A restatement of the Mesozoic Atlasic Rifting (Morocco)

Abstract Dislocation of the Pangean plate resulted in the development of extensive episodes recorded in Late Triassic-Early Jurassic synrift basins that were initiated by reactivation of older Hercynian–Alleghanian thrusts. The Central Atlantic rift system evolved from rifting to drifting with the post-rift phase beginning in the early Toarcian. The present Moroccan Atlasic belt corresponds to a Mesozoic marine basin, often described as rift basins, that were inverted and uplifted as a consequence of the Alpine Orogeny. The original geometry of these basins has not been extensively obliterated, due to shortening, allowing the distinction of the synrift and post-rift sequences and their initial geometry. The ‘Atlasic rift’ concept is still equivocal since it represents two sets of conflicting interpretations. The first considers a rifting process that began during the Upper Triassic and lasted until the Bathonian or the Cretaceous. The second distinguishes two crustal extensional events: (i) an initial event, which was initiated in Carnian time and led to the development of an Upper Triassic Atlasic true rift, with fault activity that declined in earliest Liassic time; (ii) a second event, starting early in the Toarcian, was responsible for a renewed crustal extension that disrupted the Liassic carbonate platform and led to the development of the fault-bounded Middle and High Atlas troughs. They were initiated by transtensional tectonics that ended with the development of the pre-Cretaceous regional cleavage. Field and well data were used to understand the basin evolution and timing of rifting. A post-rift sequence of Liassic age that overlapped both the Triassic sediments and the titled basement blocks has been identified. It corresponds to the first major marine incursion from the Tethys sea and it gave way to the flooding of the rifted area, emphasized by thermal subsidence. The basal surface of this sequence represents a break-up unconformity beneath the marine deposits. A renewed crustal extension led to the fragmentation of the Liassic carbonate platform and the development of the fault-bounded Middle and High Atlas troughs which were controlled mainly by transcurrent faulting in the Toarcian. This development ended during the Late Jurassic. The Atlas rift regime subsequently became transpressive. The former depocentres evolving into synclines and the ridges to anticlines. Regional cleavage developed contemporaneously. Anticlinal ridges were truncated by erosion during and after their uplift. The resulting erosional surfaces were preserved by the deposition of Callovian and Cretaceous sedimentary sequences, demonstrating that this unconformity post-dates the cleavage development. The major unconformity of Callovian and Cretaceous sedimentary rocks can therefore no longer be seen as a break-up unconformity overlain by a post-rift sequence.