PRIMED TO SLIDE: THE ROLE OF CLAY NANOCOATINGS IN THE DEVELOPMENT OF SCALY FAULT FABRICS

Scaly fabrics, composed of elongate phacoids, often host distributed slip in fault zones when deformation is not concentrated along a single slip surface. Scaly fabrics can develop in a wide range of host rocks. Here, we examine the development of scaly fabrics from the terrane-bounding, strike-slip Minas fault zone (Nova Scotia, Canada). In the low temperature (~150 ºC) domain of deformation, shear strain is localized into a phyllite-hosted fault strand ~7 m wide which contains phacoids ranging from 1 mm to 15 cm in length. Phacoid edges show slickenlines with a range of rakes, but with a dominant sub-horizontal orientation consistent with strike-slip kinematics. Between phacoids are angular fragments of wallrock and rounded spheres (~5 cm in diameter) of quartz coated in a black layer of chlorite with pervasive slickenlines. Transmission electron microscopy images of phacoids display evidence for dissolution and re-precipitation of framework silicates. However, framework silicates are coated in nm-scale layers of chlorite which indent into the framework silicates. We hypothesize that early deformation in the fault strand was accommodated by dissolution-precipitation creep followed by a switch in deformation mechanism to frictional sliding through the development of chlorite nanocoatings. Nanocoatings of clay can dramatically alter deformation over the lifetime of a fault.