Summary
On the margins of the Laurentian continent the interval
c.
800-492 Ma was dominated by (1) rifting involving continental crust and (2) rift sedimentation with or without related volcanicity and plutonism, culminating in (3) the formation of the Iapetus Ocean and succeeded by (4) initial phases of compressional tectonics. The latter involved the approach of volcanic island-arc systems and the partial or complete obduction of ophiolites of the oceanic crust cutting through the slope-rise prism and, at least in some parts of the orogen, being thrust onto the continental-shelf deposits in the Cambrian and Tremadocian. These processes led to the closing of the ocean in the mid-Ordovician Taconian and the Silurian Caledonian orogenies. The earliest and most complete pre-Arenig orogenic movements are recorded in the Finnmarkian orogeny, including obduction of ophiolites in Scandinavia (520–480 Ma). Related sedimentation gaps, preceded by deformation and ophiolite obduction and/or uplift, were accompanied by tectonic melange development and/or volcanicity. These features are recorded widely from northern Britain and Ireland (Grampian orogeny), and from the Appalachians of Newfoundland (Burlington orogenic episode), Maine, USA, and Quebec, Canada (Penobscot orogeny). Movements interpreted as relating to the Penobscotian are now recognized as widespread in the US Appalachians. The Sardic orogenic phase in southern Europe (
c.
510 Ma) is associated with plutonism and metamorphism and may be related to late Cadomian III orogenic movements.
In the southeastern landmasses of Gondwanaland (Avalonian-Cadomian-Pan-African) essentially the same time interval (
c.
800–492 Ma) was also dominated initially by rifting of continental crust overlain by late Proterozoic shelf deposits and the obduction and folding of ophiolites (Cadomian I orogeny) and underlying rocks, followed by intrusion of late- to post-tectonic plutons (Bleida). Cadomian II resulted in the deposition of coarse clastics, volcanics and glacigenic sediments (Tiddiline), and folding, erosion and deposition of a thick wedge of late Proterozoic carbonate and clastic rocks with alkaline volcanics (Adoudounian) overlain conformably by lower and middle Cambrian that are overlain disconformably by Tremadocian clastic sediments. During the rifting phase continental fragments drifted away and/or became volcanic arcs with subduction zones and back-arc basins constituting the weakly orogenic Avalonian terrane to the W and the strongly orogenic but discontinuous belt forming a terrane characterized by Cadomian fragments between the Avalonian terrane of the British Isles and the W African craton. Stratigraphic columns and schematic time-slice stratigraphic profiles with interpreted tectonic environments from various parts of the orogen provide comparisons and contrasts in stratigraphic, faunal, structural, volcanic, plutonic and metamorphic evolution.
