High-temperature shear zone formation in Carrara marble: The effect of loading conditions

Abstract

Rock deformation at depths in the Earth's crust is often localized in high temperature shear zones occurring at different scales in a variety of lithologies. The presence of material heterogeneities is known to trigger shear zone development, but the mechanisms controlling initiation and evolution of localization are not fully understood. To investigate the effect of loading conditions on shear zone nucleation along heterogeneities, we performed torsion experiments under constant twist rate (CTR) and constant torque (CT) conditions in a Paterson-type deformation apparatus. The sample assemblage consisted of cylindrical Carrara marble specimens containing a thin plate of Solnhofen limestone perpendicular to the cylinder's longitudinal axis. Under experimental conditions (900 °C, 400 MPa confining pressure), samples were plastically deformed and limestone is about 9 times weaker than marble, acting as a weak inclusion in a strong matrix. CTR experiments were performed at maximum bulk shear strain rates of ~ 2*10 -4 s -1 , yielding peak shear stresses of ~ 20 MPa. CT tests were conducted at shear stresses of ~ 20 MPa resulting in bulk shear strain rates of 1-4*10 -4 s -1 . Experiments were terminated at maximum bulk shear strains of ~ 0.3 and 1.0.Strain was localized within the Carrara marble in front of the inclusion in an area of strongly deformed grains and intense grain size reduction. Locally, evidences for coexisting brittle deformation are also observed regardless of the imposed loading conditions. The local shear strain at the inclusion tip is up to 30 times higher than the strain in the adjacent host rock, rapidly dropping to 5 times higher at larger distance from the inclusion. At both investigated bulk strains, the evolution of microstructural and textural parameters is independent of loading conditions. Our results suggest that loading conditions do not significantly affect material heterogeneity-induced strain localization during its nucleation and transient stages.

Strain Localization and Weakening Processes in Viscously Deforming Rocks: Numerical Modeling Based on Laboratory Torsion Experiments

“…Constant stress model and experimental results also do not differ significantly as shown in the supplements (Figure S1). In agreement with the results from Nardini et al (), this test likewise indicates that both loading configurations (constant strain rate and constant stress) lead to nucleation of ductile shear zones.…”

“…Two different loading conditions, constant twist rate (equivalent to a shear strain rate of 1.9 × 10 −4 s −1 at the outer periphery) and constant torque (~18.8 MPa) were tested. For each loading type, samples were tested to a final bulk shear strain γ ~ 1 (Nardini et al, ).…”

“…The local shear strain in this area is higher than in the adjacent host rock. At the inclusion tip, the local strain is up to about 30 times higher than in the neighboring matrix and ~10 times higher than the bulk strain (Nardini et al, ).…”

“…Our reference model is designed to reproduce the single inclusion experiments of Nardini et al () described above in 2‐D Cartesian coordinates. We model the laboratory shear deformation of a hollow cylinder by using periodic boundary conditions, such that material leaving one side of the model in shear direction enters again on the opposite side (see Figure c).…”

“…Here we compare the results of our numerical models to a series of laboratory tests (Nardini et al, ), in order to investigate the temporal and spatial evolution of strain localization and weakening processes in viscously deforming rocks. As mentioned above, ductile shear zones are often initiated at material heterogeneities, which is why we use a single weak inclusion torsion setup to analyze dynamics, strength, and geometry of the resulting ductile shear zone.…”