Post‐Variscan exhumation of the Central Anti‐Atlas (Morocco) constrained by zircon and apatite fission‐track thermochronology

Abstract

The origin of the Anti‐Atlas relief is one of the currently debated issues of Moroccan geology. To constrain the post‐Variscan evolution of the Central Anti‐Atlas, we collected nine samples from the Precambrian basement of the Bou Azzer‐El Graara inlier for zircon and apatite fission‐track thermochronology. Zircon ages cluster between 340 ± 20 and 306 ± 20 Ma, whereas apatite ages range from 171 ± 7 Ma to 133 ± 5 Ma. Zircon ages reflect the thermal effect of the Variscan orogeny (tectonic thickening of the ca. 7 km‐thick Paleozoic series), likely enhanced by fluid advection. Apatite ages record a complex Mesozoic–Cenozoic exhumation history. Track length modelling yields evidence that, (i) the Precambrian basement was still buried at ca. 5 km depth by Permian times, (ii) the Central Anti‐Atlas was subjected to (erosional) exhumation during the Triassic‐Early Cretaceous, then buried beneath ca. 1.5 km‐thick Cretaceous‐Paleogene deposits, (iii) final exhumation took place during the Neogene, contemporaneously with that of the High Atlas.

Post-Variscan evolution of the Anti-Atlas belt of Morocco constrained from low-temperature geochronology

“…However, it is in agreement with recent studies that have revealed the existence of major tectonic vertical motions that affected large domains on the eastern margin of the Central Atlantic Ocean (Malusà et al 2007;Ghorbal et al 2008;Ghorbal 2009;Gouiza 2011;Ruiz et al 2011;Bertotti and Gouiza 2012;Oukassou et al 2013;Sehrt 2014;Leprêtre et al 2015). Thereby, our data and the subsequent interpretation imply that the Anti-Atlas domain experienced up to 10.5 km of crustal exhumation and denudation by the Middle Jurassic, in relation with continental rifting in the Central Atlantic and the Tethyan Atlas.…”

“…This is in agreement with ZFT ages obtained on samples from the western and central Anti-Atlas, which indicate that the currently exposed basement rocks were at temperatures around 300-240 °C between 350 and 280 Ma (Fig. 7) (Sebti et al 2009;Oukassou et al 2013;Sehrt 2014).…”

“…Deformation was controlled by thick-skinned inversion of the basement with minimum estimates of shortening on the order of 15-25 km and topography in excess of 10 km (Burkhard et al 2006). However, thermal modeling constraint by AFT data from the Bou Azzer inlier in the central Anti-Atlas indicates only 5 km of exhumation during the Carboniferous-Permian time interval (Oukassou et al 2013). …”

“…However, recent findings, based on low-temperature geochronology techniques, show that the latter domains experienced km-scale vertical motion (subsidence/exhumation) contemporaneous with the Mesozoic syn-and post-rift phases in the Central Atlantic (e.g., Malusà et al 2007;Ghorbal et al 2008;Ghorbal 2009;Sebti et al 2009;Ruiz et al 2011;Oukassou et al 2013). Furthermore, well and seismic data reveal the existence of thick intervals of terrigenous siliciclastic sediments deposited in the adjacent Atlantic shelf and deep basins during Late Jurassic to Early Cretaceous time (e.g., Price 1981;Davison 2005;Gouiza 2011), which corresponds to the time interval of the aforementioned exhumation.…”

The three main steps of the Marrakech High Atlas building in Morocco: Structural evidences from the southern foreland, Imini area

“…Oligocene deposits are not observed in this region of Morocco. Southward, Paleozoic and Precambrian formations outcrop in the Anti-Atlas, due to the erosion of a previous thin Mesozoic-Cenozoic cover (Ruiz et al, 2011;Oukassou et al, 2013). Two main fault system strikes can be distinguished in the Imini area: a N45° to N90° striking group and a N110°-N130° striking group in the area (Fig.…”