LATE EOCENE UPLIFT OF THE AL HAJAR MOUNTAINS, OMAN, RECORDED BY LOW-TEMPERATURE THERMOCHRONOLOGY AND ABSOLUTE AGES OF BRITTLE STRUCTURES BY U-PB DATING OF CALCITE FIBERS

The uplift of the Al Hajar Mountains in Oman is commonly inferred to be generated by either Late Cretaceous ophiolite obduction or the Neogene Zagros (Arabia-Eurasia) collision. Both of these interpretations have problems. Firstly, the presence of Paleocene to Eocene marine sedimentary rocks that were deposited on top of the ophiolite implies that no major relief existed after ophiolite obduction. Secondly, the central mountains have low seismicity, and are currently more than 400 km away from the Zagros collision suggesting that this plate boundary is not driving uplift. The exact timing of the uplift in the Al Hajar Mountains is unknown. The results presented here indicate that a major uplift episode in the central mountains occurred in the late Eocene (c. 40 – 30 Ma). Cooling was constrained by 10 apatite (U–Th)/He, 15 apatite fission-track, and 4 zircon (U–Th)/He sample ages. Thermal modelling of these data demonstrates that up to 150°C of cooling took place in the late Eocene, interpreted to be the result of erosion due to uplift. Additionally, 8 calcite U–Pb ages from fibers which formed within thrust and strike-slip faults associated with NE–SW shortening and large-scale doming of the mountains are between 40.6 ± 0.5 Ma to 16.1 ± 0.2 Ma (2σ errors). These U–Pb ages indicate late Eocene deformation with some early Miocene reactivation. Therefore, the uplift of the Al Hajar Mountains took place at least 30 Myr after ophiolite obduction occurred, and between 28 to 5 Myr before the onset of Zagros collision. Thus, the uplift history of the Al Hajar Mountains is much more intricate than previously proposed.