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 fifth and final blog post in a series exploring the geology of Scotland, written by Alex Neches.
The Southern Uplands Terrane – a paradox explained, a lingering mystery and the assemblage of the British Isles, by Alex G. Neches
Bounded by the Southern Uplands Fault to the northwest and the Iapetus Suture to the southeast, the Southern Uplands Terrane represents a classic example of a well-preserved ‘accretionary complex’ – a defining feature of plate tectonics consisting of a succession of ocean floor and overlying sediments that have been scraped off an oceanic plate as it sank underneath a continental plate. Amidst extensive studies that lasted well over a hundred years, geologists have struggled to unravel a paradox posed by conflicting rock and fossil evidence.
The Southern Uplands Terrane consists of volcanic and sedimentary rocks. The oldest lavas and chert – a hard sedimentary rock made of quartz crystals – represent the uppermost part of the oceanic crust of the Iapetus Ocean and are as old as 480 million years. They are followed by shale – a soft sedimentary rock made of consolidated mud – deposited as early as 460 million years ago. Both chert and shale are rich in marine fossils. The first preserve radiolarians – microscopic plankton that populate world’s oceans even these days. The latter preserve a wealth of graptolites – a now-extinct large plankton group with slender bodies, whose traces resemble pencil marks. The rocks of the ocean floor and its immediate shale cover are succeeded by more recent sandstones, mudstones, siltstones and conglomerates, which are as young as 440 million years. Throughout the Southern Uplands, this succession of rocks is arranged into well-defined slices – or wedges – divided by faults. The order of rock layers within each slice indicates that their age decreases northwestwards. Fossils, however, indicate that the overall age of rocks decreases southeastwards. Scientists were confronted with one of the world’s finest instances of a geological paradox; and it had to be solved.
The Iapetus Ocean existed in the southern hemisphere. The continent of Laurentia lay to the north of the ocean, in equatorial latitudes, Baltica was to the east, in temperate settings, and Gondwana was to the south, in the polar region. Around 500 million years ago, Iapetus reached its maximum width of 1000 km – a size comparable to that of the present Atlantic – after which it began to shrink. Narrow masses of land detached from the Gondwana supercontinent started to drift towards Laurentia; among them, East Avalonia embarked on a 60-million-year journey that would play a decisive role in the final closure of Iapetus. Behind it, a new ocean – Rheic – was already opening.
The rate at which the oceanic plate of the Iapetus Ocean was sinking under the continental plate of Laurentia in a process called ‘subduction’ was very slow, with a mere 2 cm of oceanic crust consumed every year. This coupled with a long and uninterrupted period of sedimentation. The equatorial climate of Laurentia favored intense erosion of the land. Sediments were laid down by rivers in the shallow waters of the continental shelf. From there, strong marine currents carried them down the precipitous slopes of underwater canyons, creating avalanches that would come to rest on the ocean floor. The ensuing deposits, of terrestrial origin, but eventually settled in the deep marine environment, are called ‘turbidites’ – a term that evokes the image of swirling, turbulent water loaded with chaotic particles of sand, mud, silt and sometimes even boulders. The volume of turbiditic sediment was so large, it not only filled the trench – a deep and narrow fault in the ocean floor that marks the subduction area – but also extended further onto the oceanic crust, covering the increasingly younger lavas, cherts and shales that were approaching the continental margin. The ocean floor is uneven and has pronounced irregularities that rival those of terrestrial landforms, so tectonic plates do not glide smoothly past each other. As the oceanic plate was sliding beneath the continental plate, successive wedges were sliced off it and accreted to the edge of Laurentia, creating the ‘accretionary complex’.
The Southern Uplands Terrane was now formed, but the dilemma of how the rocks were becoming younger in opposite directions remained a source of debate. This caused some geologists to reconsider the nature of the Southern Uplands and even propose different tectonic models for its formation. Thorough field work was carried out, and rocks and fossils were analyzed over and over, until geologists realized the explanation might be simpler than expected.
Each new wedge of oceanic floor and sediments was stacked underneath the previous one, which caused the wedges to fold and tilt about 90 degrees northwestwards. This explains why in each individual slice the turbiditic deposits young in this direction. The southeastwards decrease in age across all wedges considered together indicates not only the path of the turbiditic sediments that were deposited in the opposite direction of the subduction, but also the order in which the individual wedges were stacked, folded and tilted: the first would, therefore, be the oldest, while the last would be the youngest.
