Geometric curvature analysis of intersecting kink bands: A new perspective on the 3D geometry of kink folds

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.