Tracking the tempo of a continental margin arc: Insights from a forearc succession in West Antarctica

The Fossil Bluff Group of eastern Alexander Island records the exceptional preservation of more than 8 km of Mesozoic sedimentary rocks deposited into an accretionary forearc basin that developed unconformably above a late Paleozoic accretionary complex, and in proximity to a continental margin arc during a prolonged phase of enhanced magmatism. Through the Mesozoic, the Fossil Bluff Group evolved from a trench-slope environment to a forearc basin sourced from the continental margin arc. During this period, the Antarctic Peninsula’s convergent margin was characterized by episodes of magmatic flare-ups that developed during tectonic compression, crustal thickening, extension, and uplift. U-Pb and Lu-Hf detrital zircon data are used to determine the provenance of the forearc succession and as a monitor of arc magmatic tempos during the late Mesozoic. The magmatic record in the adjacent arc is poorly preserved or partially absent, but the sedimentary record of the forearc basin preserves a largely uninterrupted record of arc magmatism that can be studied with detrital zircon geochronology and geochemistry. The basal succession of the Fossil Bluff Group is sourced from the adjacent accretionary complex, but thereafter it is strongly controlled by the proximal arc in western Palmer Land and is characterized by a mixed arc/recycled signature during episodes of renewed sedimentation. However, the main phases of deposition during the Early Jurassic (ca. 180 Ma), Early Cretaceous (141−131 Ma), and mid-Cretaceous (125−102 Ma) are dominated by arc-only sources. The Lu-Hf isotopic record supports a transition from convergence to extension and a return to convergence during the Mesozoic, which is consistent with accretionary orogens from elsewhere along the West Gondwanan margin. The provenance record during the depositional history of the basin points overwhelmingly to an autochthonous origin; as such, models for parts of the western province of the Antarctic Peninsula being allochthonous are unsupported.