Proof of concept for Orogenic Bridge Theory in the Fram Strait using U–Pb geochronology of syn-kinematic carbonates

Source abstract

Background Thus far, remnants of the late Neoproterozoic Timanian Orogeny (ca. 650–550 Ma) in Svalbard were thought to belong to small exotic continental blocks ripped off Baltica and/or the Pearya Terrane (Canadian Arctic) and amalgamated during the Caledonian Orogeny (ca. 465–425 Ma). The goals of this work are to test whether Timanian thrust systems (1) extend across the entire Svalbard Archipelago and (2) localized the formation of major transform faults in the Fram Strait. Methods Structural fieldwork was combined with microstructural analysis and U–Pb geochronology of syn-tectonic carbonate veins along a major fault system in western Svalbard, the Kongsfjorden–Cowanodden fault zone. A new method named “forward-casting approach” is proposed to select suitable samples to date old tectonic episodes. Results The results suggest several phases of tectonism along the Kongsfjorden–Cowanodden fault zone. These include latest Neoproterozoic–early Cambrian top-SSW thrusting during Timanian contraction at 519 ± 42.9 Ma, Ordovician–Silurian Caledonian reworking (folding) and metamorphism at 441 ± 24.8 Ma, early Cenozoic sinistral-reverse reactivation during Eurekan contraction at 54.6 ± 3.2 Ma, and earliest Oligocene proto-transform sinistral-normal faulting during the opening of the Fram Strait at 32.9 ± 9.9 Ma. Conclusions Late Neoproterozoic Timanian thrust systems extend across Svalbard and have been repeatedly reworked and reactivated–overprinted during the Phanerozoic. This suggests that the entire Svalbard Archipelago was already amalgamated to Baltica at 550 Ma, i.e., a similar plate configuration as at present. Breakup-related sinistral transform faulting in the Fram Strait localized along preexisting Timanian thrust systems in the mid–late Cenozoic, thus supporting Orogenic Bridge Theory, i.e., that major transform faults form along preexisting, rift-orthogonal thrust systems. The study showcases the potential of U–Pb geochronology of synkinematic carbonates to date Precambrian tectonics and the importance of careful sampling in the field for geochronological studies.