Tectonic and metamorphic evolution of the Temsamane units, External Rif (northern Morocco): implications for the evolution of the Rif and the Betic-Rif arc

Journal of the Geological Society, Jul 2007 by Negro, F, Agard, P, Goff�, B, Saddiqi, O

Abstract:

Located at an intermediate position in the External Rif nappe pile, the Temsamane units (northern Morocco) are characterized by an abnormally intense metamorphism and a penetrative ductile deformation. We present new metamorphic data showing that, in spite of their external position in the Rif, part of the Temsamane units underwent medium-pressure low-temperature (MP-LT) metamorphism (at c. 7-9 kbar and 330-430 �C), possibly during the Oligocene. Structural data show that the exhumation of these units, during Middle to Late Miocene times, was characterized by an intense approximately east-west stretching and by top-to-the-west shear senses. We tentatively propose two possible origins for the MP-LT Temsamane units: (1) an internal origin related to the subduction and the HP-LT event recorded in the Internal Rif (Alboran Domain), or (2) an external origin, implying a second subduction system within the External Rif, parallel to and almost contemporaneous with that of the Alboran Domain. This tectonometamorphic evolution of the Temsamane units is set within the context of the External Rif evolution. At a larger scale, we show that the exhumation history of the Temsamane units, which strongly resembles that documented in the core of the internal Betics, is compatible with the westward slab retreat occurring during the Middle to Late Miocene in the Betic-Rif region.

The Rif (northern Morocco) and the Betics (southern Spain) represent the western termination of the Alpine orogenic system, which resulted from the plate convergence between Africa and Eurasia since late Mesozoic times (e.g. Dewey et al. 1989; Jolivet et al. 2003; Chalouan & Michard 2004). The arc-shaped Rif fringes the Alboran Sea and extends eastward from Morocco to the Algerian Tell (Fig. Ia). It comprises the internal, Alboran Domain units associated with subduction-collision processes and burial (e.g. Aza��n & Crespo-Blanc 2000; Jolivet et al. 2003; Michard et al. 2006), and the non-metamorphic units from the External Rif nappe stack (Frizon de Lamette 1987; Chalouan et al. 2001).

Within the External Rif nappe stack, however, the Temsamane units (Fig. Ia) are characterized by low-grade greenschist-facies metamorphism and a penetrative ductile deformation (Frizon de Lamette 1985). Suter (1980) first imagined that they could have an internal origin related to the Alboran Domain, and preliminary ^sup 40^Ar-^sup 39^Ar constraints on metamorphism yielded an Oligocene age (Moni� et al. 1984). However, the origin, the age and the tectonic evolution of these metamorphic units have remained largely unclear and precise P- T conditions are lacking.

Such metamorphism, and associated deformation, in the external Rif challenges our understanding of the geodynamic evolution of the Rif chain, and of the Betic-Rif arc within the frame of the Africa-Eurasia convergence. We present below the results of a detailed petrological study and the first P- /"estimates for the Temsamane units, together with a coupled analysis of ductile and brittle deformation. We then discuss the tectonometamorphic evolution of these units and its implications for regional geodynamics.

Geological setting

Regional geology

The Rif chain is classically divided into several geological domains: the External Rif, the Flysch units and the Internal Rif, also known as the Alboran Domain (Fig. 1a; Suter 1980).

The External Rif nappe stack, made of Triassic to Neogene sedimentary cover units (Wildi 1983), is classically divided into the Prerif, Mesorif and Intrarif domains. Parts of the Mesorif and Intrarif were detached during the Miocene to form allochthonous nappes (External Rif nappes). All these units are non-metamorphic, except for some of the Temsamane units and the Ketama units (Fig. Ia), which recorded low-grade greenschistfacies metamorphism (Andrieux 1973; Frizon de Lamotte 1985). Serpentinites underlying the Ketama units (Beni Malek massif) are interpreted as relict mantle units exhumed during the Tethyan rifting (Michard et al. 1992). Flysch nappes (Intrarif; Fig. Ia), composed of Liassic to Oligocene sediments, were deposited on highly thinned continental or oceanic crust (Durand-Delga et al. 2000).

The Alboran Domain represents the Internal Zones of both the Rif and the Betics (Fig. Ia and b). It mainly consists of a stack of metamorphic nappes, including peridotite slivers, divided into the Sebtide and Ghomaride complexes and the Dorsale calcaire units. The Sebtide complex, equivalent to the Alpuj�rride complex in the Betics, recorded high-pressure low-temperature (HP-LT) conditions (Bouybaou�ne et al. 1995). The nonmetamorphic, Dorsale units were interpreted by some as the cover of the Sebtide units (Didon et al. 1973).

The structure and the tectonic style of the External Rif are dominated by major southward-directed thrusts that bound the various domains (Fig. Ia and b; Chalouan et al. 2001). The main tectonic phase in the External Rif is Middle to Late Miocene in age (Frizon de Lamotte et al. 2004). Two major left-lateral fault zones, Jebha and Nekor, have also been active since the Early Miocene (Late Burdigalian) and the Late Miocene (Tortonian), respectively (Leblanc 1990; Asebriy et al. 1993).