The Rharb basin is an east-west-trending trough in northwest Morocco. It separates Cenozoic Rif sedimentary sequences from older, pre-Cenozoic platform rocks. These platform deposits are relatively undisturbed; the Rif sequence is greatly deformed.

Timing of the filling of the Rharb basin suggests 3 separate and distinct events. These are: (1) pre-Rif sedimentary sequences related to the early Tertiary uplifting, folding, and faulting on the north; (2) emplacement of the Miocene "prerifaine nappe," with consequent deformation; and (3) deposition of younger, relatively undisturbed, late Miocene to Holocene sediments.

Seismic, subsurface, and surface data suggest, in addition, that nappe emplacement was severely affected by preexisting topographic highs on the rapidly subsiding pre-Cenozoic platform. One consequence of this tectonic style of deformation was the formation of a series of large faults and fault-controlled mountains (Jebel Kafs, Jebel Zerhoun, etc.). Displacement along faults locally may exceed 1,000 m; possible lateral movement could be more than several kilometers.

A strong positive arch in the eastern Rharb basin has had great effect on the structural patterns of the Rharb basin, the Sais basin on the east, and the Djfarra basin and contiguous mountains. This arch acted as a resistant buttress to laterally moving thrust sheets and the Miocene nappe. It also acted as a source for much of the Miocene sediment in the contiguous basin areas. Once stripped of sediment, the high began foundering and the tectonic patterns that already were established continued or were accentuated.

The relation of the African plate to the plate margins suggests only mild downwarping with no active zone of igneous activity. No well-developed fractures were observed. Several small oil fields are associated with the nappe; these produce from relatively weakly defined structures or structural traps.

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