Basement–cover relationships around Aberdaron, Wales, U.K.: the fault-reactivated northwestern margin of the Welsh Basin

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.

Basement–cover structural relationships in the Kaladgi Basin, southwestern India: Indications towards a Mesoproterozoic gravity gliding of the cover along a detached unconformity

“…Nevertheless, in some basins we see major deformations in the cover which is not represented in the basement. Basementcover relationships (Harris et al, 1987;Gibbons, 1989;James and Mortensen, 1992;Marquer and Burkhard, 1992;Narr and Suppe, 1994;Teixcell, 1996;Muñoz et al, 2002), may therefore indicate 'thick-skinned' tectonics (Coward, 1983;Butler et al, 2004;Pfiffner, 2006) when basement is involved or 'thin-skinned' tectonics (Gwinn, 1964;Cook et al, 1979;Kokkalas et al, 2013) when there is a detachment separating basement units from a postdetachment cover. Thin skinned deformation styles are diverse and include such types as (a) nappes riding over a rather undeformed subsurface along plane well lubricated thrust faults (e.g.…”

Welsh Basin ‘contourites’ reinterpreted as fine-grained turbidites: the Grogal Sandstones

“…Smith (1987 a, 6) and a number of earlier workers interpreted this to be the result of a tectonically-driven migration of the depocentre for sandy turbidity currents. This migration is reminiscent of the evolution of foreland basins, but was not associated with lowangle thrust sheet stacking at the northwest margin of the basin (the Menai Straits Fault System of Gibbons, 1989). However, evidence supporting a foreland basin interpretation of the Lake District Silurian to the north has been summarized by Kneller (1991).…”