Did Iapetus start to open during the Cambrian?

Abstract

The geological evolution of the British Isles during the Lower Palaeozoic was dominated by the presence of the Iapetus Ocean. Although the timing of the major events in its later history are reasonably well known, its pre-Ordovician evolution is far less clear because of the absence of rocks of that age in the area of the lapetus suture. To deduce the early history of the Ocean, circumstantial evidence from rocks originally deposited on the adjacent continental crust is needed. Petrographical evidence, from such rocks in the Dalradian, is discussed here and implies that lapetus had still not opened by ∼670 Myr ago. A dramatic increase in tectonic instability after this date, culminating in the extrusion of the Lower Cambrian Tayvallich Volcanics, is considered to date the continental rupture that led to the formation of Iapetus. This view is in contrast to the assumption that continental break-up must have preceded the earliest Dalradian sedimentation1 and that the red-bed fluviolacustrine sediments of the Torridon Group, dated at ∼810 Myr, could be a consequence of this initial rifting2.