Structure and exhumation of the Cap des Trois Fourches basement rocks (Eastern Rif, Morocco)

Abstract

The Cap des Trois Fourches (Eastern Rif, northern Morocco) metamorphic basement exposes two major tectonic units, namely the Taïdant unit underthrusting the overlying Tarjât tectonic unit. The Tarjât tectonic unit is composed of metamorphic rock originating from detrital material. This upper Tarjât tectonic unit exhibits: (1) orthogneiss bodies being Paleozoic in age and (2) evidences showing a retromorphose during the Alpine orogenesis. U-Pb SHRIMP zircon geochronology from the orthogneiss bodies has yielded Sakmarian (early Permian) ages for the intrusion of their protoliths. The lower Taïdant unit is formed of green and brown shales associated with quartzite beds. A Mesozoic age is commonly accepted for this unit.The internal structure of the Tarjât tectonic unit consists in km-scale folds trending ENE-WSW with an up-dip direction towards the SE. This compressional deformational stage was superimposed on internal thrust sheets controlled by ductile shear zones with a SW-vergent transport sense. The lower Taïdant unit shows conspicuous bedding associated with a penetrative foliation outlining a monocline structure dipping gently towards the NW. The major ductile-brittle detachment fault bounding the upper Tarjât tectonic unit from the lower Taïdant unit –i.e. the local basement– exhibits evidences for a top-to-the-west sense of motion. We assume that this specific fault is the prolongation into the Cap des Trois Fourches area of the extensional detachment previously described in the Temsamane area. Therefore, this low angle detachment fault is a major tectonic element extending hundreds of km through the Eastern Rif. We postulate that this major detachment roots deep in the Eastern Rif basement. This detachment is cut at the surface by the Nekor normal-sinistral strike-slip fault that separates thin 24–32 km thick metamorphic Eastern Rif crust from the 50–55 km thick crust of the Western Rif. This transtensive fault system exhumes the Rif middle crust, represented by the Temsamane massif units, along the Nekor sinistral STEP boundary at the southern edge of the Betic-Rif subduction system.