MICRO-SCALE DEFORMATION MECHANISMS IN KINK BANDS: PRESSURE SOLUTION MODIFIES INTERLAYER SLIP

Grain-scale deformation processes that accompany kink fold development in fine-grained, anisotropic rocks have not been well characterized, despite multiple studies of the macro-scale features of kink bands. Thin section and magnetic fabric analyses on a set of monoclinal, contractional kink bands in the Darrington Phyllite of northwestern Washington provide observations of microstructures associated with kink bands and evidence for interlayer slip and pressure solution during kinking. Dilation spaces along kink band hinges and inside kink bands are common in thin section, and void spaces are filled with unrecrystallized quartz and/or calcite, in contrast to the recrystallized grains of the host rock. In many places, mica grains protrude into void spaces along the hinges, interfingering with quartz/calcite infill. These observations suggest that interlayer slip along mica-rich horizons allowed for rotation of the foliation inside the kink band. Pressure solution is not ubiquitous inside kink bands from the study area; however, significant pressure solution is associated with “ideal, locked” kink bands, where the internal and external kink angles are equal. Magnetic fabric analyses of the anisotropy of magnetic susceptibility (AMS) show that the magnetic fabric inside kink bands was modified due to kinking, generally producing a shallower rotation than expected by the mineral fabric and a less anisotropic AMS ellipsoid; however, the deflection is less anomalous for “locked” kink bands, where pressure solution was significant. We propose that shearing due to interlayer slip along the foliation modified the magnetic fabric, producing the shallow deflection and more isotropic fabric; once kink bands locked and rotation was no longer possible, further shortening was accommodated by pressure solution inside the kink band, effectively rotating the magnetic fabric back towards its original orientation. The microstructures observed here and the inferences drawn from the magnetic fabric analyses yield insight on the unresolved kinematic models of kink band formation, and suggest that fixed-hinge rigid rotation was the main mechanism of kinking.