Influence of Heterogeneities on the Initiation of Shear Zones in the Ductile Regime
Final Report Abstract
The evolution of a high temperature shear zone by strain localization involves a nucleation and growth stage. In the brittle-ductile transition regime and at lower temperatures, shear zone formation often occurs at brittle precursors or by ductile fracturing. Local stress concentrations at initial defects and/or during propagation at the shear zone tip potentially result in local fracturing or grain size reduction by cracking even at elevated temperatures and pressures, inducing the initiation of ductile shear zones and consequently deformation localization along these high strain zones. The idea of this proposal was to simulate in the laboratory what has been observed in nature at multiple scales. In order to do that, we have developed a novel experimental assembly where single inclusions of a fine-grained, weak material (Solnhofen Limestone) were “inserted” in a coarse-grained host of similar composition (Carrara Marble). During the development of the project, we progressively increased the complexity of the geometries of these inclusions in relation to the applied stresses during the experiments. We started with simple one inclusion deformed under torsion (simple shear), and evolved to two inclusions aligned to different angles and geometries in relation to the maximum stress axis, to simulate local extensional and compressional fields between the inclusions. Nardini et al. (2018) concluded that 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 bulk strains, the evolution of microstructural and textural parameters is independent of loading conditions. The authors concluded that the loading conditions do not significantly affect material heterogeneity-induced strain localization during its nucleation and transient stages. From Nardini et al. (2020) we can learn that viscosity contrast between the matrix and the inclusions induces local stress concentration at the tips of these latter. The initial arrangement of the inclusions results in either an overpressured (contractional) or under- pressured (extensional) domain in the step-over region of the sample. At low confinement (30 and 50 MPa) abundant brittle deformation is observed, but the spatial distribution of microfractures is dependent on the kinematics of the step-over region: microcracks occur either along the shearing plane between inclusions (in extensional bridge samples), or broadly distributed outside the step-over region (contractional bridge samples). Accordingly, ductile deformation localizes along the inclusions plane in the extensional bridge samples as opposed to distributing over large areas of the matrix in the contractional bridge samples. If microcracking is suppressed (high confinement), strain is accommodated by viscous creep and strain progressively de-localizes in extensional bridge samples. The authors concluded that those brittle precursors enhance the degree of localization in the ductile deformation regime, but only if the interaction of pre-existing heterogeneities induces an extensional mean stress regime in between.
Publications
- 2018. High-temperature shear zone formation in Carrara marble: the effect of loading conditions. Tectonophysics 749, 120–139. 139
Nardini, L., Rybacki, E., Döhmann, M.J.E.A., Morales, L.F.G., Brune, S., Dresen, G.
(See online at https://doi.org/10.1016/j.tecto.2018.10.022) - 2019. Strain localization and weakening processes in viscously deforming rocks: numerical modeling based on laboratory torsion experiments. J. Geophys. Res.: Solid Earth 124 (1), 1120–1137
Döhmann, M.J.E.A., Brune, S., Nardini, L., Rybacki, E., Dresen, G.
(See online at https://doi.org/10.1029/2018JB016917) - (2020). Control of the geometric arrangement of material heterogeneities on strain localization at the brittle-to-ductile transition in experimentally deformed carbonate rocks. Journal of Structural Geology, 135, 104038
Nardini, L., Rybacki, E., Krause, M., Morales, L. F. G., & Dresen, G.
(See online at https://doi.org/10.1016/j.jsg.2020.104038)
