Geotectonic framework of the Blueschist Unit on Anglesey-Lleyn, UK, and its role in the development of a Neoproterozoic accretionary orogen

A 560-550 Ma, 5 km × 25 km Blueschist Unit extends NE-SW on the island of Anglesey, Wales, UK, and continues to the southwest for more than 70 km along the northern coast of the Lleyn Peninsula. It has the shape of a shallow-dipping slab or sheet up to a few kilometres thick that was exhumed from the subduction zone and emplaced into the accretionary complex to the northwest. Today the top boundary of the slab is commonly a thrust that was originally a low-angle normal fault at its top in the subduction zone; above it is an accretionary complex and a unit of high-grade gneisses. The bottom boundary is today a thrust (that was originally a thrust), below which is a unit of low-grade or unmetamorphosed rocks belonging to an olistostrome-type accretionary complex. This thrust was originally at the bottom of the slab when it was exhumed in the subduction zone. The sandwiched assemblage was created by the tectonic insertion of the blueschist between the other units. Folding, particularly during late doming of the tectonic sandwich, rotated the top normal-fault boundary into a thrust. Later, high-angle, NE-SW-trending normal faults displaced the tectonic stratigraphy in horst and graben. A 200 km long and 200 km wide, 680-450 Ma calc-alkaline volcano-plutonic belt extends southeastwards from the blueschist belt via northern Wales to central England, suggesting that the subduction of oceanic lithosphere was to the southeast, and this is consistent with the extrusion of the thin high-pressure slab into the accretionary complex to the northwest. This took place on the western accretionary margin of Avalonia. Finally, we outline the key steps involved in unravelling the structure of a Pacific-type accretionary complex.