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.
Highlights
Knowledge of the stress tensor state in the Earth’s upper crust is important for a better understanding of the endogenous dynamics, seismic hazard or exploitation of the underground
Stress orientation data are compiled globally in the World Stress Map database (Zoback et al, 1989; Zoback, 1992; Sperner et al, 2003; Heidbach et al, 2010, 2018). Based on such data compilations, it was assumed that patterns of stress orientation on a regional scale are more or less uniform within tectonic plates (Richardson et al, 1979; Klein and Barr, 1986; Müller et al, 1992; Coblentz and Richardson, 1995)
The main subject of this study is to investigate the impact of the variation of elastic rock properties, density and friction along faults on stress orientation in the upper crust in the given geometrical setting outlined in the previous sections (Fig. 2)
Summary
Knowledge of the stress tensor state in the Earth’s upper crust is important for a better understanding of the endogenous dynamics, seismic hazard or exploitation of the underground. Stress orientation data are compiled globally in the World Stress Map database (Zoback et al, 1989; Zoback, 1992; Sperner et al, 2003; Heidbach et al, 2010, 2018) Based on such data compilations, it was assumed that patterns of stress orientation on a regional scale are more or less uniform within tectonic plates (Richardson et al, 1979; Klein and Barr, 1986; Müller et al, 1992; Coblentz and Richardson, 1995). It was suggested that remnant stresses due to old plate tectonic events are able to overprint stress orientation on a regional scale (e.g. Eisbacher and Bielenstein, 1971; Tullis, 1977; Richardson et al, 1979) Such old basement structures present geomechanical inhomogeneities and discontinuities, which have the potential to perturb the stress pattern.
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