Structural evolution of the Lake District Boundary Fault Zone in west Cumbria, UK

SUMMARY The Lake District Boundary Fault Zone (LDBFZ) lies at the boundary between the Permo-Triassic East Irish Sea Basin and the Lower Palaeozoic Lake District Block. It divides northwards, within west Cumbria, into a network of fault strands that lie within the cover sequence of Upper Palaeozoic and younger rocks and Lower Palaeozoic basement rocks, and terminates at the St Bees Fault Zone. The earliest evidence of movement across the LDBFZ during the Ordovician is drawn from the component Thistleton Fault which has a volcanotectonic origin and bounds blocks of distinctive stratigraphy and structure. Reactivation of the LDBFZ in response to regional tectonic events occurred during the Late Palaeozoic, Mesozoic and Cainozoic. Oblique reverse displacement during the late Carboniferous was associated with Variscan basin inversion. East–west extension and syndepositional normal displacement accompanied the formation of the East Irish Sea Basin during Permian and early Triassic times. Analysis of fracture mineralization phases from the fault zone strands demonstrates repeated fault activity from mid-Triassic to early Cretaceous times. Slickenside data from the Sellafield area indicate these phases of Mesozoic faulting accommodated dip-slip displacement and south-west-directed extension. Regional uplift during the Cainozoic was accompanied by basin inversion; oblique-reverse displacement across the LDBFZ at this time was accompanied by Cainozoic folding of the hanging-wall block. The structural evolution and displacement history noted for the LDBFZ is likely to have been similar to that of other major faults of north-north-west trend in northern England, notably the Pennine Fault.