Ken Thomson writes: The suggestion that the Silverpit Crater is a pull-apart basin (Smith 2004) and not an impact structure (Stewart & Allen 2002) demonstrates that its origin requires further investigation. As noted by Underhill (2004) the coincidence of the structure with a salt withdrawal basin suggests that halokinesis may provide an alternative mechanism. Although the model provided by Smith involves salt movement it is overly complex as strike-slip detachment tectonics are not required. Instead gravitational collapse of the post-salt cover provides a simpler mechanism.

The Southern North Sea is dominated by the development of large linear salt swells (Jenyon 1988; Stewart & Coward 1995). Elevation and tilting of the swell flanks results in gravitational instability and a range of deformation styles can develop (Jenyon 1988; Stewart & Coward 1995; Thomson 2004). The instability commonly leads to listric faulting of the post-salt cover down slope, towards the adjacent salt low (Jenyon 1988; Stewart & Coward 1995; Thomson 2004). Detachment levels for the listric faults occur at a number of stratigraphic levels. The top of the Upper Permian Zechstein Group salt, which forms the swells, provides a detachment at the base of the post-salt cover whilst detachment within the post-salt cover in both the Triassic Röt Halite and within the Upper Cretaceous chalk have also been recognized (Jenyon 1988; Stewart & Coward 1995; Thomson 2004). The majority of the flank collapse faults dip in the same direction as the cover rocks they displace (i.e. towards the salt withdrawal basin and away from the swell), but occasionally listric faults can be found which dip away from the salt withdrawal basin, towards the swell (Jenyon 1988). Associated with the listric faulting rollover anticlines can develop in the hanging walls (Stewart & Coward 1995) whilst slump folds …