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

Hypothesis 1 (Fig. 10a) assumes that the MP-LT Temsamane units are of internal origin and were subducted together with the Sebtides and later thrust onto the External Rif nappe pile as allochthonous nappes. The major sinistral displacement of the Alboran Domain relative to the African margin during the Miocene (Andrieux et al. 1971; Frizon de Lamotte 1985; Leblanc 1990) could have favoured such a juxtaposition of the Alboran Domain units with the Mesorif.

Hypothesis 2 (Fig. 10b) implies the existence of a second, short-lived subduction zone within the Mesorif margin, located further to the south and parallel to the main Sebtides subduction system. Hypothesis 3 (Fig. 10c) suggests that the tectonic positions of the three units result from basement thrusts affecting the Mesorif (Frizon de Lamotte et al. 2004). The last hypothesis does not account for the existence of MP-LT metamorphism or for the similarity between Ras Afraou and Khebaba metamorphic imprints. Therefore, we retain Hypotheses 1 and 2 in the discussion below.

Early emplacement of the MP-LT Temsamane units in the External Rif nappe stack. Whatever the correct interpretation regarding the geodynamic scenario (i.e. Hypothesis 1 or 2 described above), the thrusting of the MP-LT Temsamane units necessarily took place after their exhumation at 15-13 Ma and before the emplacement of the Aknoul nappe of supra-Ketama origin (which post-dated the Tortonian; Frizon de Lamotte 1981). The timing of nappe emplacement is shown in Fig. 10a and b, with the successive emplacement of the MP-LT Temsamane, Bou Haddoud- Senhadja and Aknoul nappes. Late thrusts and pop-up structures locally reworked these contacts in the vicinity of Khebaba (Fig. 10a and b; Frizon de Lamotte 1987; Darraz & Leblanc 1989).

A major uncertainty concerns the relationship between the Khebaba and the Ras Afraou-Tres For�as units: whereas the two latter units are readily linked, the tectonic position of the Khebaba unit is more difficult to assess. Given its structural position and its metamorphic imprint, the Khebaba unit could either belong to the MP-LT Temsamane units proper or represent the metamorphic base of the Senhadja nappe (Figs 10a, b and 11).

A possible tectonic time frame for the Temsamane units. The following tectonometamorphic scenario can be proposed for the Temsamane units.

(1) A first metamorphic stage took place under MP-LT conditions, possibly during the Oligocene or somewhat later, but its age is at present poorly constrained.

(2) The second, major tectonometamorphic event corresponded to the exhumation of the MP-LT Temsamane units during the Middle Miocene (c. 15-13Ma; Negro 2005). This event was accompanied by east-west stretching and top-to-the-west movements along a palaeo-Nekor fault, whose offset was sinistral or normal with respect to the Intrarif. Consistently, apatite fissiontrack dating for the Ketama units points to ages between 20 and 14 Ma (Azdimousa et al. 1998), suggesting that these units were near the surface while the MP-LT Temsamane units were being exhumed.