The Edinburgh Geologist
The Geology of Devolution by Phil Stone
If the Earth's plate tectonic system had seized-up 500 million years ago Scotland might now be enjoying a sub-tropical climate. The palm-fringed beaches along our southern shores would be a holiday Mecca for the inland inhabitants of the Laurentian Federation. With this tourist trade at stake the recent devolution debate would have had a different focus. Should we go it alone? Should we go in with near-neighbours Newfoundland and Labrador or was the grass greener in Greenland? Paradoxically, a simultaneous debate might have occurred 1000 km to the south as Nova Scotia considered whether to quit the Avalonian Federation. In the event, the Iapetus Ocean closed, the Atlantic Ocean opened, and we finally arrived at our present geography, geological immigrants on the fringe of Europe. Somehow auld Scotland had passed Nova Scotia going the other way. You might well wonder how this bizarre crustal shuffle came about and just what is the evidence that enables us to work it out.
It was the fossils that started it all. Cambrian and Ordovician trilobites from Scotland are pretty similar to those in parts of North America, from Newfoundland down the Appalachians. Trilobites of the same age from England are quite different but have affinities with others in Nova Scotia and Morocco. Several other fossil groups show a comparable bipartite distribution and it was this sort of evidence which led Tuzo Wilson to ask 'Did the Atlantic close and then re-open?' in his seminal Nature paper of 1966. Up until then proponents of continental drift had been largely filed away with flat-earth enthusiasts and the victims of alien abductions, but things were about to change. The plate tectonic revolution swept scientific orthodoxy aside and almost any geological convolution seemed possible. Before long it was commonly accepted that an early Palaeozoic Ocean opened between 600 and 500 million years ago hut then closed again before the Atlantic split the newly-merged continents along not-quite-the- same line. The old ocean was named after lapetus, the father of Atlas. On its northern, subtropical shores lay the continent of Laurentia; to the south was temperate Avalonia the margin of the circum-polar continent of Gondwana (Figure 1). page 8 Figure I. The drif't of'Avalonia away from Gondwana and across the lapetus Ocean relative to a stationary Laurentia. (after Fig.5 in Pickering & Smith, l995) The mismatch between closing of the Iapetus Ocean and opening of the Atlantic left some bits and pieces trapped on the 'wrong' side (Figure 2). One of those bits is Scotland hut our Laurentian credentials are only really convincing from the Hebrides and the northern Highlands down to the Highland Border. South of there, things are a wee bit less certain as the foundations of the country turn out to he the geological flotsam of the Iapetus Ocean, swept up and accreted onto the edge of Laurentia as the oceanic lithosphere was subducted. A big contribution was added about 470 million years ago when an oceanic volcanic arc collided with an early Midland Valley continental fragment and shunted it northwards to crush the oceanic rocks of the Highland Border Complex and deform the Dalradian. Part of the arc causing all the trouble ended up as an obducted ophiolite fragment on top of the Midland Valley basement block; we see a bit of it now at Ballantrae. Thereafter, with subduction established under the new and extended Laurentian margin, the end of the lapetus Ocean was inevitable. The Southern Uplands was sequentially scraped off the sea floor and built-up into a thrust belt advancing south. The sand grains in its rocks tell of other volcanic arcs, now lost for ever. Its northern, Ordovician belt still has convincing Laurentian faunal links but by the time the southern margin was added, in the middle Silurian, Laurentia and Avalonia had converged; Iapetus was no more. Scotland came out on top in this first meeting with England as the margin of Laurentia over-rode Avalonia. The Southern Uplands thrust front marched on southwards into Avalonia with a load-induced foreland basin ahead of it. Some of the rocks deposited therein are now preserved as part of the Windermere Supergroup in the south of the English Lake District.
Beneath the Southern Uplands thrust belt the plane of collision, the Iapetus Suture, dived northwards (Figure 3). We can now image this plane seismically and see that eventually it intersects the Moho somewhere under Edinburgh. So, at about 30 km depth, Edinburgh is underlain by Avalonian mantle. Above that, but under a surface skin of sedimentary rocks between 5 and 10 km thick, the associations of the crystalline crust beneath southern Scotland are also a bit ambiguous. The Midland Valley crust apparently extends south of the Southern Upland Fault to underlie the northern, Ordovician part of the Southern Uplands. Farther south, under the Silurian sector, the crust is significantly more magnetic and this character is shared with Avalonian crust, south of the Solway and in the footwall of the suture. Did a detached piece of Avalonia get trapped on the wrong side during the final closure of Iapetus? There certainly seems to have been some late movement between this suspect block and the Midland Valley crust beneath the Ordovician part of the Southern Uplands. The surface expression of this movement is the Moniaive Shear Zone, up to 4 km across, which stretches along the Southern Uplands beneath Glenluce, Moniaive and Drumelzier, more or less coincident with the Ordovician Silurian boundary.
All of this means that the geological join between Laurentian Scotland and Avalonian England is not too easy to define. At one extreme, the influence of the Southern Uplands thrust belt extends into the southern Lake District and inliers of Southern Uplands rock sensu stricto span the border at Carter Bar. The Iapetus Suture itself does not make it to the contemporary level of erosion but remains buried beneath the Carboniferous strata of the Solway Northumberland basin. In fact, since the suture plane was reactivated as a normal fault during the development of that basin, one important manifestation of the suture might be the Maryport Stublick Fault system across northern England. At deeper, mid-crustal levels, bits of Avalonia might get as far as Moniaive; still deeper and we have Avalonian mantle under the whole Southern Uplands. This is complicated enough but remember that there were other players in the great Caledonian terrane exchange. Europe might have got Laurentian Scotland and the northern half of Ireland (thats half the whole island, not just Northern Ireland) but North America got the southern bit of Newfoundland (paradoxically, the Avalon peninsula, type area for Avalonia), Nova Scotia, large chunks of New Brunswick and Maine and even Boston, quintessential New England; arguably it got bits of Carolina and Florida as well as Scandinavia, originally part of Baltica, the third participant in this geotrading, got hits of the Norwegian coast but thats another story. This is probably not the place to debate who got the best of the bargain, which all comes down to quality versus quantity. Until the Atlantic goes into reverse and provides the opportunity to swap back there is not a lot to he done about our present arrangements. However, in that respect the future doesnt look too bright since we now live on a continental margin. Thats not too healthy a place to be during subduction and inevitable continental collision. When auld Scotland is finally reunited with Nova Scotia who will come out on top then?
following could be a start if you want more details of the geological act of union:
Dr Phil Stone works for the British Geological Survey at Murchison House and is the geologist in charge of the Southern Uplands Project. He has worked on the Southern Uplands and Lake District for near on 20 years. This is the second of Phils articles published in the Edinburgh Geologist taking a light-hearted look at geology.
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