Magnitude of rift‐related burial and orogenic contraction in the Marrakech High Atlas revealed by zircon (U‐Th)/He thermochronology and thermal modeling

Abstract

The Atlas of Morocco is a continental rift developed during the Triassic‐Jurassic and moderately inverted during the Cenozoic. The High Atlas south of Marrakech, with exposures of basement and Triassic early synrift deposits, has been viewed as a high during the Mesozoic rifting. First zircon (U‐Th)/He ages and thermal models obtained from 42 samples in the Marrakech High Atlas following two NNW‐SSE transects across the mountain belt reveal that in contrast to previous models, the Triassic‐Jurassic rift was well developed in the Marrakech High Atlas (with more than 4.5–6 km of rift‐related deposits). Middle Jurassic‐Early Cretaceous zHe cooling ages obtained indicate that rift‐related subsidence in the Marrakech High Atlas finished in the Middle Jurassic and was followed by a period of exhumation where 2–3 km of rock were eroded. Thermal models from zHe data provide the first thermochronologic clue for a Late Cretaceous initiation of the Atlas compression‐driven exhumation in the inner parts of the Marrakech High Atlas. The Triassic‐Jurassic basin reconstruction assisted by thermochronology highlights a key role of inherited basement anisotropy in rift orientation and evolution, and on its subsequent inversion. Comparison of present‐day and restored sections to the rifting stage aided by thermochronology suggests minimum values of total orogenic shortening in the Marrakech High Atlas of 13 to 14 km (21 to 17%), with exhumation of 1 to more than 5 km of rocks. Similar zHe ages on both sides of the Tizi n'Test fault evince minor vertical movements along the fault during the Atlas orogeny.

Reconstructing Extensional Basin Architecture and Provenance in the Marrakech High Atlas of Morocco: Implications for Rift Basins and Inversion Tectonics

“…The reported stratigraphic thicknesses for Atlas rift basins varies. Although up to 5‐km stratigraphic thickness has been reported (Brown, ; Beauchamp et al, ), other estimates are thinner: 1 km (Mattis, ), <1 km (Baudon et al, ), and ~2.6 km (Domènech et al, ; this study, Figure ). Limited Triassic synrift magmatism is reported within the Atlas rift, but Permian calc‐alkaline magmatism has been documented in the Meseta domain (El Hadi et al, ; Youbi et al, ).…”

“…Such a fault was potentially deformed or cut by Cenozoic reverse faults situated to the south, such as the Oukaimeden or Meltsene reverse faults. The steeply north dipping Ourika fault (Figures , a, and ) retains the Triassic normal offset and may represent an original major basin‐bounding fault that was part of the north dipping fault system, as reported further west along the MHA (Domènech et al, ). Post‐Precambrian rocks have been eroded from much of the outcropping reverse faults' hanging walls, preventing precise reconstructions of the normal fault position.…”

“…These opposing paleoflow directions have been interpreted as evidence that the West Moroccan Arch (incorporating the Massif Ancien), a proposed paleostructural high situated between the Argana and Oukaimeden‐Ourika Valleys, was a potential sediment source based on the paleocurrents that seem to flow away from this paleostructural high (Michard, ; Laville & Piqué, ; Laville et al, ; Frizon de Lamotte et al, ; Baudon et al, ; Mader and Redfern, ). Recent thermochronometric and structural studies suggest the West Moroccan Arch was not emergent during the entire Triassic‐Jurassic rifting and, thus, potentially not a sediment source for Atlas rift basins (Domènech et al, ; Ghorbal et al, ; Saddiqi et al, ).…”

“…The Paleozoic (Carboniferous‐Permian) intrusive and volcanic rocks in the Meseta domain are typically calc‐alkaline and bimodal, consistent with orogenic and postorogenic tectonic setting, and attributed to the Variscan/Hercynian orogeny (Youbi et al, ; Ait Chayeb et al, ; El Hadi et al, ). The High and Middle Atlas are relatively narrow ENE‐WSW to NE–SW trending intracontinental mountain ranges with elevations over 4,000 m. Estimates of the timing (Cretaceous, Domènech et al, ; Neogene, Balestrieri et al, ) and magnitude of shortening vary (15–24%, Teixell et al, ; 4%, Missenard et al, ). A phase of mid‐Jurassic to Early Cretaceous uplift, attributed to thermal doming, predated the onset of shortening in the MHA and adjoining regions (Bertotti & Gouiza, ; Domènech et al, ; Frizon de Lamotte et al, ).…”

“…The High and Middle Atlas are relatively narrow ENE‐WSW to NE–SW trending intracontinental mountain ranges with elevations over 4,000 m. Estimates of the timing (Cretaceous, Domènech et al, ; Neogene, Balestrieri et al, ) and magnitude of shortening vary (15–24%, Teixell et al, ; 4%, Missenard et al, ). A phase of mid‐Jurassic to Early Cretaceous uplift, attributed to thermal doming, predated the onset of shortening in the MHA and adjoining regions (Bertotti & Gouiza, ; Domènech et al, ; Frizon de Lamotte et al, ). The High and Middle Atlas compressional belts developed by stress propagation into the African plate during Cenozoic collision between Africa and Eurasia (Dewey et al, ; Frizon de Lamotte et al, ; Mattauer et al, ).…”