The Pontesford Lineament, Welsh Borderland

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Abstract
The NE-trending Pontesford Lineament centres on the Pontesford-Linley and adjacent faults in the Shelve/Longmynd inlier. To the NE it is usually drawn along the SE edge of the Cheshire Basin and to the SW along a variety of lines parallel to or converging with the Church Stretton Fault. New mapping and reassessment of published data suggest that SW of the Pontesford-Linley Fault the lineament is logically drawn along the Clun Forest Disturbance, a post-Pridoli (late Silurian) fold/fault belt, then across the NW side of the Llandrindod inlier to flank the Tywi Lineament. The lineament was intermittently active at least from mid-Ordovician through to Triassic time, with both strike-slip and dip-slip displacements that involved the upper part of the Precambrian basement. Late Ordovician and early Silurian strike-slip was particularly important. The lineament controlled the sites of several Silurian sedimentary basins. Ordovician contrasts across the lineament can be explained either by contemporary fault control or by subsequent large displacements juxtaposing unlike terranes. The long history of reactivation and basement control on the Pontesford Lineament is typical of lineaments throughout Wales and the Borderland. It suggests that both Caledonian and Variscan structural trends N of the Variscan Front may be inherited from older, pre-Silurian or even Precambrian, basement structures.

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