Significance of transport-parallel strain variations in part of the Raft River shear zone, Raft River Mountains, Utah, USA

Abstract

This paper examines part of the Raft River shear zone (RRSZ) in northwestern Utah that exhibits an extreme transport-parallel increase in strain intensity coupled with a transition from flattening to constrictional strain. Detailed geologic mapping and finite-strain, quartz- c-axis-fabric, and kinematic-vorticity analyses demonstrate local necking of the shear zone associated with an increase in transport-parallel elongation accommodated by a stretching fault at the base of the shear zone. The domain of intense deformation and necking of the shear zone is localized where the basal stretching fault cuts rheologically weak rocks. This domain is characterized by strain in the constrictional field caused by transport-perpendicular flow into the area of high transport-parallel elongation. Where rocks cut by the stretching fault are rheologically strong, the RRSZ locally records flattening strain and lower strain intensities, limiting the amount of stretching needed at the base of the shear zone in any one direction and/or recording transport-perpendicular flow into adjacent highly extended domains. The rheology of rocks cut by the stretching fault directly controlled the amount and style of zone-normal shortening and transport-parallel elongation, and these observations provide an example of deformation partitioning in a crustal-scale structure driven by local rheological transitions.