Strain localization in direct shear experiments on Solnhofen limestone at high temperature – Effects of transpression

Some features of natural shear zones formed under non-coaxial strain geometries, including some effects of transpression, can be simulated in the laboratory by using the direct shear experimental configuration. Slices of ∼1 mm thick Solnhofen limestone were deformed in direct shear between two stronger forcing blocks of cores of Tennessee sandstone pre-cut at 45° to the cylinder axis. Experiments were run dry at 600 °C, 200 MPa confining pressure and bulk shear strain rates of ∼5 × 10−3 s−1, at which conditions Solnhofen limestone deformed by dislocation creep with a stress exponent of 4.7. When loaded, strain concentrates in the limestone band, producing non-coaxial deformation as one pre-cut block slides past the other. The orientation and intensity of the shape fabric developed in calcite grains indicate that strain is heterogeneous across the specimen, with the formation of two high-strain shear bands close to the limestone–sandstone interface, separated by a central zone of low strain. Crystallographic preferred orientation patterns in the calcite grains measured by electron backscatter diffraction are consistent with a switch in deformation geometry from flattening-dominated in the middle of the specimen towards shear-dominated in the high-strain bands. From tests on thin slices of the same material compressed axisymmetrically (without shearing) normal to the layer, heterogeneous thinning of the slice develops, from a maximum in the centre of the slice to zero at the edges. The formation of the paired shear zones observed in the sheared experiments is interpreted in terms of superposed strain fields, with shearing in the centre of the slice being inhibited by the strain hardening that accompanies the higher flattening strain in the centre of the specimen.