Curved slickenfibers: a new brittle shear sense indicator with application to a sheared serpentinite

Abstract

Brittle fault zones are commonly characterized by penetrative fracturing of the rock to form an aggregate of blocks, so that the rock becomes, in essence, like a granular material composed of rigid ‘grains’. The surfaces of the blocks, or ‘grains’, have a wide distribution of orientations, and shearing of the blocks past one another on these planes accommodates the large-scale motion, i.e. the macromotion, in the fault zone. During a macromotion that is a non-coaxial deformation, the rigid blocks and their surfaces may undergo a progressive rigid rotation that is distinct from the macromotion and is described by the microspin. If a macroscopic non-coaxial deformation has monoclinic symmetry, this symmetry should be reflected in the sense of rigid rotation of the blocks and their surfaces. On the surfaces of the blocks, which are local shear planes, the history of displacement may be recorded by mineral fibers that grow progessively with displacement. Because of the rigid rotation of the local shear planes, slickenfiber lineations commonly are curved. The sense of curvature from the youngest to the oldest part of the fiber, looking down the normal to the local shear plane, is different for those planes whose normals are on opposite sides of the unique monoclinic symmetry plane for the macroscopic shearing. The intersection of the symmetry plane with the plane of the fault zone defines the macroscopic slip direction, and the sense of rigid rotation of the local shear planes determined from the lineations defines the shear sense. Application of this technique to the disrupted and sheared margins of the Feather River Peridotite in the northern Sierra Nevada of California indicates that late deformation involved dextral-normal oblique slip along the Melones fault zone.