The Scottish Geology Trust is keen to offer people involved with Scottish geology the chance to share information about Scotland’s geology with a wider audience. This is the forth in a series of blog posts exploring the geology of Scotland, written by Alex Neches.

Previous blog posts: The Hebridean Terrane | The Northern Highlands Terrane | The Grampian Terrane

The Midland Valley Terrane – jumbled fragments of an ocean floor and a puzzle with a twist, by Alex G. Neches

The Midland Valley Terrane lies in mainland Scotland between the Highland Boundary Fault to the northwest and the Southern Upland Fault to the southeast. It has been studied for more than a hundred years, and although it is younger than the blocks located northwest of it, so far it has proven to be the most enigmatic and controversial. In fact, it has been a battleground for geologists holding different views in their attempt to solve a series of mysteries and paint a complete picture of its past.

Simplified map of the Midland Valley Terrane. The sedimentary rocks resulted from the erosion of the Caledonian Mountains are known as the Old Red Sandstone – an informal and very general name, since not all of these rocks are sandstone and not all have a distinctive red colour. Volcanic rocks resulted from eruptions of lava. Igneous intrusions represent crystallized magma within older rocks. Image info: created using QGIS 3.4 software; contains British Geological survey materials (BGS Geology 1:625000) © UKRI [2020]

When faulted, Earth’s crust is divided into individual tectonic blocks that often move both horizontally and vertically. When vertical movements occur, blocks can be either raised or lowered. The Midland Valley represents a ‘graben’; that is, a lowered area caught between higher blocks – the Grampian terrane and the Southern Uplands. The geology of the Midland Valley Terrane, however, is far from being so simple.

Field evidence is critical in reconstructing distant events and establishing a chronological framework. There are times when a puzzle piece appears to be missing; in this case, it is the old basement rocks. Without a doubt, a basement exists, but its nature and origin have long been a subject of speculation. In the Midland Valley, the ancient bedrock is hidden deep under a thick cover of more recent rocks deposited as sediments and lavas in a range of environments, throughout the Paleozoic, more than 250 million years ago. This era, the first of the Phanerozoic, brought major geological, climatic and biological changes.

In the Lower Paleozoic (540-440 million years ago), the notorious glaciers that characterized the end of the Precambrian had long melted. The climate was warm and sea levels were rising. The Iapetus Ocean reached its maximum width, then started to close. The continent of Laurentia was in equatorial latitudes, where it would remain for the rest of the era. Large amounts of sediment – gravel, sand, silt and mud – carried by powerful rivers were deposited in shallow and deep marine waters on its continental margin. The oldest sedimentary rocks of the Midland Valley Terrane date back from this period. The Grampian phase of the Caledonian orogen occurred around 470 million years ago when a volcanic arc collided with Laurentia.

In the Middle Paleozoic (440-360 million years ago), the Gondwana supercontinent, which had amassed most of the continental blocks, moved over the South Pole. At the margin of Laurentia, Midland Valley sediments were now laid down in arid conditions on land. A period of uplift, folding and erosion was followed by more deposition. The Scandian phase of the Caledonian orogen occurred around 430 million years ago, when Baltica collided with Laurentia to form Laurussia. Around 400 million years ago, the Iapetus Ocean closed.

In the Upper Paleozoic (360-250 million years ago), Laurussia was separated from Gondwana by the now-closing Rheic ocean. Organic matter gathered into vast swamps, where it formed peat that would eventually make coal. Volcanic eruptions had been common throughout the era, so lavas and pyroclastic material – a very hot and dense mixture of ash, gases and rocks spewed during eruptions – also appear interspersed with the mostly sedimentary rocks of the Midland Valley Terrane. All existing continental masses eventually merged into the Pangaea supercontinent, which caused some folding and faulting of the recently deposited sediments.

Old Red Sandstone near Seaton, Angus. A gap of about 10 million years exists between the smooth layers (Lower Old Red Sandstone) and the coarse layers (Upper Old Red Sandstone). Copyright of Anne Burgess (https://www.geograph.org.uk/photo/5496384, https://creativecommons.org/licenses/by-sa/2.0/)

Having acquired a picture of the climate and environment in which deposition occurred, geologists had to answer a familiar question: where did all these sediments come from? And another, less familiar question: what can these sediments tell about the basement rocks? Samples of sandstone gathered from the north and south of the Midland Valley Terrane were dated using zircon – a mineral with minuscule, but very durable grains that can be recycled over and over despite extreme conditions of heat, pressure and erosion.

The older zircon grains from both sets of samples, although far more abundant in the north, could be linked to the Grenville orogen – the ancient mountain chain that formed on the Laurentian margin as the Rodinia Supercontinent assembled about 1 billion years ago. Some grains were even as old as 3 billion years! There was a good chance that the source of the sediments had been the Dalradian Supergroup of the neighboring Grampian Terrane, whose zircon grains could also be traced to the Grenville orogen and as far back as 3 billion years. This also opened up the possibility that a basement of similar age existed. Moreover, geologists discovered old metamorphic ‘xenoliths’ – literally ‘foreign rocks’ wrenched from the basement during magmatic intrusions. These strengthened the possibility that the Midland Valley Terrane could be underlain by a metamorphic basement of Precambrian age.

