Suspect terrane definition in Anglesey, North Wales

Abstract

The late Precambrian/early Cambrian Monian basement to the lower Paleozoic cover in Anglesey does not form one coherent stratigraphic unit but is divisible into at least three separate suspect terranes. Where they are exposed, the boundaries to these terranes range from ductile shear zones to brittle faults, all of which have an undetermined amount of displacement. Terrane 1 is represented by the Monian Supergroup: a thick sequence of marine sedimentary rocks with subordinate basic volcanic rocks, gabbros, and serpentinites, overlain by olistostromic mélange. Terrane 2 is identified as the Coedana Complex of central Anglesey: high-grade gneisses and the ca. 600-Ma Coedana Granite with an associated hornfels. Terrane 3 is represented by the southeast Anglesey schist belt and includes the ca. 550 to 560-Ma Anglesey blueschists. These suspect terranes are bounded to the southeast by the Menai Strait Fault System, to the southeast of which rocks of the Sarn Complex are placed within a separate terrane which may represent the northwest margin of the Avalon terrane in Britain. Terrane 1 and possibly terrane 2 are traceable into southeast Ireland (Cullenstown Formation and Rosslare Complex). Monian terranes are not known from elsewhere within the British Isles, although broadly coeval rocks occur in different terranes as the Avalonian/Cadomian rocks of southern Britain and the Channel Islands, and the Dalradian Supergroup of Scotland and northwest Ireland. No Monian rocks have yet been identified from the Tornquist line in the east, whereas several attempts at correlation have been made between Anglesey and the eastern seaboard of North America. Such attempts have failed to appreciate that there is not one "Mona Complex," but a collage of disparate terranes, some of which are juxtaposed by shear zones comparable in scale to major crustal shears developed along transcurrent plate boundaries.

Bedrock controls on subglacial landform distribution and geomorphological processes: Evidence from the Late Devensian Irish Sea Ice Stream

“…The pre-Quaternary geology of the island can be divided into five main units ( Fig. 5b): (i) a pre-Ordovician basement assemblage referred to as the Mona Complex (Greenly, 1919;Gibbons, 1983Gibbons, , 1989Howells, 2007); (ii) an Ordovician overstep sequence consisting of coarse to finegrained sandstones, conglomerates and mass flow deposits (Bates, 1972(Bates, , 1974Beckly, 1987), occupying a Y-shaped tract in central Anglesey; (iii) a volumetrically restricted Silurian succession of deep marine mudstones and volcanic rocks exposed in a small (c. 6 km x 1 km), elongate belt centred on Parys Mountain in northern Anglesey; (iv) an unconformable ?Silurian to Devonian fluviatile red-bed succession comprising tabular to trough cross-bedded conglomerates, cross-laminated to planar laminated sandstones and locally calcrete-rich siltstones (Greenly, 1919;Allen, 1965;Davies, 2005); and (v) Carboniferous sedimentary succession which can be divided into an older (Dinantian) shallow marine/lagoonal carbonate sequence comprising bioclastic limestones intercalated with subsidiary terrestrial terrigenous sandstones (Walkden and Davies, 1983;Davies, 1984Davies, , 1991Davies et al, 2004), and a younger (Pennsylvanian), poorly exposed coal-bearing succession of fluviatile to deltaic sandstones and conglomerates (Greenly 1919); the crop of the latter being restricted to the low-lying area beneath Malltraeth Marsh (Fig. 5b).…”

“…The Mona Complex has been subdivided into three terranes (Gibbons, 1989) that are separated by low-angle to subvertical, northeast-southwest-trending brittle faults and ductile shear zones. The most prominent of these is the Berw Fault (Fig.…”

“…The second terrane, the Coedana Complex of central Anglesey (Gibbons, 1989), is a relatively narrow, northeast-southwest-trending belt of poorly exposed Neoproterozoic (Strachan et al, 2007) metasedimentary and metamafic gneisses intruded by the Coedana Granite (Fig. 3b).…”

“…The southeastern margin of this terrane is formed by a 1.5 to 2 km wide belt of gently to moderately southeast-dipping schistose rocks referred to as the Central Anglesey Shear Zone (Howells, 2007). The final terrane, the 'blueschist terrane' of southeast Anglesey (Gibbons, 1989;Howells, 2007), is dominated by highly schistose Neoproterozoic metasedimentary rocks that contain disrupted lenses of blueschist facies metabasalts, the famous Anglesey or Monian blueschists (Greenly, 1919;Kawai et al, 2006). Bedding and deformation structures (folds, cleavages) present within the Monian Supergroup, as well as the schistosity within the 'blueschist terrane', are typically oriented northeast-southwest, parallel to the major terrane-bounding faults.…”

Mineral isograds and metamorphic zones of the Anglesey blueschist belt, UK: implications for the metamorphic development of a Neoproterozoic subduction–accretion complex

“…Shackleton (1969Shackleton ( , 1975 in re-evaluating the Anglesey stratigraphy with way-up criteria reversed Greenly's stratigraphy putting the South Stack Group at the bottom, but regarded the Groups above to be in normal stratigraphic order, whereas Barber & Max (1979) reassessed the tectonic stratigraphy, in particular proposing that the less deformed South Stack Group was thrust beneath the more highly deformed New Harbour Group. Gibbons (1983) suggested that the evidence in Anglesey for palaeo-subduction including the blueschists was weak, and Gibbons (1989) interpreted the tectonic belts in Anglesey in terms of suspect terranes. Gibbons & Mann (1983) first reported lawsonite in mafic blueschists, and Hora´k & Gibbons (1986) reclassified the blueschist amphibole in particular as crossite and barroisite.…”