Permo-Triassic structural evolution of the Argana Valley, impact of the Atlantic rifting in the High Atlas, Morocco
Résumé
This paper provides an alternative model to explain the structural evolution of the Argana Valley, located in the Western High Atlas of Morocco. Most Triassic basins in the High Atlas have been described as isolated rift basins distributed along an ENE-WSW oriented trough and developed as a series of halfgrabens. They are characterised by two main sets of syn-sedimentary faults striking ENE-WSW and NNE-SSW, generally attributed to the Atlantic and Tethys rifting. The Permo-Triassic sediments of the Argana Valley are offset by similar trending faults and, as such, they have been interpreted as typical infill sediments of narrow rift-basins in Morocco, controlled by syn-sedimentary faulting due to the Atlantic rifting. Our study investigates the structural evolution of the area using newly acquired field analysis of the main faults and unconformities. This data, together with published information, has been used to construct a series of structural cross-sections across the valley. Three main unconformities have been recognised. The first unconformity separates Permian sediments from the underlying deformed Palaeozoic rocks. The second is an angular unconformity between Late Permian sediments and the overlying Triassic sequence, attributed to relaxation and orogenic collapse that occurred after the Hercynian Orogeny. The geometry of Late Permian units suggests that the large E-W to ENE-WSW faults occurred after deposition of the Late Permian and before the Triassic, and were associated with a major phase of tectonic activity and associated erosion. The third angular unconformity occurs between the latest Triassic sediments and the overlying basalts or Jurassic sedimentary sequence. Significantly, this study suggests that the ENE-WSW faults and most of NNE faults were not active during sedimentation of the Triassic. We conclude that the Triassic sediments of the Argana Valley were not deposited in a half graben, but within a slowly subsiding domain, such as a sag basin or a wide rift. Comparison with other Permo-Triassic basins on the Atlantic margin and in the High Atlas suggests that the structural evolution in the western part of the High Atlas has been mainly affected by Atlantic rifting. The influence of Tethys rifting is confined to the central part of the High Atlas, the Massif Ancien acting as a structural buffer between the two realms.