Synopsis

Although modest in size and topographic expression, the Cumbrae islands hold a key to better understanding of the structurally anomalous portion of the Midland Valley of Scotland underlying the Firth of Clyde. Situated beneath the western edge of the largest and thickest block of the Clyde Lava Plateau, the islands occupy the nexus between mainland and offshore and serve as a stepping stone to more remote outliers of the lavas and their underlying sedimentary formations in Bute and Arran. The Cumbrae islands carry representatives of three major groups of faults that have been regional controls on sequence preservation, outcrop distribution, and geomorphology since their initiation as proto-Variscan structures in the Late Devonian. Moreover, the islands are shaped by two sets of folds, one of inferred early to mid-Carboniferous age, the other of later Carboniferous age, both also present in other parts of the Firth. Finally, long accepted as a matchless locality for intersecting dyke swarms of Carboniferous (Viséan and latest Westphalian) and Palaeogene (Paleocene–Eocene) age, the interplay of dykes and faults on Great Cumbrae allows a refined history of fault reactivation to be reconstructed. In turn, analysis of the islands’ structures and their evaluation in regional context enables a local tectonic history to be formulated and presented in six informal stages between the Late Devonian and Holocene.

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