CROSS-SECTION RESTORATION – SPICIER WITH A TOUCH OF SALT

Cross-section restoration depends on a number of assumptions and thus has its limitations. Add some mobile salt and the uncertainties mount. One of the primary assumptions, that of plane-strain deformation, has to be abandoned for salt because of its three-dimensional flow. Geologists consequently restore supra- and subsalt domains separately without constraining the area of salt to be maintained over time. Nevertheless, there are often large uncertainties in just how to handle salt and even the surrounding strata. Interpretative bias and a regional approach, always important components of restoration, become even more critical.

Here, we briefly examine four examples of this intractable problem. First, basinward-dipping rollover geometries can be restored either as salt-evacuation structures or as counterregional normal faults. Nothing in the local geometry is diagnostic, and only knowledge of laterally equivalent or more distal provinces can help constrain the solution. Second, diapirs in contractional terranes get squeezed during shortening, but by an unknown amount. Sections through adjacent fold structures can be used to estimate the amount of cryptic shortening, but steep lateral strain gradients make even this an underestimate. Determining the amount of shortening in the case of salt walls is even more problematic because the lateral strain gradient itself can be entirely hidden in salt. Third, even where diapirs have not been shortened, the application of different restoration algorithms leads to varying interpretations of the onset of diapirism. Fourth, the presence of multiple salt layers provides its own set of idiosyncrasies. Accommodation for a shallow minibasin can be restored in the deep autochthonous salt, in the shallow canopy, or in a combination of the two. Moreover, if the base of the canopy has been deformed since its emplacement, it is impossible to determine the timing if the ‘growth’ is hidden in salt. In all these examples, and in countless more, good geological reasoning using all available data is crucial if restorations are to have any chance of representing the evolution of salt structures accurately.