Using stratal geometries above prerift to early synrift salt to constrain crustal fault interpretations in the distal domains of magma-poor rifted margins

The presence of pre- to early synrift salt leads to varying degrees of decoupling between supra- and subsalt deformation during rifting. Decoupling is favored by thick salt or small fault displacement. This has been examined in detail in low-𝛽settings such as the southern and central North Sea and is applicable to the proximal domains of rifted margins. In addition, the role of late syn-rift salt on margins has been extensively studied. But the behavior of pre- to early synrift salt in the high-𝛽 necking, hyperextended, and exhumed mantle domains remains poorly understood.

A common suprasalt geometry in the necking and hyperextended domains of the western Iberian margin is that of strata that dip and thicken basinward. These might be mistaken for growth strata adjacent to a landward-dipping fault bounding a horst or for salt evacuation structures in a half graben, with both interpretations invoking low-𝛽, high-angle normal faults. However, they more likely record extension associated with large-offset detachment faults, but with thickening onto the top of the hanging wall instead of the fault. Slip ceases on the low-angle, basinward-dipping fault between the hanging- and footwall cutoffs of the salt, with continued extension on the deeper part of the fault transferred to slip on the steeper, landward-dipping hanging-wall salt in a zig-zag pattern like that of fish-tail thrusts. This simple concept can guide interpretations in areas with inadequate imaging.

The same idea also explains the presence of significant volumes of pre- to early synrift salt in the exhumed mantle domain, as seen in the Mauléon Basin of the NW Pyrenees. This relationship is enigmatic because mantle represents new real estate that formed after salt deposition and, moreover, any salt should be highly attenuated. The solution is that as mantle is exhumed from beneath the upper plate, extension on the landward-dipping exhumation detachment is transferred to the basinward-dipping salt detachment on that upper plate, thereby generating a zig-zag fault geometry. Effectively, the upper plate moves out from between both detachments, which merge at the hanging-wall cutoff of the upper plate such that salt and suprasalt strata end up juxtaposed above the footwall of the exhumation detachment. That part of the detachment becomes locked and the salt above the mantle does not get attenuated by further extension.