CONTINENTAL BREAKUP
Geometric aspects of continental breakup: How do continents break ? Pure shear vs simple shear ...
The mode of extensional deformation in the continental crust is strongly controlled by the depth of the brittle ductile transition and the degree of coupling between the brittle upper crust and the ductile lower crust. Décollement surfaces are brittle/ductile shear zones running parallel to the rheological layering of the crust. This contrast with detachment surfaces which cut through the layering.
The brittle/ductile transition in the crust is a major rheological discontinuity therefore focussing shear stress and shearing. During extension, a décollement surface may develop at the brittle/ductile transition, mechanically decoupling the upper from the lower crust. When the rheological profile is such that the brittle/ductile transition is not a sharp dicontinuity but a thicker transition zone the upper crust is strongly coupled to the lower crust. In this instance a crustal-scale detachment rather than a décollement is to be expected.
On the examples on the right, all normal faults dip toward the ocean. This is not always the case as we will see later on...