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.