Mechanical framework of rifted margin salt tectonics: A review and new insights from physics-based forward models

Abstract:

Salt tectonics in rifted margins is the result of complex interplay among key factors such as margin subsidence, salt mobility and sediment loading. Salt tectonics is predominantly gravity driven and comprises two end-member deformation processes. These include “gravity gliding” due to the tilt of the basal surface of the salt layer, and “gravity spreading” caused by the differential loading from a sediment overburden. Commonly, both the processes operate simultaneously. Their relative contributions to halokinesis are controlled by the width of the rift margin, rifting rate, sediment loading and strength of the continental crust. In addition, the timing of salt deposition vis-à-vis that of rifting and salt composition are the other key controls.

Within a margin-scale mechanical framework, it is reasonable to assume that the maximum horizontal stresses in post-salt sediments are (1) sub-parallel to the margin in proximal regions and (2) normal to the margin in the distal portions. At the scale of individual basins, interactions between pre-existing salt structures (e.g., salt walls, stocks) and active sediment fairways can cause significant departures in the in-situ stress state from that at the margin-scale. Finally, at the prospect-scale, the tectonics stress states can vary in a complex manner even around an individual salt body.

In the present work, we provide a review of mechanical controls on rifted margin salt tectonics ranging from the margin-scale to the prospect-scale. We also present results from basin-scale 3D geomechanical forward models that highlight the complex distribution of tectonic stress states around commonly observed salt structures. Our models also demonstrate the effect of salt heterogeneity on the overall evolution of stress regimes in the surrounding sediments. We propose that a multiscale mechanical framework, that takes into account the contributions from key factors listed above, can provide key insights into rifted margin salt tectonics and can enhance kinematic interpretations in such settings.