Permian and Triassic rifting in northwest Europe

Abstract

The Permo-Triassic roughly coincided with the life span of Pangaea. So far as Europe is concerned, Pangaea's creation began in the early Carboniferous (Visean) when the northward-drifting megacontinent Gondwana began to collide with the Iberian portion of the slower moving Laurussia, while Proto-Tethys was subducted beneath the southern margin of central Europe. Creation was complete by the end of the Carboniferous or early in the Permian with the final development of the Variscan orogenic belt, which trends from Brittany eastward through central Europe, and with the addition of western Siberia along the line of the Ural orogen (Ziegler 1989). Despite, or perhaps because of, its bulk, Pangaea was not a stable megacontinent. No sooner had it formed than it tried to break apart again. The disintegration of Pangaea had already started before the end of the Triassic with the westerly extension of Tethys between Iberia and Africa, though not yet underlain by oceanic crust, and, by early in the Jurassic, rifting was taking place between Africa and the Americas in the newly forming Central Atlantic Ocean (Ziegler 1988). Indeed, E-W extensional movements within a Proto-Atlantic Ocean possibly began as early as the Late Carboniferous (Haszeldine & Russell 1987), whilst mid-Permian extension is well documented in East Greenland (Surlyk et al. 1984). These extensional movements may have propagated southward to initiate fracturing in the Viking and Central Grabens of the North Sea, along which the Late Permian Zechstein Sea was to break into the subsiding Rotliegend basins, and towards the Central Atlantic, where a shallow seaway eventually developed early in the Jurassic. Thus in the northern half of Pangaea, the continued existence of the former Laurussia was already at risk in the Permian, although crustal separation in the North Atlantic, which possibly started in the Rockall Trough in the Early Cretaceous, was finally achieved along the line of the Reykjanes Ridge only in the Paleocene. These major events on the periphery of what is now Europe, separately and jointly, were factors that probably controlled a whole sequence of tectonic events within the continent, which, in turn, controlled its patterns of mostly terrestrial sedimentation. Climatically, the northward drift of Laurussia had carried NW Europe from a region of equatorial rain forest during the later Carboniferous to the latitudes of a trade wind desert, like the modern Sahara, in the Permian. The Late Permian basins of Upper Rotliegend and Zechstein deposition were arid. Rotliegend sediments are characterized by dune sand and the saline mudstones of a semipermanent desert lake, and the Zechstein by shallow-water carbonates and anhydrite and deeper-water halite. During the Triassic, however, brackish-water fluvial and lacustrine sediments occupied the basinal areas, although even here, halite horizons (e.g. Rrt, Muschelkalk and Keuper in the North Sea area) associated with local transgressions from Tethys, testify that arid conditions were never far away.