GEOMETRY OF KINK BANDS IN THE DARRINGTON PHYLLITE, NORTHWESTERN WASHINGTON

Kink bands in the Cretaceous Darrington Phyllite (Easton Terrane) deform the well-defined foliation on Samish Island, NW WA, where coastal outcrops and a quarry give 3D exposures. The kink bands are late-stage contractional features that post-date the development of local folds and multiple cleavages. Two sets of kink bands are present: the dominant set (A) has axial surfaces that dip moderately NE and deforms the south-dipping phyllitic foliation; the less common set (B) dips steeply SW and deforms a secondary cleavage. In places where both sets are present, the SW-dipping set appears to crosscut the dominant NE-dipping set and may be younger. Overall, the kink bands run perpendicular to the axial planes of larger folds, and some thin or die out in the fold hinge. Intersections within set A are observed in the plane of the foliation and include crossing X, merging Y, and truncating T geometries. In cross section, the kink bands of a single set are straight and roughly parallel. The kink bands of set A are generally closely spaced, averaging 3-5 cm between bands (range: 0.2-52 cm). There is no apparent relationship between spacing and width of the bands: the kinked width of the bands ranges from 0.1-4.8 cm with a mode of 0.5 cm. The kink bands of set B are more widely spaced, averaging 11 cm (range: 3-29.5 cm), and show greater variability in other characteristics. The angular relationships between the kinked and unkinked foliation are complex and do not conform to published idealized geometric models of kink bands. For both sets, the external kink angle (α) between the undeformed foliation and the kink band boundaries clusters closely between 65-80°, and the internal kink angle (β) ranges from 58-90°; however, α for set A has a mode of 80°; 65° for set B. An idealized kink band propagates or locks at α=β: thus we predict well-developed kink band sets to show β/α=1 and early stage bands to show β/α>1. Just over a third of all bands measured on Samish Island (n=188) show 0.9≤β/α≤1.1, and about a third have β/α<1, not allowed by the models. The deviation of the kink bands from the ideal model may be explained by the presence of quartz and calcite within the kink bands and along the kink band boundaries, indicating volume and rheology changes during the development of the bands, and/or rotation of the external foliation during deformation.