Structural evolution of the Moroccan Central High Atlas Syncline-Topped Anticlinal Ridges: Insights from micro-structural analysis of Tirrhist and Anemzi ridges

Abstract

<p>The Moroccan High Atlas mountain range is an aborted Mesozoic rift basin that was moderately shortened during the Late Cretaceous‒Cenozoic inversion. The range is currently featured in its central part by the presence of conspicuous S-shaped open gentle synclines where Middle Jurassic strata crop out, with sub-horizontal bottom, separated by 15-to-80-km narrow faulted anticline ridges with two distinct directions: ENE and NE. The tight anticline ridges are cored by Triassic continental red-beds intruded by the CAMP basalts and subsequently by Upper Jurrasic‒Lower Cretaceous alkaline magmatism. Regional cleavage with very low-grade anchi- to epi-zonal metamorphism are depicted along several structures of the High Atlas, particularly the NE-trending anticlines. The sedimentary layers thickness, on the other hand, gets thinner towards the faulted anticlines with the development of intraformational truncations. The structural history of the High Atlas syncline-topped anticlinal ridges remains a controversial matter. Any attempt to reconstruct the evolutionary process of such folded structures must take into consideration the following circumstances:</p><ul><li>After a Triassic rifting episode followed by the establishment of Liassic carbonate platform, the High Atlas basin underwent a wide spread exhumation event at the time interval between the Middle Jurassic and Lower Cretaceous leading to the deposition of continental detrital series and sedimentary hiatus;</li> <li>The upward motion was accompanied with the emplacement of alkaline magmas in the Central High Atlas;</li> <li>A complex halokinetic history characterizes the Central High Atlas salt province during both pre-orogenic and orogenic stages;</li> <li>During the Late Cretaceous‒Cenozoic, the High Atlas experienced a moderate crustal shortening which was focused essentially within the range’s borders;</li> </ul><p>In order to bring new insights to the structural history of the High Atlas folded structures, a structural investigation was carried out in Tirrhist and Anemzi ridges. In each station, fractures measurements were taken, and oriented samples were collected for micro-structural analysis. First paleo-stress inversion in some stations reveals the presence of pre-folding bedding-parallel maximal horizontal stress oriented NE to NNE. For a deep analysis of pre syn and post-folding stresses history, we use a calcite stress inversion technique, namely Etchecopar’s method, to unravel the paleo-stresses orientations and to quantify the differential stresses during the different episodes of deformation. The present work is a preliminary attempt to quantify tectonic stresses in the hinterland of an arguably weakly deformed orogenic belt.</p>