La Mamora, charnière entre la Meseta et le Rif: son importance dans l'évolution géodynamique postpaléozoïque du Maroc / The Mamora Plain, a hinge between the Meseta and the Rif. Its importance in the post-Paleozoic geodynamic evolution of Morocco

Abstract

The Mamora area (Morocco) is located in the northern part of the Meseta and the southern part of Rharb. The recent formations (Mesozoic to Quaternary) lie unconformably on a Paleozoic basement. This study based on hydrogeological, sedimentological, drilling data and seismic reflection profiles interpretation, proposes new interpretations of geodynamical evolution of this area particularly in terms of tectonic patterns. The most ancient formations recognized in this region are Paleozoic schists and quartzites in the Tifíete sector. They represent the basement of the basin on which Triassic conglomerates and red mudstones associated with basalts lie unconformably. Jurassic and Cretaceous sediments are limestones and marls. The Mio-pliocene formations are open marine blue marls. Plio-quaternary sediments are limestones and sands containing gravel and pebbles. Miocene to Pliocene blue marls facies corresponds to deep marine marls (bathyal as indicated by planctonic foraminifers) with an attributed age from upper Miocene [9] to middle Pliocene [7]. A facies distribution map of the top of the blue marls has been realized where four main facies—conglomerate, shelly sandstone, limestone and marls—indicate a major regression in the Mamora basin. The datation of the formations was mostly realized by Wernli [22, 23, 24, 25], and Cirac [10], In the Mamora area, Hercynian faults show two main structural directions, N020°E-N040°E (Agadir-Rabat), N120°E (Rabat- Tiflète) [5], and a new major Hercynian fault (K2S). The seismic profiles have been studied between Sidi Slimane and Sidi Yahia area, to illustrate the structure of the Mamora, and to replace it, in the geodynamical evolution. The seismic reflection lines and drilling data show that the eastern Mamora was subdivided into two sectors: i) the southern sector is affected by Hercynian faults which create horsts and grabens in the Paleozoic. Mio-Pliocene formations infill these depressions and are covered by Quaternary sediments; ii) the northern sector is constituted by various formations: 1. Paleozoic formation as basement covered by autochthonous Mesozoic to Miocene, 2. Prerifain nappes (marls and evaporites), 3. Mio-Plio-Pleistocene formations as subautochthonous to autochthonous. These two sectors are separated by a major fault (K2S). On the other hand, in the occidental Mamora, the facies distribution and the Plio-Pleistocene thickening seem to be induced by faults with a NE-SW and NW-SE trends which affect the Paleozoic basement. Then, between the Meseta domain and the septentrional Rharb basin, two major Hercynian initially dextral shear zones, Rabat- Tiflete and K2S, have been recognised. During the Atlantic Ocean opening, they are probably senestral shear zones. At the same time the subsidence in Rharb basin is active, major action of these faults is normal. Therefore, Mamora represents a real hinge between stable Meseta and unstable septentrional Rharb basin. © 2001 Éditions scientifiques et médicales Elsevier SAS