Stress rotation – impact and interaction of rock stiffness and faults

Abstract

Abstract. It has been assumed that the orientation of the maximum horizontal compressive stress (SHmax) in the upper crust is governed on a regional scale by the same forces that drive plate motion. However, several regions are identified where stress orientation deviates from the expected orientation due to plate boundary forces (first-order stress sources), or the plate wide pattern. In some of these regions, a gradual rotation of the SHmax orientation has been observed. Several second- and third-order stress sources have been identified in the past, which may explain stress rotation in the upper crust. For example, lateral heterogeneities in the crust, such as density and petrophysical properties, and discontinuities, such as faults, are identified as potential candidates to cause lateral stress rotations. To investigate several of these candidates, generic geomechanical numerical models are set up with up to five different units, oriented by an angle of 60∘ to the direction of shortening. These units have variable (elastic) material properties, such as Young's modulus, Poisson's ratio and density. In addition, the units can be separated by contact surfaces that allow them to slide along these vertical faults, depending on a chosen coefficient of friction. The model results indicate that a density contrast or the variation of Poisson's ratio alone hardly rotates the horizontal stress (≦17∘). Conversely, a contrast of Young's modulus allows significant stress rotations of up to 78∘, even beyond the vicinity of the material transition (>10 km). Stress rotation clearly decreases for the same stiffness contrast, when the units are separated by low-friction discontinuities (only 19∘ in contrast to 78∘). Low-friction discontinuities in homogeneous models do not change the stress pattern at all away from the fault (>10 km); the stress pattern is nearly identical to a model without any active faults. This indicates that material contrasts are capable of producing significant stress rotation for larger areas in the crust. Active faults that separate such material contrasts have the opposite effect – they tend to compensate for stress rotations.

Competition between 3D structural inheritance and kinematics during rifting: insights from analogue models

“…The en echelon grabens developing along an oblique VD (Figs. 7d, 8d) are typical for such model set-ups, as the effective stretching direction is locally deviated by the oblique VD (e.g., Tron & Brun 1993;Bonini et al 1997;Clifton et al 2000;Van Wijk 2005;Morley 2010;Brune & Autin 2013;Ruh et al 2019;Ducleaux et al 2020;Reiter 2021;Zwaan et al 2021a). The varying effects of the simulated crustal weaknesses in our models are in agreement with previous model results.…”

3D crustal stress state of Germany according to a data-calibrated geomechanical model

“…The model geometry reflects the contemporary distribution of rock properties such as density and stiffness and Poisson's ratio. An appropriate initial stress equilibrates the gravitational stresses and resembles a reference stress state (Fischer and Henk, 2013;Hergert et al, 2015;Reiter and Heidbach, 2014). The orientations of the lateral model boundaries where the displacement boundary conditions are applied are chosen in such a way that the mean S Hmax orientation (Fig.…”

“…6c). This procedure has been used and described several times (Buchmann and Connolly, 2007;Hergert, 2009;Hergert and Heidbach, 2011;Reiter and Heidbach, 2014). The resulting stress state represents the initial stress state, which is subsequently perturbed by applying displacement boundary conditions that impose the tectonic stress.…”

“…Nevertheless, our model results also show no impact of mechanical contrasts on the orientation of S Hmax , e.g predicted by Grünthal and Stromeyer (1994), Marotta et al (2002) or Cacace (2008) despite mechanical contrast, e.g., a Young's modulus difference of 40 GPa between the sedimentary (30 GPa) and the upper crust units (70 GPa). Probably a lateral stiffness difference and a weak unit seems to be necessary to get some perturbation due to a stiffness contrast (Reiter, 2021). To test this thesis, we defined an unrealistic low Young's modulus of 30 GPa to the upper crust unit of Avalonia (Fig.…”

“…The model is calibrated using appropriate initial conditions and displacement boundary conditions to find a best fit with respect to the stress orientation and magnitude datasets described above. This modeling approach has been used for a wide range of scales and different tectonic settings (Buchmann and Connolly, 2007;Heidbach et al, 2014;Hergert and Heidbach, 2011;Hergert et al, 2015;Reiter and Heidbach, 2014).…”