Uplift of the Al Hajar Mountains in Oman has been related to either Late Cretaceous ophiolite obduction or the Neogene Zagros collision. To test these hypotheses, the cooling of the central Al Hajar Mountains is constrained by 10 apatite (U‐Th)/He (AHe), 15 fission track (AFT), and four zircon (U‐Th)/He (ZHe) sample ages. These data show differential cooling between the two major structural culminations of the mountains. In the 3 km high Jabal Akhdar culmination AHe single‐grain ages range between 39 ± 2 Ma and 10 ± 1 Ma (2σ errors), AFT ages range from 51 ± 8 Ma to 32 ± 4 Ma, and ZHe single‐grain ages range from 62 ± 3 Ma to 39 ± 2 Ma. In the 2 km high Saih Hatat culmination AHe ages range from 26 ± 4 to 12 ± 4 Ma, AFT ages from 73 ± 19 Ma to 57 ± 8 Ma, and ZHe single‐grain ages from 81 ± 4 Ma to 58 ± 3 Ma. Thermal modeling demonstrates that cooling associated with uplift and erosion initiated at 40 Ma, indicating that uplift occurred 30 Myr after ophiolite obduction and at least 10 Myr before the Zagros collision. Therefore, this uplift cannot be related to either event. We propose that crustal thickening supporting the topography of the Al Hajar Mountains was caused by a slowdown of Makran subduction and that north Oman took up the residual fraction of N‐S convergence between Arabia and Eurasia.

Direct dating of brittle structures is challenging, especially absolute dating of diagenesis followed by a series of superimposed brittle deformation events. We report 22 calcite U-Pb ages from tectonites and carbonate host rocks that date 3 diagenetic and 6 brittle deformation events. Results show that U-Pb dating of calcite fibers from these structures is compatible with overprinting relationships. Ages indicate that diagenesis occurred between 147 ± 6 Ma and 103 ± 34 Ma, and was followed by top-to-the-south, layer-parallel shearing due to ophiolite obduction at 84 ± 5 Ma (2σ errors). Sheared top-to-the-northeast, layer-parallel veins were dated as 64 ± 4 Ma and are interpreted to have developed during postobduction exhumation. After this event, a series of strike-slip structures, which crosscut and reactivated older faults due to northwest-southeast horizontal shortening, were dated as 55 ± 22 Ma and 43 ± 6 Ma. Eight ages from strike-slip faults and thrusts resulting from northeast-southwest shortening range from 40.6 ± 0.5 Ma to 16.1 ± 0.2 Ma. The youngest ages are from minor overprinting fibers ranging in age between 7.5 ± 0.9 Ma and 1.6 ± 0.6 Ma. Our results show that U-Pb dating of calcite fibers can be successfully used to constrain a complicated succession of brittle deformation structures that encompasses two orogenies and an intervening extension period. BRITTLE STRUCTURES IN THE AL HAJAR MOUNTAINS Carbonate rocks at the base of the Al Hajar Mountains in Oman (Fig. 1) record several generations of overprinting brittle structures (Fig. 2) containing calcite fibers (Gomez-Rivas et al., 2014), and provide a unique opportunity for dating calcite that crystallized from the Cretaceous to Quaternary. The Al Hajar Mountains, situated on the northeast margin of the Arabian continental plate, consist of a pre-Permian basement that is unconformably covered by a middle Permian to Late Cretaceous carbonate platform (Glennie et al., 1974). In the Late Cretaceous, continental slope-rise sedimentary rocks and the Semail Ophiolite were thrust on top of the carbonate platform (Searle and Cox, 1999). Subsequently, the Al

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