Strain localization by shear heating and the development of lithospheric shear zones

• Published in: Tectonophysics 2019-08, Vol.764, p.62-76
• Main Authors: Willis, Katy, Houseman, Gregory A., Evans, Lynn, Wright, Tim, Hooper, Andrew
• Format: Article
• Language: English
• Subjects: Aquatic reptiles; Continental tectonics; Deformation mechanisms; Depth; Domains; Fault zones; Fugacity; Geological faults; Heat flow; Heat transmission; Heating; Instability; Lithosphere; Lithospheric deformation; Localization; Magma; Mathematical models; North Anatolian Fault Zone; Numerical experiments; Numerical models; Olivine; Parameterization; Plagioclase; Power law; Rheological properties; Rheology; Robustness (mathematics); San Andreas Fault Zone; Seismology; Shear; Shear zone; Stability; Stability criteria; Strain localization; Strain rate; Stress; Strike-slip shear zone; Temperature; Temperature dependence; Thermal diffusion; Two dimensional models
• ISSN: 0040-1951; 1879-3266
• DOI: 10.1016/j.tecto.2019.05.010

Analogue and numerical models show that strong or weak domains in a deforming ductile material cause stress concentrations that may promote strain localization. Such domains commonly occur in the lithosphere through variations in composition or mineral fabric. Here we use a 2D plane-stress, non-Newtonian, viscous model to explore how strain localization develops from an initial isolated weak inclusion. We use a temperature-dependent rheological law for which the material weakens as a result of work done by shear converted to heat. The progress of strain localization follows a power-law growth that is strongly non-linear and may be regarded as an instability. Although this localization mechanism is ultimately limited by thermal diffusion, this parametrization permits a robust criterion for the conditions in which localized shear zones can form within the lithosphere. Shear zones in the lower crust are typically depicted as the downward continuation of faults. We argue that the depth-extent of narrow shear zones within the lithosphere is limited by the stability criteria that we infer from 2D numerical experiments. When applied to the rheological laws for common lithospheric minerals, the combination of temperature and stress-dependence provides a direct means of predicting the depth below which the localization instability does not occur. For an olivine based rheology, the maximum depth at which rapid localization is expected is in the range of ~20 to 60 km, depending on heat flow, strain-rate and water fugacity. We apply our calculations to two major continental strike-slip zones, the San Andreas Fault and North Anatolian Fault, and compare our predicted maximum localization depths with published seismological images. Strain localization in the lower crust requires a dry rheology comparable to plagioclase. Observations that imply localized strain in the uppermost mantle beneath these fault zones are consistent with the localization criteria and the rheological properties of dry olivine. •Shear-heating enhances local strain concentration and promotes further localization.•Thermally activate localization in a shear zone follows a power-law type growth.•Strain localization rate depends on background temperature, depth and lithology.•Maximum localization depths are consistent with seismic images of major shear zones.•Thermal localization of mantle below continental Moho to ~60 km is possible.