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- W2107737757 abstract "We re-evaluate the Neoproterozoic, Pacific-type accretionary complex on Anglesey and in the Lleyn peninsula (Wales, UK), by reconstructing its ocean plate stratigraphy (OPS). Three types of distinctive OPS were successively emplaced downwards in an accretionary wedge: the oldest at the top formed when an ocean opened and closed from a ridge to a trench, the central OPS was subjected to deep subduction and exhumed as blueschists, and the youngest at the bottom is an olistostrome-type deposit that formed by secondary gravitational collapse of previously accreted material. The three types formed by successive eastward subduction of young oceanic lithosphere at the leading edge of Avalonia. The downward growth of the accretionary complex through time was almost coeval with exhumation of the blueschist unit at 550– 560 Ma in the structural centre of the complex on Anglesey. From balanced sections we have reconstructed the ocean plate stratigraphy on Llanddwyn Island from which we calculate that about 8 km of lateral shortening of ocean floor took place during imbrication and accretion; this is comparable with the history of plate subduction around the Pacific Ocean. We also calculate that the age of the subducted lithosphere was very young, probably less than 10 Ma. Ocean plate stratigraphy (OPS) is the fundamental, first-order structure of accretionary orogens that are forming today on the Pacific margins (Isozaki et al. 1990; Kusky et al. 1997a, b). It is a composite stratigraphic succession of the ocean floor reconstructed by means of microfossil biostratigraphy (mostly radiolaria, but also fusulinids and conodonts) from protolith sediments in accretionary wedges and melanges. It consists of the following sediments in ascending order. At the base are cherts deposited under deep-sea pelagic conditions at or near a mid-oceanic ridge. The cherts may be overlain by limestones if the ridge rises above the carbonate compensation depth. The basal cherts are transported across the ocean as it opens, and are overlain by more and more cherts, as the pelagic ocean floor sediments are transported towards a subduction zone. When the pelagic sediments reach a hemipelagic environment on the offshore side of a trench typically near a continental margin, the oceanic cherts are succeeded by hemipelagic siliceous shales and mudstones that consist of radiolaria and fine-grained continental-derived detritus. Finally, when the sediments enter a trench, they are covered by voluminous clastic sediments in the form of shales, sandstones, conglomerates and turbidites that were deposited by turbidity currents that transport continental-derived detritus into the trench. The classic OPS section is represented by mid-ocean ridge basalt (MORB)– chert (limestone)–hemipelagic shale/mudstone– sandstone/conglomerate/turbidite (Matsuda & Isozaki 1991; Fig. 1). Moreover, when an oceanic plate passes over a plume, a seamount or volcanic island is created that is characterized by ocean island basalts overlain by reef limestones, given the appropriate latitude and carbonate compensation depth (Okamura 1991; Ota et al. 2007; Takayanagi et al. 2007). When cherts on the ocean floor pass over the plume head, they may be intruded by sills of alkali basalt, as illustrated in Figure 1. A new component to the OPS story was provided by Bradley et al. (2003), who pointed out that the subduction of a mid-oceanic ridge can explain the interbedding of trench sediments and pillow lavas, and the development of high-grade metamorphism of an accretionary wedge. Although the OPS model is an idealized reconstruction, it is important to emphasize the fact that the idea that OPS represents a transition of sediment from a ridge to a trench has been extremely tightly constrained and supported by an enormous databank of material derived first from dredge and/or drill-hole material from ridges, the ocean floor and from trenches, and second from the presence of complete OPS in accretionary wedges either in still-evolving subduction–accretion complexes in the western and northern Pacific or in accreted From: Kusky, T. M., Zhai, M.-G. & Xiao, W. (eds) The Evolving Continents: Understanding Processes of Continental Growth. Geological Society, London, Special Publications, 338, 55–75. DOI: 10.1144/SP338.4 0305-8719/10/$15.00 # The Geological Society of London 2010. material in Palaeozoic–Cenozoic accretionary and collisional orogens, the origin of that material having being well constrained by biostratigraphy and trace element geochemistry. Accordingly, it is well accepted that OPS represents a viable and indeed key plate-tectonic indicator (Wakita & Metcalfe 2005). OPS has proved useful as a means of determining the width of oceans, because the age range of chert deposited between a mid-ocean ridge and a trench documents the duration of oceanic plate migration (Xenophontos & Osozawa 2004). The OPS is present in all the subduction– accretion complexes across Japan from the Nankai Trough to the Japan Sea (Isozaki et al. 1990). The classic or indeed type-section of OPS is at Inuyama in Central Japan; this consists of a series of stacked thrust sheets, for which the stratigaphy was first worked out by Matsuda & Isozaki (1991) using radiolaria. Each thrust sheet is composed of Early Triassic to Middle Jurassic lithological units (bottom cherts to top clastic deposits), all well defined by radiolaria into nine assemblage zones. The lower cherts and upper sandstones each have calculated original stratigraphic thicknesses of about 100 m. However, offscraping accretion gave rise first to a decollement in the hemipelagic mudstones, followed by in-sequence thrusting, then imbrication to produce up to six thrust sheets in a Sandstone/conglomerate (turbidite) Shale/mudstone (hemipelagite) Bedded chert (pelagic sediment)" @default.
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- W2107737757 date "2010-01-01" @default.
- W2107737757 modified "2023-10-12" @default.
- W2107737757 title "Ocean plate stratigraphy and its imbrication in an accretionary orogen: the Mona Complex, Anglesey–Lleyn, Wales, UK" @default.
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- W2107737757 doi "https://doi.org/10.1144/sp338.4" @default.
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