Abstract
On mainland North Wales basement rocks emerge from beneath a Lower Ordovician cover along the west side of the Llyn peninsula. The basement contains steeply dipping mylonites (Llŷn shear zone) that separate plutonic and gneissic rocks (Sarn Complex) from a melange (Gwna mélange). The western edge of the Ordovician outcrop follows the basement shear zone, and new trenching data confirm that only a faulted relationship exists between cover and basement along this northwestern extremity of the Welsh Basin. Deformation along this margin has propagated into the Arenig cover to produce southeasterly verging thrusts, asymmetric folds and northwesterly dipping cleavage. A prominent steep fault (Daron Fault) cutting the Ordovician succession follows an eastern splay of the Llŷn shear zone and again therefore records brittle reactivation of an underlying mylonite belt. The likelihood of syn-Arenig fault movements is provided by the presence of a prominent late Arenig coarse clastic unit, containing boulders of the basement mylonites, that is found only to the west of the Daron Fault. Steep basement structures such as the Llŷn shear zone, initially generated as major transcurrent faults, are interpreted as having exerted a strong control over the deposition and subsequent deformation of the Ordovician cover sequence.