many kink bands also taper out along strike. Geometric curvature analyses were performed on millimeter-resolution DEMs of hand samples containing intersecting kink bands. Maps of curvature parameters (e.g. mean curvature, geologic curvature) clearly outline kink bands in the samples and illuminate the geometry of kink band hinges in each type of intersection. Shortening across hand samples varies where kink bands intersect. Correlations among geometric parameters corroborate rigid rotation as a kinking mechanism for these bands. Quantitative geometric description is the first step toward understanding the three-dimensional mechanics of kink bands. [Copyright &y& Elsevier]

Abstract: Field, thin section, and magnetic fabric analyses of monoclinal, contractional kink bands in the Darrington Phyllite of northwestern Washington, USA, provide evidence of kinematic evolution and microstructural processes during kinking. Relationships among internal, external, and rotation angles of kink bands suggest that rotation was a primary mechanism of kinking, and a significant number of kink bands deviate from the “ideal” kink band geometry (equal internal and external kink angles). Dilation spaces (voids) filled with unrecrystallized quartz and/or calcite inside kink bands and along the boundaries are commonly observed in kink bands with unequal kink angles. Evidence for localized interlayer slip inside kink bands includes mica grains protruding into the hinge voids and stair-step geometry of the voids. Interlayer slip allowed for rotation of the internal foliation during kinking. Pressure solution is not ubiquitous inside kink bands from the study area; however, significant pressure solution is associated with “locked” kink bands (equal kink angles). A new approach using the anisotropy of magnetic susceptibility (AMS) shows that the magnetic fabric and susceptibility inside the kink bands differ more from the outside fabric than can be explained by simple rotation of the outside fabric into the kink attitudes. Combined, our observations suggest that fixed-hinge kinking operated at this location, and that veining, interlayer slip, and pressure solution were the dominant deformation mechanisms. [Copyright &y& Elsevier]