DIAPIRS AND MINIBASINS AS TRACKERS OF THE TECTONIC DEVELOPMENT OF THE ATLAS OROGENIC BELT, MOROCCO

The style of the High Atlas is controlled by precursor diapirs that initiated during preorogenic rifting episodes and created a structural inheritance that influenced the subsequent orogenic architecture. We document the geometry and along-strike variation on a particularly well-exposed salt ridge (the Aberdouz salt wall) and flanking minibasins, as trackers of the tectonic evolution through linear diapiric structures from extension to contraction. We present a geological map, a new syn-growth stratigraphy and serial cross-sections with sequential restorations, to illustrate how salt structures may influence an orogenic building.

The Aberdouz salt wall, cored by Triassic Keuper, is 38 km long and flanked by minibasins containing Lias and Dogger growth strata up to 5 km thick, displaying a deepening to shallowing upward trend. Tectonosedimentary relationships indicate salt migration during deposition of the entire Jurassic megasequence in an extensional setting. The general trend is governed by regional subsidence events in the Atlas rift but is modulated by salt withdrawal in depocenters. Low sedimentation rate episodes or even interruptions are marked by synchronous salt-sheet extrusion on the diapir flanks.

The diapiric core is welded along much of the ridge but is partly open where inclusions (Triassic basalts and Jurassic carbonates or gabbros) prevented closure. Gabbro plutons intruded into salt-rich Keuper bodies that were expelled during the regional shortening. Rare Cenozoic clastic minibasins sinking into diapir feeders suggests possible salt extrusion during the shortening.

Steeply upturned stratal flaps flanking the salt wall occasionally contain homoclinal, near-isopachous Jurassic sequences concordant with the Keuper, indicating original deposition in minibasin floors followed by upthrust and rotation to a diapir-flanking position, although there is significant along-strike variation. Diapiric stocks transverse to the main salt wall preserve halokinetic sequences and an along-strike turtle structure, attesting for less distortion of these structures during the shortening.

The excellent exposure in the Atlas Mountains illustrate how evaporites are a source of heterogeneity in the kinematic evolution of sedimentary basins, playing a crucial role in their later evolution into orogenic belts.