INCREMENTAL AND FINITE STRAINS WITHIN DUCTILE SHEAR ZONES, NARRAGANSETT BASIN, RHODE ISLAND (STRUCTURAL, APPALACHIAN TECTONICS)

A geometric technique is presented for determining sense and amount of shear and finite displacements from superposed shear-related cleavages. These geometric models are applied to a system of ductile shear zones in the Narragansett Basin of southern Rhode Island. Multiple shear-related crenulation cleavages that form by bulk non-coaxial rotational deformation are used as kinematic indicators within ductile shear zones. Quantitative models explore the effects of anisotropy/ shear zone geometry and incremental and finite strain histories on resulting planar fabrics.; Pennsylvanian pelitic metasediments of the Narragansett Basin record a complex history of Alleghanian deformation during the Permian. Two early compressive events (D(,1) and D(,2)) related to basin closure are accompanied by prograde Barrovian regional metamorphism. Two later deformations (D(,3) and D(,4)) are related to left-lateral and right-lateral shear, respectively, within a system of strike-slip ductile shear zones that comprise the Beaverhead Shear Zone. The D(,3) and D(,4) events were progressive in nature and produced multiple phases of folding. Progressive deformation of graphitic schists within the zones formed superposed crenulation cleavages. Overprinting crenulations track movement histories and sense and amount of shearing deformation within the zones. Microscopic examination of the deformed fabric indicates the crenulations formed by buckling along the foliation with minor flattening and no appreciable volume loss or solid-state diffusion.; Using the models presented, the three-dimensional geometries of successive crenulation cleavage planes are used to calculate shear strains. Left-lateral shear strains are generally greater than right-lateral shear strains. Values for left-lateral shear strains range from 0.5 to 19.6; those for right-lateral shear strains range from 0.2 to 25.0. Integration of shear strains across individual stirke-slip ductile shear zones yields minimum estimates for the bulk finite displacement across the bulk Beaverhead Shear Zone of approximately 3 km in a left-lateral sense and 5 km in a right-lateral sense.