Absolute ages of multiple generations of brittle structures by U-Pb dating of calcite

Abstract

Abstract 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.