On the reactivation of extensional fault systems

In terranes that have undergone substantial extension, three sets of faults dominate: (a) shallow- to steep-dipping, commonly rotational normal faults; (b) a really extensive, shallow-dipping, normal detachment faults; and (c) steep-dipping transfer faults that strike at high angles to the normal faults. These fault systems may extend through a large fraction of the crust. Reactivation of these fault systems will depend primarily on the relative strengths of the faults (shear zones) and their host rock, and their orientation in the prevailing stress field. It is concluded that reactivation is generally mechanically favoured, but that it will probably only take place when the fault-shear zones are in near-ideal orientations. Consideration of the tectonic setting of extended terranes and of the limited number of well described examples suggests that reverse (thrust) reactivation of the normal and detachment faults and wrench reactivation of transfer faults are the most likely styles. Examples of these styles are described from the Bass Strait Basins of southeastern Australia. Because extended terranes commonly underlie sedimentary basins (for example, on passive continental margins), reactivation of extensional faults may be a key control on the tectonic evolution of such basins (i.e. basin inversion).