LITMIR.BIZ

Figure 1.32. Evaporite deposition at different stages of rifting, shown just prior to the onset of sea-floor spreading: a) syn-stretching (early syn-rift) salt; b) syn-exhumation (late syn-rift) salt. Rift template modified from Ribes et al. (2019); vertical exaggeration 3:1

      Salt has physical characteristics and mechanical properties (Jackson and Hudec 2017) that make it special in rift evolution and thus a critical consideration when interpreting subsurface geometries. First, it has a constant low density (2.160 g/cm³ for pure halite) and is thus incompressible. Other sediments become denser with burial and compaction/cementation, so there will be a density inversion for the deeply buried salt. Much more important, however, is that halite has a low viscosity that allows it to easily flow, compared to surrounding rocks. Where the encasing rocks behave as relatively strong, brittle materials, salt is best viewed as a pressurized fluid that flows in a viscous manner (Vendeville and Jackson 1992). The drivers for salt movement are extension, contraction and differential loading, and the consequent way in which salt deforms is known as salt tectonics, halokinesis or halotectonics.

In a rift/rifted margin context, the style of halokinesis largely depends on whether the salt is pre-rift, early syn-rift or late syn-rift (Rowan 2014). In the first two cases, the deformation is thick-skinned in that the basement, salt and its cover are all extended. The salt tends to decouple the supra- and subsalt deformation to some degree, with a greater degree of coupling for thin salt and/or large-displacement faults (Withjack and Callaway 2000). In cases of extreme extension and mantle exhumation, salt can completely decouple low-angle detachment faults in the presalt from thin-skinned detachment faults in the suprasalt (Jammes et al. 2010). In less extended settings, typical structural features (Figure 1.33a) include listric or conjugate faults that are detached on the salt, drape folds and faulted drape folds over basement faults, and diapirs located over or slightly in the footwall of subsalt faults. A salt diapir is a body of salt with a truncating relationship with its overburden; in rifted settings, they are usually triggered by extension, going through reactive, active and passive stages (Vendeville and Jackson 1992). Passive diapirs continue to grow at or just beneath the surface as sediment is deposited around them.

      In the case of late syn-rift salt, salt tectonics is dominated by post-rift gravitational failure of the salt and its overburden (see Rowan 2020). The primary drivers are the basinward tilt of the margin, caused by the thermal subsidence of oceanic crust, and proximal sediment loading. The deformation is thin-skinned, comprising of linked systems of proximal extension, translation and distal contraction (Figure 1.33b). Extension is accommodated by both basinward- and landward-dipping normal faults, contraction is manifested by both salt-cored folds and thrust faults as well as the squeezing of diapirs and the translational province has both symmetric and asymmetric salt-evacuation structures. Relief on the base salt can generate ramp-syncline basins. Diapirs are triggered by various processes, and the salt may break out and laterally flow to form salt sheets and canopies.

Salt has commonly been associated with geophysical imaging problems. Firstly, salt bodies may be non-reflective or have complex internal structures depending on the original layering and mode of deformation. Secondly, salt structures such as diapirs, typical of many rifted margins worldwide, usually have steep sub-vertical geometries that are often poorly imaged and that often also generate geophysical noise, making their exact shape and edge-diapir location difficult to define. Thirdly, the contrast between shallow high-velocity salt and low-velocity sediment causes migration problems and artifacts. Finally, there are many cases where little orno energy penetrates salt sheets/canopies into subsalt domains and then back to the surface receivers. However, new acquisition and processing techniques now allow much better imaging (Figure 1.34).

      Further reading.– The above descriptions are abbreviated and often simplified. If interested in reading and learning further, the reader is referred to the following list of publications and references: