The Inception and evolution of wide salt-bearing rifted margins – insights from numerical modelling and natural systems

Abstract

Rifted margins are often associated with widespread and thick evaporite (salt) deposits, typically formed during the latest stages of rifting, immediately prior to continental breakup. These margins are also characterized by pronounced salt tectonics, which is commonly attributed to gravity-driven salt flow and characterized by kinematically-linked domains of updip extension, translation and downdip shortening. The precise spatial and temporal links between these processes, their relative contributions and the role of rifting and rifted margin architecture on salt deposition and tectonics are still a topic of debate on many margins. We apply 2D thermo-mechanically coupled finite-element modelling of lithospheric extension to investigate the evolution of salt basins along wide rifted margins and the interplay between rifting and salt basin geometry with syn- to post-rift salt tectonics. The models use a geodynamically self-consistent approach where the geometries of the lithosphere and salt basins are not prescribed. They show that late syn-rift salt basins form as a single large basin across both conjugate margins that are later separated by continental breakup and oceanic spreading. This produces syn-depositional salt flow and stretching of the distal salt over an outer margin trough with emplacement of a syn-breakup allochthonous salt nappe over newly-formed seafloor (i.e., oceanic crust and/or exhumed mantle). The post-rift evolution is characterized by updip extension that is balanced by downdip diapir shortening, and pressure-driven nappe advance, which is largely independent of the other two processes. The results are comparable to examples from various salt-bearing rifted margins, including the South Atlantic and Gulf of Mexico, and help us understand their genesis and evolution.