Tectonic evolution of Anglesey and adjacent mainland North Wales

Abstract

The Appalachian–Caledonian Orogen preserves a complex record of piecemeal trans-oceanic terrane transfer and accretion during the early Paleozoic collision between West Gondwana and Laurentia, whilst the intervening Iapetus oceanic tracts were largely destroyed. The now preserved terranes include arc fragments of Laurentian and Gondwanan affinity, oceanic fragments incorporated into the Gondwanan continental margin, and remnants of the Gondwanan continental slope apron and adjacent platform (both Ganderia and Megumia). A new tectonostratigraphic synthesis for the island of Anglesey (and adjacent NW Wales) reveals a comprehensive record of the Appalachian orogenic cycle in the UK segment of the orogen of the peri-Gondwanan margin prior to amalgamation into the Laurentian margin. We identify elements of Late Neoproterozoic accretion forming the pre-Appalachian basement; Cambrian extension, deposition and continental margin growth; Early Ordovician accretion and renewed extension; and, finally, terminal Caledonian collision and continental foreland-basin development.

Heterogeneous Subgreenschist Deformation in an Exhumed Sediment‐Poor Mélange

“…Exhumation of the Gwna Complex at Llanddwyn Island is associated with an antiformal stack of variable metamorphic grade, up to blueschist facies (Kawai et al., 2007; Schofield et al., 2021). Though difficult to discern due to several later high‐angle strike‐slip faults, the sub‐blueschist facies units of the Gwna Complex at Llanddwyn Island have been interpreted to sit structurally above this blueschist unit (Kawai et al., 2007).…”

“…Recently, a reinterpretation of lithologies across Anglesey has resulted in the Gwna Complex at Llanddwyn Island being reclassified as the Llanddwyn Island Volcanic Member, part of a mega-conglomerate called the 10.1029/2022JB024353 3 of 20 Bodorgan Formation (Schofield et al, 2021). The Bodorgan Formation is thought to record NW-vergent folding and thrusting associated with Early Devonian subduction, and the Llanddwyn Island Volcanic Member is said to lie in strands of the strike-slip Berw Fault Zone which extends across Anglesey (Schofield et al, 2021). While recognizing this broader context, our current work focuses on processes of subduction deformation recorded in structures with a dip-slip shear sense clearly preserved at Llanddwyn Island (Figure 1; Kawai et al, 2008;Maruyama et al, 2010).…”

Inherited Heterogeneities Can Control Viscous Subduction Zone Deformation of Carbonates at Seismogenic Depths

“…The Gwna subduction complex at Llanddwyn Island (Anglesey, Wales, UK) contains lenticular slices of ocean plate stratigraphy (OPS) metamorphosed to subgreenschist conditions during subduction between 488 and 448 Ma (Kawai et al., 2007 ). Lenticular slices of OPS are bound on each side by vertical mélange‐bearing shear zones <15 m wide with a SE‐upward shear sense (Figure 1e ), defining an imbricated structure consistent in lithology, metamorphic grade, and kinematics with SE‐ward subduction and subsequent passive block rotation of the complex into a vertical dip within regional strike‐slip zones (Leah et al., 2022 ; Schofield et al., 2021 ). Massive carbonates and volcanics are commonly deformed alongside varied lithologies in shear zones throughout the complex.…”

Terrane history of the Iapetus Ocean as preserved in the northern Appalachians and western Caledonides

“…However, identifications of trace fossils, together with increasingly abundant detrital zircon data, have shown that some of the rocks involved are Cambrian (McIlroy and Horak, 2006). The deformation is now known to be early Paleozoic (Schofield et al, 2020). An upper limit to the deformation is provided by a Floian overstep sequence, suggesting that most of the Monian deformation occurred at around 480 Ma, simultaneously with Penobscottian deformation in the northern Appalachians.…”

“…Still earlier episodes of deformation affected several peri-Gondwanan terranes during the Ediacaran, producing multiple unconformities, typically in association with voluminous arc-related intrusive and extrusive rocks. Local blueschists and other highpressure low-temperature metamorphic rocks (White et al, 2001;Schofield et al, 2020) suggest that both lower-plate and upper-plate settings were preserved. Some authors have related these convergent events to an Avalonian Orogeny related to Pan-African orogenic events more widely recorded in Gondwana (e.g.…”