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

For the Khebaba units, peak conditions are given by multi-equilibrium calculations based on chlorite-chloritoid-quartz-water equilibria (Fig. 6a and b).

The P- T conditions in the three units are very similar, with pressures around 7-9 kbar for temperatures ranging between 330 and 430�C (Fig. 6a), and are in agreement with the lack of carpholite in these rocks. Therefore, part of the Temsamane units underwent medium-pressure low-temperature (MP-LT) metamorphism.

Tectonic patterns accompanying metamorphism

To precisely assess the relation between deformation and metamorphism, we complemented the detailed study of Frizon de Lamotte (1985) with some new observations on ductile and brittle structures. Figure 7 synthesizes the structural data collected in the Temsamane units.

Ductile deformation

In the Temsamane units, ductile deformation is characterized by a homogeneous ENE-WSW to NE-SW stretching lineation as described by Frizon de Lamotte (1985) (Fig. 7). In the southern very low-grade metamorphic units, this lineation is marked by clay minerals and rare muscovites or stretched sedimentary clasts and pebbles. Towards the north a more penetrative lineation appears together with increasing metamorphic grade, and is marked by chlorite or micas. The penetrative character of ductile deformation also increases towards the north. In the Ben Tieb and Taferhsit areas (Figs 7 and 8a) deformation is almost coaxial, and characterized by symmetric boudinage structures in the schist or the quartz veins. S-C structures are scarce and the angle between schistosity and the shear planes is low. These structures are also present at a larger scale in the Jurassic marbles of the Taferhsit and Imzirene units. Fold axes are subparallel to the main stretching direction and compatible with a major flattening component in the v;-plane of the finite strain ellipsoid (Fig. 7).

In the northern units, sigmoidal quartz veins and S-C shear bands become more frequent (Figs 7 and 8b) and indicate top-to-the-west movements. The Tres For�as unit, located further to the east, presents similar deformation patterns (Fig. 7). These structures are observed at the microscopic scale and shear bands or sigmoid porphyroblasts are present within the main foliation (Fig. 8c and d). The shear bands are well developed and very penetrative in the Ras Afraou and Tres For�as units. Deformation is generally more coaxial in the core of the units and becomes more non-coaxial towards tectonic contacts bounding the various units. Intense stretching is observed in the Ras Afraou unit, near Cape Afraou (Fig. 8e and f)> with an important flattening component marked by N80-90 folds and boudinage structures (Fig. 8e and g).

In all the units, and particularly in the northern units, small sigmoidal veins, sheared towards the SE, appear in the vr-plane. However, these SE-directed structures do not overprint the main foliation and the west-directed structures. Therefore, an additional component of SE shearing seems to be contemporaneous with approximately east-west stretching and a top-to-the-west sense of shear.

The evolution towards brittle deformation

Numerous brittle structures (fibrous veins, tension gashes, fractures), which rework ductile structures, also characterize the Temsamane units (Fig. 7).

The general orientation of these structures is perpendicular to the main ENE-WSW stretching direction observed regionally in all the Temsamane units, including the Tres For�as outcrop (Fig. 7). Fibrous veins of quartz or calcite crosscut the foliation, and vein poles are generally parallel to the stretching lineation (Figs 7 and 8h). These structures are more numerous near major tectonic contacts. Poles to late fractures also exhibit the same orientation (Fig. 7). Therefore all the ductile and brittle structures indicate the same roughly east-west stretching direction.