Rheologically Driven Strain Partitioning in the Footwall Shear Zone of a Metamorphic Core Complex, Raft River Mountains, Utah

The Raft River shear zone (RRSZ) is a footwall shear zone of an east–west-trending, megamullion-shaped metamorphic core complex in the northernmost Great Basin, Utah, U.S.A. This paper examines part of the RRSZ that exhibits a large transport-parallel gradient in strain intensity coupled with a transition from flattening to constrictional strain. Detailed geologic mapping and finite-strain, microstructural, 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 Elba Quartzite in the center of the RRSZ uniformly records 50–60% noncoaxial deformation. Rocks at the lower boundary and uppermost preserved levels of the RRSZ record simple-shear-dominated flow. Coaxial, highly constrictional deformation is localized in channel-form quartz-cobble-metaconglomerate beds at the base of the quartzite. At the same structural level rheologically weaker phyllonites that envelop the metaconglomerate beds record simple-shear-dominated deformation. The domain of intense deformation and necking of the shear zone is localized where the basal stretching fault cuts rheologically weak schist. This domain is characterized by strain in the constrictional field caused by transport-perpendicular flow into the area of high transport-parallel elongation. Where the basal stretching fault cuts rheologically stronger granite, 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. These data indicate that the rheology of rocks cut by the stretching fault directly controlled the amount and style of zone-normal shortening and transport-parallel elongation, and they provide an example of deformation partitioning and deformation-path partitioning in a crustal-scale extensional structure driven by local rheological transitions.