Strain localisation in mechanically layered rocks beneath detachment zones: insights from numerical modelling

Abstract

Abstract.We have designed a series of fully dynamic numerical simulations aimed at assessing how the orientation of mechanical layering in rocks controls the orientation of shear bands and the depth of penetration of strain in the footwall of detachment zones. Two parametric studies are presented.In the first one, the influence of stratification orientation on the occurrence and mode of strain localisation is tested by varying initial dip of inherited layering in the footwall with regard to the orientation of simple shear applied at the rigid boundary simulating a rigid hanging wall, all scaling and rheological parameter kept constant. It appears that when MohrCoulomb plasticity is being used, shear bands are found to localise only when the layering is being stretched. This corresponds to early deformational stages for inital layering dipping in the same direction as the shear is applied, and to later stages for intial layering dipping towards the opposite direction of shear. In all the cases, localisation of the strain after only γ = 1 requires plastic yielding to be activated in the strong layer.The second parametric study shows that results are lengthscale independent and that orientation of shear bands is not sensitive to the viscosity contrast or the strain rate. However, decreasing or increasing strain rate is shown to reduce the capacity of the shear zone to localise strain. In the later case, the strain pattern resembles a mylonitic band but the rheology is shown to be effectively linear.Based on the results, a conceptual model for strain localisation under detachment faults is presented. In the early stages, strain localisation occurs at slow rates by viscous shear instabilities but as the layered media is exhumed, the temperature drops and the strong layers start yielding plastically, forming shear bands and localising strain at the top of the shear zone. Once strain localisation has occured, the deformation in the shear band becomes extremely penetrative but the strength cannot drop since the shear zone has a finite thickness.

The Impact of Matrix Rheology on Stress Concentration in Embedded Brittle Clasts

“…Details of the 2D version of the code are given in Appendix A, as well as in Jourdon et al (2018) and Ioannidi et al (2021). We consider the structures observed in the field to be scale independent, as suggested by recent studies (Fagereng, 2011;Grigull et al, 2012;Ioannidi et al, 2021;Le Pourhiet et al, 2013), therefore we upscale field outcrop scales to map scales (from a cm-scale to a m-scale). For more details, see Discussion on scaling (Section 5.1.1).…”

“…Previous work (Fagereng, 2011;Grigull et al, 2012;Ioannidi et al, 2021;Le Pourhiet et al, 2013) showed that the dimensions of blocks/structures do not affect the results of a model, as long as block distribution, strain rate and deformation mechanisms are the same at the different scales. Here, we created a numerical map representing the average block distribution of the field observations (7%).…”

M2Di: Concise and efficient MATLAB 2‐D Stokes solvers using the Finite Difference Method

“…Experimentally derived flow laws characterizing the rheology of single mineral phases often exhibit a power law behavior. Modeling the deformation of heterogenous power law viscous materials allows to unravel the dynamics of shear localization and the formation of rock fabrics [Mancktelow, 2002;Jessell et al, 2009;Deubelbeiss et al, 2011;Dabrowski et al, 2012;Le Pourhiet et al, 2013]. We have set up a power law model consisting in randomly located highly viscous elliptical inclusions.…”

Author Response to Reviewers and Comments

“…It is clear that continental break-up can only be achieved if there is localization of deformation (Le Pourhiet et al, 2013;Moresi et al, 2007;Regenauer-Lieb et al, 2008). This is achieved by feedback and possibly the presence of magma (Corti et al, 2003).…”