Salt tectonics at passive margins: Geology versus models – Discussion

Brun and Fort (2011) use mechanical analysis, experimental models, and geologic data to suggest that deformation in passive-margin salt basins is dominantly a result of gravity gliding rather than gravity spreading. They claim that only seaward tilt of the salt layer is effective in driving basinward translation of the salt and overburden and that differential loading alone requires extreme conditions that do not occur in nature. In this Discussion, we refute many of their arguments and conclusions. We show that: i) a more thorough mechanical analysis indicates that gravity spreading is effective if the proximal overburden is at least three times thicker than the distal overburden, a common occurrence on passive margins; ii) more realistic analogue models also demonstrate that extreme thickness variations are not necessary for gravity spreading; iii) their analysis of structures or structure associations is sometimes misleading; and iv) there is abundant evidence that gravity spreading is dominant on some margins. In particular, modern data from the northern Gulf of Mexico confirm traditional interpretations that Cenozoic failure was mainly due to downslope movement driven by sedimentary loading, not SW-directed gliding driven by tilt of the deep salt as claimed by Brun and Fort (2011). We conclude that both gravity gliding and gravity spreading are common processes which may vary spatially and temporally in any one salt basin.