| The style of deformation is dependent on the orientation of the stress ellipsoid. In the Earth's crust one can demonstrates that one of the three principal axes of the stress ellipsoid is always vertical. On that basis, three tectonic regimes can be defined that correspond to the three main types of plate boundaries. |
Tectonic Regime in Contraction. |
| σ3 the smallest axis is vertical. Rock formations tend to thick and fold. This is the usual regional tectonic regime at convergent plate boundaries, the one associated with the formation of mountain belts->
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