The Upper Neoproterozoic-Lower Cambrian continental extension in the Anti-Atlas (Morocco)

Introduction. – In the Anti-Atlas, south of Morocco (fig. 1), the Precambrian terrains are usually divided into several “series” (fig. 2) : the Paleoproterozoic (PI) is an old crystalline basement, at least Eburnean ; the Neoproterozoic (PII) is constituted by metasedimentary rocks, quartzites and limestones, indicative of a shelf, in which volcano-sedimentary and volcanic flows are intercalated, laterally grading to an ophiolitic complex along the Sirwa-Bou Azzer axis. These PII rocks have been deformed in the course of the Panafrican orogeny ; above the underlying upper Proterozoic terrains and in major unconformity on the Panafrican structures, the Saghro group (PII3) and Ouarzazate group (PIII) series are volcanic and volcano-clastic sequences, often considered as late-Panafrican molasses. Above them, the Tata group (Adoudounian), constituted by marine carbonates and siltstones, represents the earliest Cambrian. Recent structural and sedimentological observations Recent observations have been realized through all the Anti-Atlas, of which the present note gives only examples that are the most significant and easily accessible. They show that the PII3 conglomerates were not everywhere deposited around Panafrican paleoreliefs ; they often contain large bodies of quartzites embedded within the conglomerates (fig. 3). Clearly, the PII3 is an olistostrome at the base of the PIII détrital and volcanic series, which were deposited at the base of active faults. The development of these reliefs took place several tens of millions of years after the end of the Panafrican paroxysm and therefore the PII3 and the PIII are post-Panafrican deposits, unrelated to the Panafrican orogeny. Study of synsedimentary structures (folds, faults, progressive unconformities : fig. 4 to 7) reveals the extensive character of this faulting event that extends even in basal Cambrian. Between the PII3 series and PIII an angular unconformity due to tilting can exist, but we did not find there plicative structures clearly related to the compressive late-Panafrican « B2 » phase sometimes described in the litterature. In the western Anti-Atlas, the extension is pure, with a NW-SE direction ; it is N-S in the central Anti-Atlas and it is transtensive according to N070°E faults, en échelon between sinistral N110°E trending faults in the central-eastern Anti-Atlas. In the detail, nevertheless, the synsedimentary structures suggest slidings from raised zones that correspond to the future inliers (fig. 7). Magmatic and metallogenic activity This extension accompanies various events : (1) a marine transgression, from west to east ; (2) the emplacement of extrusive magmas, first calco-alkaline then tholeiitic ; (3) an hydrothermal activity responsible for the concentration of Co, Au, Cu, etc. These concentrations were in the past attributed to various episodes, from the pre-Panafrican extension to the Hercynian compression. In fact, they result from the circulation of hydrothermal solutions that deposited, in the superficial levels of the crust, products extracted from the PIII magmas or the PII Proterozoic serpentines. The circulations took place in the old compressive structures (e.g. the Panafrican foliation) reopened during the extensive episode described above. Discussion and conclusion : the late Proterozoic-early Cambrian rifting The crustal extension that affected the Anti-Atlas started during the late Proterozoic, after the end of the main Panafrican deformation. Its tectonic significance is discussed with regard to the Panafrican orogeny : either a late Panafrican extension, bracketed between two compressive deformations and possibly related to a thinning of the orogenic crust, or a post-Panafrican extension, unrelated to compressive phases, described as a synrift event. In the Anti-Atlas, it developed through late Proterozoic and early Cambrian times. It aborted at the end of the early Cambrian. Evidences of a comparable extension are found in northern Morocco, western Europe and as far in the Middle East, i.e. all along the northern margin of the paleo-Gondwana.