Penobscottian type accretionary tectonics in southern Britain [abstract only]

Conventional models for tectonic and stratigraphic evolution of the Southern Britain
\nenvisage Cambrian to Early Ordovician sedimentary basin development in terms of
\nvertical movements of essentially autochthonous crustal units. However, this is clearly
\nat odds with the wider understanding of the Gondwanan margin of Iapetus elsewhere
\naround the northern Atlantic region, where development of complex subduction
\nsystems and patterns of dispersal and assembly are preserved.
\nIn the Northern Appalachians of New England and Atlantic Canada, evolution of the
\nperi-Gondwanan margin during the Ordovician is thought to have involved
\nemplacement of the Penobscot (513-486 Ma), Victoria (478-460 Ma) and Exploits
\n(473-455Ma) arcs and accretionary complexes against a composite Ganderia –
\nAvalonia continental margin, significantly pre-dating latest Silurian, Acadian
\ntectonism associated with terminal Laurentia – Gondwana collision.
\nIn their attempt to synthesise Iapetus evolution throughout the transposed remanats of
\nthe belt, Van Staal et al. (1998) suggested that deformation in the ensalic Welsh Basin
\nduring the Tremadoc, followed by formation of a late Tremadoc/Early Arenig arc was
\nrelated to diachronous Penobscottian collision and a renewed cycle of
\nsuprasubduction zone magmatism equivalent to the Popelogan arc.
\nWithin Southern Britain, evidence in support this hypothesis is relatively sparce.
\nHowever a new, concordant, U-Pb date for the Twt Hill Granite of the Arfon Sub-
\nBasin of NW Wales of 615.2 ± 1.3 (2σ) Ma contrasts with an Rb-Sr isochron age of
\n491 ± 12 (2σ) Ma from the same body. The latter is thought to result from isotopic
\nresetting during regional low grade metamorphism. In this context, the Rb-Sr isochron
\nage is taken to reflect the onset of latest Cambrian to Early Tremadoc tectonic uplift
\nthat ultimately led to basin inversion prior to the Arenig overstep at around 478 Ma,
\nand that this in turn reflects plate-scale processes analogous to the Penobscottian
\nevent.
\nAlthough the extent and influence of this event cannot be fully assessed at present, it
\nperhaps puts into context some earlier studies that have noted the presence of pre-
\nArenig folding and early cleavage formation within the Harlech Dome area of the
\nNorthern Welsh Basin. Also, it questions the earlier conclusion that the outboard
\nMonian Composite Terrane represents a single, purely pre-Iapetan, accretionary
\ncomplex. In the light of this data and in response to a need to develop new expertise
\nand support promotion of geology within the region (such as the GeoMôn UNESCO
\nGeopark initiative), further investigation of the tectonic assembly of this region will
\nform part of a multi-disciplinary project that the BGS has just commenced in NW
\nWales.

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