Abstract The Ribblesdale fold belt, representing the Variscan inversion of the Bowland Basin, is a well-known geological feature of northern England. It represents a crustal strain discontinuity between the granite-underpinned basement highs of the northern Pennines and Lake District in the north, and the Central Lancashire High/southern Pennines, in the south. Recent seismic interpretation and mapping have demonstrated that the Ribblesdale fold belt continues offshore towards Anglesey via the Deemster Platform, beneath the Permo-Triassic sedimentary cover of the southern part of the East Irish Sea Basin. The Môn–Deemster fold–thrust belt (FTB) affects strata of Mississippian to late Pennsylvanian age. Variscan thrusts extend down into the pre-Carboniferous basement but apparently terminate at a low-angle detachment deeper in the crust, here correlated with the strongly sheared Penmynydd Zone exposed in the adjacent onshore. Up to 15% shortening is observed on seismic sections across the FTB offshore, but is greater in the strongly inverted onshore segment. Pre-Carboniferous thrusting post-dates formation of the Penmynydd Zone, and is probably of Acadian age, when basement structures such as the southward-vergent Carmel Head Thrust formed. Extensional reactivation of the Acadian structures in early Mississippian time defined the northern edge of the offshore Bowland Basin. The relatively late brittle structures of the Menai Strait fault system locally exhume the Penmynydd Zone and define the southern edge of the basin. The longer seismic records from the offshore provide insights to the tectonic evolution of the more poorly imaged FTB onshore.

Abstract The outer parts of collision mountain belts are commonly represented by fold and thrust belts. Many of the key concepts in the structural geology of fold and thrust belts have origins in ancient orogens such as the Appalachians and Caledonian chains of Europe, together with the Alps. Impetus in thrust belt research then came from the desire to exploit geological resources that reside in the subsurface, especially arising from hydrocarbon exploration in the foothills of the Canadian Cordillera in the 1960s and 1970s. Notwithstanding decades of exploitation, continental fold and thrust belts are still estimated to hold reserves of 700 billion barrels of oil equivalent. But exploration will focus increasingly on small, hard-to-resolve structures. Basic geological understanding remains as important today as it did for the pioneering explorers in the Canadian foothills. It is a theme that runs throughout this Special Publication.