Clues of a Cryptic Collapse: New extension indicators identified from the Early Ordovician margin of Avalonia constrained as post Penobscotian Orogenic collapse

The Caledonian orogeny is characterised by several distinct episodes of subduction, back arc basin development, continent/arc collision, and orogenic collapse. Here, we present evidence for four distinct deformation events to have affected the northern margin of Avalonia as recorded in the New Harbour Group accretionary complex on Anglesey, North Wales. We contextualise this deformation within the Caledonian plate tectonic framework to show that: S1, possibly an enhanced bedding related fabric, likely developed in response to subduction (D1), and for the first time we recognise F1 folds in bedding and possibly F1 folds of sedimentary fabrics within S1. We demonstrate that S2 cleavage, which is not present in Arenig sequences, developed prior to 478-467 Ma and attribute S2 to the Penobscotian Orogeny (D2). This marked the return of Ganderia and the overlying Penobscot volcanic arc during its collission with Avalonia between ~490-474 Ma. Following D2, a widespread set of quartz veins, parallel to and overprinting S2 crenulation cleavages, reveals a newly recognised episode of post-orogenic collapse. During collapse, extension was almost perfectly aligned with the earlier compressional episode, demonstrating counteraction of the previous crustal shortening. These extensional structures are, however, difficult to recognise due to the level of overprinting deformation, particularly D3 folding. This third compressional event is inferred to correspond to the Shelvian Orogeny, which followed closure of the Tornquist Sea and collision of Avalonia with Baltica. Widespread F3 folds have developed in bedding, S1 fabrics, S2 crenulation cleavages, S2 folds, and S2 parallel quartz veins related to orogenic collapse. In addition, a final compressional event, D4, sporadically results in the development of open folds and may have resulted from the Acadian orogeny and final closure of the Iapetus Ocean.

The north Avalonian margin experienced multiple phases of collision, subduction, and extension during the Caledonian Orogeny. As a result, North Wales offers an excellent natural laboratory for the study of orogenic collapse and subduction derived plate driving forces. Ultimately, the study of these processes is paramount as they fuel the enigmatic plate tectonic engine and the Wilson Cycle.