EVOLUTION OF THE PERI-GONDWANAN MARGIN OF SOUTHERN BRITAIN

Synthesis of geological data from the Late Neoproterozoic through to Early Devonian geological record of southern Britain has revealed an ‘event stratigraphy' that provides a powerful tool for local and regional tectonic interpretation of the peri-Gondwanan margin.

Comparison the disparate Late Neoproterozoic geological successions of southern Britain reveals a number of common events which together preserve an overall geological history similar to other Avalonian terranes and suggest that they preserve a collage of decoupled upper-plate fragments formed during subduction along the outboard margin of Gondwana.

The Cambrian to Tremadocian succession is marked by subsidence in a number of discrete basins and formation of a Tremadoc arc (<489 to >478 Ma) illustrating attenuation of the Gondwanan margin in response to both the onset of Iapetan subduction and opening of the Rheic ocean. Palaeogeographic affinities of these basins are poorly understood although inversion in the Harlech Dome (<489 to >466 Ma), low grade metamorphism within the Arfon Basin (490 Ma) and folding in the Monian Supergroup attest to tectonism analogous to Penobscottian accretionary events on the Gander margin of the northern Appalachians.

Renewed subsidence recorded in the Ordovician record of the Welsh Basin is interrupted by intrabasinal uplift during Sandbian times (460 Ma) associated with development of volcanic edifices in the north and margins of the basin. Following a Katian highstand (>445 Ma), glacioeustatic regression that reached its acme during Hirnantian times is interrupted by localised deformation in the basin margin (‘Shelvian Orogeny') that may record the distant effects of Avalonia/Baltica collision.

Good preservation of Silurian strata within the Welsh Basin and adjacent shelf area has enabled more detailed sequence stratigraphic analysis in which anomalous high frequency lowstands can be interpreted with respect to intrabasinal tectonics. Events include an intra-Telychian event (<436 to >428 Ma), an Early Wenlock event (<428 to >426 Ma), enhanced subsidence during Pridoli times (<418 to >416 Ma) and final, Early Devonian penetrative deformation (400 Ma) that together record closure of Iapetus and the possible influence of Rheic ocean subduction.