The geology of Turkey

Abstract

Detailed mapping of the Sidi Flah-Bou Skour inlier in the central Saghro, of which the initial aim was to determine the metallogenic potential of the area, enabled us also to challenge the former tectono-lithostratigraphic classifications on the basement units immediately underneath the P.III cover, as well as their relationships with successive magmatic events (effusive and/or intrusive). These new results from this area, compared with data from the literature (and sometimes from additional field trips to other inliers in the central and eastern Anti-Atlas), allow us to revise the tectono-lithostratigraphic classifications of most of the basement units immediately underneath the P.III-cover in the eastern Anti-Atlas. In actual fact, they were generally found to differ only slightly from those in the central Anti-Atlas: prior to the deposition of the P.III-cover, the basement unit of both areas experienced two similar tectonic events (same types of folding: sub-parallel axial trends, foliations, schistosities—with or without associated metamorphism—similar or identical magmatic series, successions and dates of emplacement. Thus, as in the central Anti-Atlas, most of these basement unit must be classified as belonging to the P.II-2 unit and not to the P.II-3 as was previously thought (Choubert's nomenclature, 1963–1979): only rarely do inliers contain P.II-3 basement units.During the period of extension, prior to the first Pan-African compressive phase, mafic volcanics were poured out onto the floor of subsiding basins characterized by detrital terrigenous sedimentation (deltaic fans). Locally, the volcanics are associated with sulfide mineralisation, which show features (alteration pipes and feeding stockworks) suggestive of massive sulfide deposits; similar geometrical relationships with the host rocks, hydrothermal alteration and sulphide parageneses characterise these showings. The presence of such a type of mineralisation is an additional argument for classifying their host series as P.II-2 (by comparison with that of Bleida in the central Anti-Atlas).Moreover, this revision leads us to propose a new palaeogeographical scheme for the evolution of the Middle- to Late-Proterozoic and, subsequently, a new geotectonic model. The central and eastern Anti-Atlas forming, during this period, a diamond-shape, mainly intra-continental, subsiding basin with internal shoals and troughs whose directions were determined by those of pre-existing fractures or zones of weakness: mostly N130° and N70°, (sometimes N110°). These same trends were re-activated again during the Upper- and the Late-Proterozoic and controlled the two-stage pre-P.III Pan-African tectogenesis.The diamond-shape of the basin, including its shoals and troughs, was caused, through wrench-faulting, by transtensional movements inside a multiphase approximately NS compressive stress field generating a mainly SWNE extension. In the earliest phase (Middle- to Upper-Proterozoic), the distension reached a maximum (associated with a thinning of the continental crust), creating an epicontinental ensialic basin between two passive margins (the West African Shield to the southeast and southwest, and an ill-defined continent to the northeast and the northwest: the “European” continental plate. The thinning of the continental crust caused it to rupture locally, and here the upper-mantle came to form the bottom of the basin; this occurred along the Anti-Atlas Major Fault ( =AAMF) in the central Anti-Atlas, e.g., the Bou Azzer-El Graara and Siroua areas. However, the southwestern and northeastern zones remained controlled by the continental crust. Then, during the Upper-Proterozoic, compressive events (the two-stage Pan-African tectogenesis) the basins progressively closed, folding the volcano-sediments contained in them and causing local “intrusions” of remnants of oceanic crust as subvertical slabs (ultramafic rocks at Bou Azzer-El Graara and Siroua). The basins were completely closed at the end of the Upper-Proterozoic first Pan-African event; the second Pan-African phase and the subsequent Upper-Proterozoic events were governed by strictly continental conditions throughout the Anti-Atlas.Thus, the proposed geotectonic model consists of early wrench-faulting and the subsequent creation, though transtension of successive, more or less deep and active grabens and rifts (the present-day inliers), separated by horsts, within a continental environment. This extension is bound to an approximately NS compressive stress-field acting along inherited zones of weakness (mainly N70° and N130° faults). Only one rift opening reached the upper-mantle, the others remained confined to a continental crust thinned to a greater or lesser degree. During the later compressions (approx. EW stress-fields), which led to the pre-P.III tecto-orogenesis of the Anti-Atlas, no subduction of the sea-floor is though to have occurred, but the thinned continental crusts (passive margins) faced each other along pre-existing transform and/or strikeslip faults, after an early and local obduction of small upper-mantle (sea-floor) slices along the northern margin of the West African Shield. This hypothesis, which is an alternative to subduction (either under active margins or under island arcs) for calc-alkaline magmatic generation, the type to which the late- and post-Pan African magmatisms belong, recalls the one proposed for the Neogene evolution of the zone of the Rif-Betic Cordilleras-Alboran Sea and has also been suggested for other Upper-Proterozoic Belts in Central Africa.