UNDERSTANDING THE EVOLUTION OF PASSIVE MARGINS WITH THICK MOBILE SALT: A SUMMARY OF THE KEY ROLE PLAYED BY STRUCTURAL BALANCING

Passive margins where salt not only acts as a detachment but is also present in allochthonous nappes & canopies provide a particular challenge in terms of complexity of geology & also with respect to geophysical imaging of the often highly heterogeneous velocity field. Any technique that provides some constraint to the interpretation of such environments will clearly have value, as has been the case with section balancing in thrust belts with similar imaging/interpretation challenges. The well established rules of structural balance - simple conservation of area/volume/bed length in the spatio-temporal strain path combined with appropriate models to guide process and structural style, together provide powerful tools with which to project, construct and retro-deform interpretations in under-constrained subsurface settings and hence test their validity and reduce uncertainty in subsurface characterisation. Many passive margins also exhibit regionally 'balanced' systems whereby gravitational failure driven by the post-rift basinward slope and/or the sediment wedge entering the post-rift basin accommodation space has to balance up-dip extension with down-dip compression, salt canopy emplacement and squeezing of existing diapirs and feeders. This 'closed-system' provides constraints when extrapolating to new areas in different parts of the gravitationally failing passive margin. I will show examples from the Gulf of Mexico and West African margins and include a retrospective look at the application of geometrical balance & the integration of the key steps in our understanding of salt tectonic processes on exploring and developing hydrocarbons in these systems.