In a theologically layered crust, compositional layers have an upper,
elasto-plastic part and a lower, viscous one. When broken, the upper
elastic part undergoes flexure, which is upward for the foot-wall and
downward for the hanging wall. As a consequence of bending, stresses
will develop locally that can overcome the strength of the plate and,
therefore, impose the migration of active fault. In the lower, viscous
part of each compositional layer, rocks can potentially flow. Numerical
modelling of the behaviour of a crust made up of two compositional
layers, during and following extension, shows that flow can take place
not only in the lower crust but also, and more importantly, in the lower
part of the upper crust. The ability of crustal rocks to flow in
influences the style and kinematics of rifted regions. When no flow
occurs, subsidence will affect the extending areas, both hanging wall
and foot-wall will subside with respect to an absolute reference frame
such as sea level, and there will be a strict proportionality between
extension and thinning. In addition, the downward movement of fault
blocks will decrease the local stresses created in the foot-wall and
increase those of the hanging wall, thereby imposing a migration of the
active fault towards the hanging wall. This is the behaviour of
extensional settings developed on stabilised crust and which evolved in
a passive margin. When flow does take place, middle crustal rocks will
move towards the rifting zone causing isostatically driven upward
movements that will be superimposed on movements associated with crustal
and lithospheric thinning. Consequently, fault blocks will move upwards
and the crust will show more extension than thinning. The upward
movements will decrease the stresses developed in the hanging walls and
increase those of the foot-wall. Faults will then migrate towards the
foot-wall. Such a mode of deformation is expected in regions with
thickened crust and has its most apparent expression in core complexes.
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