STRUCTURAL EVOLUTION OF THE SKIDDAW GROUP (ENGLISH LAKE DISTRICT) ON THE NORTHERN MARGIN OF EASTERN AVALONIA

The Skiddaw Group comprises a marine sedimentary sequence deposited on the northern margin of eastern Avalonia in Tremadoc to Llanvirn times . It is unconformably overlain by subduction-related volcanic rocks (t he Eycott and Borrowdale Volcanic groups) of mid-Ordovician age, and f oreland basin marine strata of late Ordovician and Silurian age. The S kiddaw Group has a complex deformation history. Syn-depositional defor mation produced soft sediment folds and an olistostrome. Volcanism was preceded (in late Llanvirn to Llandeilo times) by regional uplift and tilting of the Skiddaw Group, probably caused by the generation of me lts through subduction-related processes. The Acadian (late Caledonian ) deformation event produced a northeast- to east-trending regional cl eavage, axial planar to large scale folds, and a later set of southwar d-directed thrusts with associated minor folds and crenulation cleavag es. This event affected the northern Lake District probably in the lat e Silurian and early Devonian. The Skiddaw Group structures contrast s trongly with those formed during the same event in the younger rocks o f the Lake District inlier. The contrasts are attributed to differing rheological responses to varying and possibly diachronous stresses, an d to possible impedence of thrusting by the combined mass of the Borro wdale Volcanic Group and the Lake District batholith.