Conall Mac Niocaill Projects

Projects

Palaeozoic Palaeogeography and the Evolution of the Caledonian-Appalachian Orogen

The following sequence of reconstructions incorporate palaeomagnetic, structural, faunal, chronological and tectonic constraints on the evolution of the Iapetus Ocean in the Early Palaeozoic. Rifting and opening of the Ocean took place in latest Cambrian time, with final closure occuring in the Late Silurian. Continuing research is focussing on delineating the drift history of many of the Intra-Iapetan terranes. These maps are the results of extensive collaborations with colleagues in the US, Canada, Norway, the UK, and Ireland. PDF versions are available by clicking on the links in the right-hand column. You will need to have Adobe Acrobat installed to read the PDF versions

	The Iapetus ocean opened in the late-Precambrian, with some debate as to the precise palaeogeographic configuration of the continents during opening, and the precise timing of rifting. The construction depicted here is for the latest Cambrian to early Ordovician and it is at this time that the Iapetus seems to have reached its maximum extent. Arc subduction had already commenced on the Laurentian margin, with an arc being built on continental crust with an age of about 1.1 billion years, likely to be the North American margin. Subduction also seems to have been active at this time on the southern (Avalonian) margin (not depicted on this diagram). [PDF Version]
	By the late Early Ordovician subductions was well under way on both margins and Avalonia had started to rift away from the Gondwanan margin. [PDF Version]
	By the middle Ordovician the ocean had started to narrow significantly. There is extensive evidence for the collision of the arc outboard the Laurentian margin with the margin, possibly accompanied with a flip in the polarity of the subduction zones, marking the "Grampian" orogeny in the British Isles. Available palaeomagnetic data also indicate that there were several seamounts and / or island arcs present within the Iapetus ocean, which is also indicated by analyses of the faunal distributions within the Appalachian-Caledonian fold belt (see diagram at end of page). [PDF Version]
	Late Ordovician time marks the rapid narrowing of the ocean and faunal provinciality starts to break down. We can also see a raft of rifted terranes to the south of Avalonia, which will become involved in the later collisions that mark the Hercynian, or Variscan, orogeny in central Europe. [PDF Version]
	By the Early to middle Silurian the Iapetus Ocean had narrowed to below the limits of palaeomagnetic resolution. Sediments start to flood across the suture, though it is likely that there were stilll a number of small successor basins. [PDF Version]

Much of the evidence for the presence of arcs and oceanic islands within the Iapetus comes from palaeomagnetic analyses carried out in collaboration with Rob Van der Voo, Ben van der Pluijm and various students in the Tectonophysics Group at the University of Michigan, notably Arlo Weil and Allen McNamara. The Figure below illustrates the pattern of terranes derived from palaeomagnetic studies in the Northern Appalachians on the right, compared with the faunal provinces indicated by statistical analyses of brachipod distributions on the left. The brachipod analyses were carried out in collaboration with Dave Harper of the Palaeontological Institute of the Danish Natural History Museum.