In the Variscides of SW Britain, the Start-Perranporth line (SPL) trends E-W and shows a history of dextral transpression. It is proposed that during the Devonian the SPL marked a major basement fault which formed the northern margin of a continually infilled group of dextral pull-apart basins, locally floored by oceanic crust. In SW Britain transtension may have arisen due to dextral motions along a terrane boundary which separated the ORS Continent from an Armorican microplate to the south. The faulted continental margin may then have acted as an oblique buttress to the advancing Variscan nappes, inducing dextral transpression, large scale backfolding, and forming the SPL.

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