Structural geology of the Llandovery Series in the type area, Dyfed, Wales

The type area of the Llandovery Series lies structurally between the Ordovician core of the Tywi ‘anticline’ to the northwest and the higher Silurian rocks of the Myddfai steep belt to the southeast. Major SE-verging anticline–syncline pairs dominate the structure in the northeast and southwest of the area but they are not continuous with each other along strike. Their non-cylindricity results from nucleation on lenticular sand-rich units. The steep belts are more laterally continuous and are thought to be localized above major basement faults. Fault tips have propagated up into the steep belts. Components of SE-down dip-slip and sinistral strike-slip operated during fold and fault development. Accompanying NW–SE shortening is recorded by a steep NW-dipping cleavage, here axial planar to the major folds but clockwise transecting them further northwest. This transpressive event climaxed in Early to early Mid Devonian time. Relict structures, basin-down faults and growth folds remain from earlier syndepositional extension or transtension.