Collapse of the Caledonian orogen and the Old Red Sandstone

Abstract

The classic Caledonian red sandstones, breccias and conglomerates of the Devonian ‘Old Red Sandstone’ sequences of north-west Scotland, Norway and East Greenland have long been regarded as traditional ‘molasse’-type deposits generated by the uplift of the Caledonian mountain chain1–4 and controlled either by extensional5,6 or strike-slip7–9 faulting. Here we propose that the Old Red Sandstone was deposited in extensional basins developed as a result of the collapse of the overthickened crustal welt which resulted from Caledonian compressional tectonics. The Devonian basins developed within the Caledonian mountain chain very soon after the cessation of compressional tectonics, locally reactivating these compressional structures as extensional fault systems. We review the tectonic setting of the Old Red Sandstone in the Caledonian orogen and suggest that the most applicable tectonic analogue is the intra-orogen collapse and extension model proposed for the mid-Tertiary Basin and Range province in the western United States10. This tectonic model has important implications for subsequent Mesozoic extension during the evolution of the North Sea basin.