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. It has been assumed that the orientation of the maximum horizontalcompressive stress (SHmax) in the upper crust is governed on aregional scale by the same forces that drive plate motion. However, severalregions are identified where stress orientation deviates from the expectedorientation due to plate boundary forces (first-order stress sources), or theplate wide pattern. In some of these regions, a gradual rotation of theSHmax orientation has been observed. Several second- and third-order stress sources have been identified in thepast, which may explain stress rotation in the upper crust. For example, lateral heterogeneities in the crust, such as density and petrophysicalproperties, and discontinuities, such as faults, are identified as potentialcandidates to cause lateral stress rotations. To investigate several of thesecandidates, generic geomechanical numerical models are set up with up to fivedifferent units, oriented by an angle of 60∘ to the direction ofshortening. These units have variable (elastic) material properties, such asYoung's modulus, Poisson's ratio and density. In addition, the units can beseparated by contact surfaces that allow them to slide along these verticalfaults, 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 allowssignificant stress rotations of up to 78∘, even beyond the vicinity ofthe material transition (&gt;10 km). Stress rotation clearlydecreases 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 stresspattern at all away from the fault (&gt;10 km); the stress pattern isnearly identical to a model without any active faults. This indicates thatmaterial contrasts are capable of producing significant stress rotation forlarger areas in the crust. Active faults that separate such materialcontrasts have the opposite effect – they tend to compensate for stressrotations.

Stress rotation – impact and interaction of rock stiffness and faults