Anatomy and evolution of a salt wall from halokinesis to contraction (Central High Atlas, Morocco)

The structural style of the High Atlas fold and thrust belt is controlled by precursor diapirs that initiated during preorogenic rifting episodes. In this work, we document and interpret the geometry and the along-strike variation of salt tectonic features on a particularly well-exposed salt ridge (the Aberdouz salt wall) that records tectonic evolution for linear diapiric structures from an extensional to contractional regime. The Aberdouz salt wall, cored by Triassic Keuper salt, was created during the Jurassic rifting of the Atlas domain, and was subsequently shortened during Cenozoic mountain building. The study presented is based on field observations, including geological mapping, definition of syn-growth stratigraphy, and the construction of serial cross-sections and sequential restorations.

The Aberdouz salt ridge trends ENE-WSW, is ca. 38 km long and is flanked by minibasins containing Jurassic growth strata up to 5 km thick. The minibasin fill displays a deepening to shallowing upward facies trend, from shallow water carbonates in the lower Lias, calciturbidites and shales (with marginal reefs) during the upper Lias-Dogger, grading finally into terrestrial red beds in the Bathonian-Callovian. Tectonosedimentary relationships indicate salt migration during deposition of the entire Jurassic megasequence. Although this sequence is modulated by salt withdrawal in depocenters, the general trend is governed by regional subsidence events in the Atlas rift. Lower sedimentation rates or interruptions during early Bajocian time are marked by synchronous salt-sheet extrusion on both diapir flanks, overlapped by condensed-fauna intervals.

The Aberdouz diapiric core is welded in many places along the length of the ridge, but is still partly open where inclusions (Triassic basalts, Jurassic carbonates or late Jurassic gabbro-syenite bodies) prevented complete welding. Keuper red-green shale and gypsum is locally preserved, but halite is never exposed. The absence of metamorphic aureoles around the gabbro-syenite plutons suggests magmatic intrusion into weak salt-rich Keuper bodies, which were subsequently expelled during the Cenozoic shortening. Steeply upturned stratal panels flanking the diapirs or welds contain homoclinal, near-isopachous, but thickened Jurassic sequences concordant with the Triassic, indicating original deposition on the minibasin floor followed by upthrust and rotation to a diapir-flanking position. In contrast, diapiric stocks oriented transverse and splaying from the main salt wall preserve halokinetic sequences and along-strike turtle structures, indicating they have experienced less distortion and stratal rotation during Cenozoic shortening.

Finally, the Aberdouz ridge offers the opportunity to study salt wall terminations, which are different from each other: at one end of the diapir, the termination is marked by a large Q-tip stock (with inclusions) and at the other end the salt wall and megaflap are terminated against by a sharp tear fault.