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Abstract

The present study introduces the first comprehensive thermochronological research at the south-ern Moroccan passive continental margin. The established low-temperature thermochronological methods apatite and zircon fission-track (AFT, ZFT) as well as apatite and zircon (U-Th-Sm)/He (AHe, ZHe) dating combined with time-temperature (t-T) path modelling have been applied to resolve the long-term landscape evolution of the Tarfaya Basin and western Anti-Atlas. The Tarfaya Basin is the northern part of the Tarfaya-Laâyoune-Dakhla Basin that extends over 1000 km along the Moroccan Central Atlantic margin. The basin is characterised by vast subsidence since Mid-Triassic times, whereby up to 12 km of Mesozoic to Cenozoic sedimentary rocks have been accumulated. In the northeast, the basin is bounded by an ENE-trending Palaeozoic fold belt, the Anti-Atlas. The mountain belt consists of numerous widespread Precambrian basement inliers surrounded by a thick Palaeozoic sedimentary succession that is folded during the Variscan orogeny. Due to massive surface uplift and exhumation since the Upper Carboniferous–Lower Permian, the Anti-Atlas reaches elevations of more than 3300 m and therefore, constitutes a potential source area for the surrounding basins. The study intends to analyse and interpret thermochronological data to constrain the pattern and history of subsidence and exhumation at the southern Moroccan passive continental margin. The main research objective was focused on the thermal, subsidence and exhumation history of the Tarfaya Basin in order to better comprehend the hydrocarbon generation in time and space. To determine the t-T development, thermochronological analyses were performed on 66 outcrop and well samples from Mesozoic–Cenozoic sedimentary rocks. The results reveal a continuous subsi-dence phase in the offshore Tarfaya Basin from Mid-Triassic onward to recent times. In contrast, AHe and AFT data as well as thermal modelling point to a basin inversion in the northeastern onshore basin starting in the Palaeogene at 65–50 Ma. The rock uplift and exhumation period resulted in the erosion of a 1.2–1.6 km thick Cretaceous–Palaeogene sedimentary pile at an average rate of 0.025 mm/a corresponding with peak Atlasian surface uplift in the Cenozoic. Detrital AFT ages from 92 (±16) to 237 (±35) Ma of the Upper Cretaceous–Neogene succession indicate no heating above 60 °C confirming immature to early mature Cenomanian to Campanian and Eocene source rocks in the onshore Tarfaya Basin. The second objective dealt with the thermal, subsidence and exhumation history of the western Anti-Atlas mountain belt. Thermochronological data of 34 Precambrian–Lower Carboniferous sam-ples propose a common geological evolution of the western Anti-Atlas. ZFT ages from 287 (±23) to 331 (±24) Ma point to a main exhumation in the Upper Carboniferous–Lower Permian related to the Variscan folding and post-folding erosion. The rock uplift and exhumation phase lasted up to the Lower Cretaceous, whereby 9 km of Precambrian–Palaeozoic overburden has been eroded at an average rate of 0.046 mm/a. In the late Lower Cretaceous to Upper Cretaceous, the western Anti-Atlas underwent a minor subsidence phase in accordance with the widespread transgression across the North African continent in the Cenomanian–Turonian. AHe ages between 49 (±3) and 89 (±5) Ma as well as t-T path modelling indicate the final exhumation period starting in the Upper Cretaceous, contemporaneously with the earliest record of surface uplift in the Atlas system during Senonian (90–65 Ma). The exhumation process and erosion of the 1.5–2.5 km thick Cretaceous overburden continued until present time at an average rate of 0.045 mm/a. Except for a hiatus in the Upper Cretaceous, the extensive denudation events in the western Anti-Atlas suggest a continuous clastic sediment flux to the Tarfaya Basin from Triassic to recent times. The third objective considered the establishment of a correlation between subsidence history of the Tarfaya Basin and exhumation history of the western Anti-Atlas. Hence, a provenance analysis of the Cretaceous to Neogene sedimentary succession has been performed. Thermochronological data suggest a continuous sediment transport from the western Anti-Atlas to the Tarfaya Basin from Lower Cretaceous onward to present time. Furthermore, due to Precambrian ZHe and ZFT single grain ages, a concurrent sediment input from a cratonic area, i.e. the Reguibat Shield, occurred. During the early Upper Cretaceous, the influx from various source areas into the Tarfaya Basin decreased. In the Neogene, the sediment input from the cratonic area reduced and an influx emerged from the High Atlas probably by a coastal longitudinal flow, i.e. the Canary Current. Finally, the northeastern onshore Tarfaya Basin delivered clastic material to the offshore and southern onshore basin in the Neogene.