Evolution of an Accretionary Complex (LeMay Group) and Terrane Translation in the Antarctic Peninsula

Abstract

The LeMay Group accretionary complex of Alexander Island (Antarctic Peninsula) comprises a 4 km thick succession of variably deformed turbidites associated with thrust slices of ocean floor basalts. The depositional age and provenance of the succession is uncertain with estimates ranging from Carboniferous to Cretaceous. The accretion history is also poorly established and whether the LeMay Group developed allochthonously and accreted during an episode of Cretaceous terrane translation. We have examined the geochronology and geochemistry of 22 samples from across the entire accretionary complex to determine its depositional, provenance and accretion history. The accretionary complex has been subdivided into four separate groups based on detrital zircon U‐Pb age and Lu‐Hf provenance analysis. Groups 1 and 2 are interpreted to be a continuation of the extensive Permian accretionary complexes of West Gondwana and have a depositional age of c. 255 Ma, with volcaniclastic input from the extensive silicic volcanism of the Choiyoi Province. Accretion of the LeMay Group to the continental margin developed during the mid‐Triassic, potentially related to the Peninsula Orogeny and an episode of flat‐slab subduction of the proto‐Pacific plate. Group 3 is only identified from an island to the west of Alexander Island and has a mid‐Cretaceous depositional age and provenance akin to offshore sequences from Thurston Island. A para‐autochthonous origin is suggested, with mid‐Cretaceous accretion associated with the melange belts of central Alexander Island. Group 4 is also a distinct unit with an Early Jurassic depositional age and a source more closely related to the Antarctic Peninsula.

Subduction of active spreading ridges and the disappearance of Andean-type cordilleras

“…Early Tertiary (A) and present-day (B) configurations of the South American plate and West Antarctica Peninsula. (A) Paleoelevation of South American plate and supposed continental connection to the West Antarctica Peninsula at 60-50 Ma [from Boschman, 2021, Ghiglione, 2016, Reguero et al, 2013, Riley et al, 2023. Cross section A-A ′ represents the large-scale 2D structure of the Andean magmatic arc and backarc domains [simplified from Armijo et al, 2015] and the cross-section B-B ′ is a proposed structural view of the former elevated Cordillera (including the Cordillera Darwin) connecting the South American plate to the West Antarctica Peninsula [after Cunningham, 1993, 1995, Ghiglione, 2016, Ronda et al, 2019.…”

Geological insights from the newly discovered granite of Sif Island between Thwaites and Pine Island glaciers

“…In contrast, placing this boundary along Pine Island Rift (e.g. Gohl et al 2007, Riley et al 2023 would incorrectly include Sif Island in the Marie Byrd Land crustal block. Sif Island represents the south-western-most extent of exposed coastal rocks of Early-Middle Jurassic age, which probably extend beneath Pine Island Glacier (Simões Pereira et al 2020), as well as the south-western-most exposure of Jurassic arc-related rocks.…”