Dynamic link between the level of ductile crustal flow and style of normalfaulting of brittle crust

In a theologically layered crust, compositional layers have an upper, elast o-plastic part and a lower, viscous one. When broken, the upper elastic par t undergoes flexure, which is upward for the foot-wall and downward for the hanging wall. As a consequence of bending, stresses will develop locally t hat can overcome the strength of the plate and, therefore, impose the migra tion of active fault. In the lower, viscous part of each compositional laye r, rocks can potentially flow. Numerical modelling of the behaviour of a cr ust made up of two compositional layers, during and following extension, sh ows 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. Whe n 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 exte nsion 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 t ake place, middle crustal rocks will move towards the rifting zone causing isostatically driven upward movements that will be superimposed on movement s associated with crustal and lithospheric thinning. Consequently, fault bl ocks 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 thic kened crust and has its most apparent expression in core complexes. (C) 200 0 Elsevier Science B.V. All rights reserved.