Abstract - Basement controlled normal faulting has been studied extensively in the past using a variety of approaches, such as physical (sand, clay), numerical (FEM) and analytical modelling. Typical boundary conditions include a rigid basement, containing pre-cut faults, which is overlain by a (brittle) cover. The base cover interface is typically horizontal and the (sand, clay) model is extended under plane strain conditions. Under these simple boundary conditions the growth pattern – details of which depend on the dip of the basement fault and the rheology of the overburden - within the cover sequence can be summarized as follows: (i) A steep precursory fault develops, which is often convex up and is therefore a reverse fault close to the top of the surface of the cover. (ii) A shallower dipping masterfault later develops in the footwall of the precursory fault. (iii) If the masterfault within the cover has a steeper dip than the pre-cut fault, antithetic adjustment faults develop. In summary, faults that develop under these boundary conditions often comprise multiple slip surfaces, partly due to free surface effects and fault refraction, i.e. fault dip variations between pre-cut and cover fault. Systematic stepping of fault segments is not expected under these boundary conditions. In this talk we show that systematic and predictable stepping a fault segments above a pure dip slip normal fault can be generated in the cover using an inclined base cover interface.
We have performed a series of sandbox experiments in order to investigate the impact of a dipping base cover interface on fault growth and geometry. Normal faults developing in the analogue material have fault dips of ca 65°. We varied both the dip of the basement faults and the dip of the base cover interface: the basement faults have dips of 45 and 70° and the base cover interface has dips of 0, 10 and 20° and its strike was always normal to the strike of the pre-cut faults. Using the stereonet it can be easily shown that under boundary conditions where the base cover interface is inclined, strike changes of the faults within the cover are expected, if the fault dip within the cover is not exactly the same as the pre-cut fault dip. This is due to the fact that a continuous fault that refracts across the interface has to have the same fault/interface intersection lineation. An alternative outcome is, however, that the cover faults are comprised of dip-slip fault segments that exhibit systematic stepping.
Our simple geometrical predictions of the orientation of cover faults are verified with our sandbox experiments. In addition to the typical growth sequence of basement controlled normal faults described above we observed either fault strike changes or systematic stepping of fault segments within wedge-shaped sand covers. The frequency of relays that develop between the overlapping segments increases with increasing dip of the base cover interface. Our results highlight the fact that systematic stepping of faults above a basement fault is not necessarily a kinematic indicator of oblique-slip reactivation and we advise caution regarding interpretation of fault kinematics from stepping directions alone. Our model also suggests that refracting normal faults in inclined multilayers will most likely be segmented.
Abstract of talk given to:
Tectonic Studies Group Annual Meeting, Glasgow, January 2007.