A serpentinite ridge in a collisional paleomargin setting: the Beni Malek massif, External Rif, Morocco

Abstract

In the Eastern Rif Belt of northern Morocco the Beni Malek serpentinised peridotite massif is included in a low-grade metasedimentary tectonic pile, which was thrust southward onto the Atlas foreland, and which belongs to the external zones of the Maghrebides belt. We describe serpentinite clasts within calcareous deposits that are transgressive onto the ultramafic (lherzolitic) massif and detrital serpentinite layers that occur within greenschist formations. Reworking of the serpentinite clasts occurred at the beginning of the Late Jurassic-Early Cretaceous accumulation of the overlying, marginal sedimentary prism. We connect the ultramafic outcrop with a regional, NE-SW trending magnetic anomaly (the Temsamane anomaly), and conclude that the source of the serpentinite clasts extended roughly parallel to the local, NE-SW trend of the North African margin. By comparison with the submarine serpentinite ridge of the Galician margin, we suggest that the Beni Malek ultramafics were part of a similar ridge of serpentinised mantle rocks exhumed by the Tethyan rifting off northwestern Africa. The extensional emplacement of this ridge is correlated with the early stage of uplift of the Ronda-Beni Bousera lherzolite massifs under the Alboran thinned continental block, and with ocean-floor spreading on both sides(?) of the Alboran block. The seismic Nekor fault parallels the Temsamane anomaly to the southeast and is interpreted as a former extensional fault of the North African paleomargin that was reactivated as a sinistral wrench fault during the Miocene by the Iberian-African collision, which caused the obduction of the Beni Malek massif itself.