The younger zircon grains from the south of the Midland Valley, however, indicated a major igneous source, about 470 million years old. More specifically, large amounts of sediment seemed to originate from the erosion of a volcanic arc – a chain of volcanic islands that developed in the once-closing Iapetus Ocean. This indicated that the Midland Valley Terrane might not, in fact, be underlain by an old metamorphic basement, but a more recent igneous basement.

These contradictory results pushed geologists to make further investigations. The old ages returned by zircon grains from the northern half of the Midland Valley Terrane, as well as the links with the Dalradian Supergroup and the Grenville orogen soon became clear. The Himalayan-sized peaks of the Caledonian Mountains had been eroded at a fast pace in the hot and humid equatorial climate, and the resulting debris, rich in iron oxide, had been deposited as sandstones that now cover a considerable part of the northern Midland Valley Terrane. This, however, did not explain the old and young ages of zircon grains from the south area, and also left unresolved the issue of the mysterious basement. For this, geologists had to dig further. Their attention focused on a special rock assemblage that contains relicts of a peculiar plate tectonic phenomenon.

The Ballantrae Ophiolite Complex near Knockdolian, South Ayrshire. The pink (foreground) and greenish (middle ground) rocks represent pieces of ocean floor that were emplaced onto the continental margin. Copyright of Anne Burgess (https://www.geograph.org.uk/photo/6625892, https://creativecommons.org/licenses/by-sa/2.0/)

When tectonic plates collide, it is almost always the oceanic crust that slides underneath the continental crust. The oceanic crust is thinner, but denser as it is made of igneous rocks whose minerals are rich in heavy elements. The continental crust is thicker, but lighter as it is made of igneous rocks whose minerals contain light elements. In exceptional circumstances, the continental crust may sink underneath the oceanic crust, which, in turn, is thrust over it. The fragments of oceanic crust and ocean floor that become emplaced onto continents are called ‘ophiolites’. These relicts – which can sometimes be found in high mountains! – are rare, but valuable indicators of lost ocean basins.
Few rock assemblages in Scotland preserve such remnants. The Ballantrae Ophiolite Complex, located in the southeast of the Midland Valley Terrane, consists of a wide range of igneous and fine-grained sedimentary rocks that formed 490-470 million years ago and contains vestiges of an expanding ocean floor, magmatic intrusions and a volcanic arc. As the Iapetus Ocean was closing, tectonic plates carrying oceanic crust collided. The older, more mature and, therefore, heavier crust was subducted underneath the younger, lighter one. Hot magma ascended through the overriding crust and solidified at the surface, creating a volcanic arc, which eventually collided with Laurentia about 470 million years ago, causing the first phase of the Caledonian orogen. Upon impact, most of the constituents of this igneous assemblage – ocean floor, magmatic intrusions and lavas – were heavily dismembered and forced underneath Laurentia, with small fragments being scraped off and accreted onto the continental margin.

This crucial evidence caused the puzzle pieces to fall into place. The once probable existence of an old Precambrian basement became questionable. Geologists undertook further research, which brought ground-breaking discoveries. New zircon analyses performed on metamorphic ‘foreign rocks’ indicated that not one, but quite possibly a couple of volcanic arcs developed at regular intervals; some geologists hypothesize that the Caledonian orogen may, in fact, have involved multiple collisions between volcanic arcs and Laurentia. Indeed, with little variation, the original scenario seems to have repeated itself about 450 million years ago. A volcanic arc was born where oceanic crust of the Iapetus Ocean was being consumed. Gradual erosion supplied sediments that were deposited at its front base – an area called ‘fore-arc basin’ – while additional sediments were also brought from nearby areas. The volcanic arc and sediments were pushed underneath the continental margin of Laurentia and were metamorphosed around 400 million years ago during the final phase of the Caledonian orogen. The magmatic intrusions that followed the closure of the Iapetus Ocean – the ‘Newer Granites’ – ripped fragments from this igneous basement and contained them.

With new research being carried out, it is becoming more and more plausible that the basement of the Midland Valley Terrane is made up of remnants of Paleozoic volcanic arcs and not ancient Precambrian rocks. The sedimentary cover, with the exception of the northern part, did not originate from erosion of the Dalradian Supergroup, but of the volcanic arcs and other sources within Laurentia. It was concluded that the 1 billion years old zircon grains encountered in the sandstone samples must have been recycled. In other words, the Grenville mountains were a distant source for the sediments, perhaps a ‘grandparent’ or even ‘great-grandparent’.

Geologists have managed to reconstruct yet another episode in the past, with vital clues gathered from one of the most fascinating and least understood rock assemblages in the whole of Scotland. But, as geologists themselves admit, the recounting of these events is an over-simplification. Even in light of new evidence, the full story is far more complex and remains to a great extent, enigmatic.

Timeline illustrating major reference points in the geological history of the Midland Valley Terrane (dates are approximate): A. Archaean eon; MA. Mesoarchaean era; MP. Mesoproterozoic era; S. Stenian period; NP. Neoproterozoic era; T. Tonian period; Sandstones p. / p. max. Average age / Maximum age of sandstone parent rocks; Gr. Grampian phase; Sc. Scandian phase

Alex G. Neches

Based on information from the following sources:

Badenszki, E., Daly, J.S., Whitehouse, M.J. and Upton, B.G.J., 2015. The mystery of the Scottish Midland Valley basement: Solved! Conference paper, 20 Feb 2015, Irish Geological Research Meeting
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