This explanation required many years of discussion and the collaboration of specialists from different geological disciplines. The paradox was no longer a paradox. However, geologists would soon face a new challenge.
The ever-lasting question of sediment source has posed another interesting, and perhaps even more difficult, problem. Where did the vast amounts of the now-compacted turbiditic sediments originate? Their terrestrial provenance was not to be doubted, but what landforms had been eroded? And where? A good rule of thumb is that the larger and coarser the rock particles are, the closer their source is, while the smaller and more rounded they are, the more weathering and erosion they endured. Grains of the hard and durable mineral zircon revealed that the turbiditic sediments derived from volcanic and metamorphic sources.The grains of volcanic origin, in particular, were rigorously studied and geologists were surprised to see that most of them were not contemporaneous with the formation of the sandstones, but were much older. In fact, only the very rare, but considerably large boulders of granite found among the turbiditic sandstones are thought to originate form erosion of the nearby volcanic arc of the Midland Valley Terrane, to which the Southern Uplands had been accreted. The other grains of volcanic origin were 1000-500 million years old, a considerable interval that corresponds to the formation and progressive evolution of Avalonia as a volcanic arc. By around 460-440 million years ago, East Avalonia had advanced close enough to Laurentia for its gradual erosion to feed the turbiditic sediments that were laid down in the same period.
The grains of metamorphic origin generated even more surprising and mysterious results. Most zircon grains date back 1 billion years, some are even 2 billion years old, while one grain holds a record age of 3.6 billion years – older than the oldest known age of the Lewisian Gneiss! This time, an Avalonian origin was out of question; such ancient grains could only have come from Laurentia. About 1000 million years ago, the supercontinent Rodinia assembled and the Grenville Mountains formed as a result. Their subsequent erosion shed vast amounts of sediments that would later become the Caledonian Mountains. Therefore, it would make perfect sense that erosion of the Caledonian Mountains would bring recycled grains of Grenville age all the way to the Southern Uplands. There was an inconvenient problem, though. These grains would have had to cross the entire width of the Midland Valley Terrane, which separates the Grampian and Southern Uplands, but this is improbable, as in the Midland Valley itself zircon grains of Grenville age are very scarce.
To accommodate this scenario, some geologists proposed that the terranes involved must have existed at disparate locations and ended up in their present configuration upon significant lateral slip along fault lines. This is also implausible, as the accretionary complex of the Southern Uplands formed at the edge of the Midland Valley Terrane, which at the time represented the margin of Laurentia. This mystery is yet to be solved.
While the Southern Uplands Terrane was developing, a related series of events had started to unfold. East Avalonia – composed of nowadays England and Wales, but also Belgium, Netherlands and Luxembourg, and small parts of France, Germany and Poland – was fast approaching Baltica and Laurentia; it was both pulled forward by the subducting oceanic crust of the Iapetus Ocean and pushed from behind by the growing oceanic crust of the Rheic Ocean. Around 440 million years ago, East Avalonia collided with Baltica. Ten million years later, also with Laurentia. The latter was not a head-on collision, but a rather soft one, in an oblique direction. Some of the sediments were compressed and compacted into rocks, but their mineralogical composition remained largely unaltered. The northwest margin of East Avalonia sank underneath the Southern Uplands Terrane, whose rocks deformed into folds that draped over it.
Around 400 million years ago, West Avalonia – East Avalonia’s sister block that nowadays represents a strip of land in eastern Canada and United States – also collided with Laurentia, while Armorica – another mass of land detached from Gondwana, comprised of the nowadays Armorican Massif in France and the Channel Isles – caught up with East Avalonia and collided with it. The distant effects of this collision were minor, but some of the faults separating the individual wedges of the now stable accretionary complex of the Southern Uplands were reactivated.
The closure of the Iapetus Ocean formed the British Isles as we know them now, by joining Scotland and Ireland from its Laurentian shore with England and Wales from its East Avalonian shore. The line that marks the closure, known as the Iapetus Suture, follows closely the borderline between Scotland and England, but is mostly hidden by more recent layers of rock.
The Southern Uplands paradox – a once insurmountable problem – was solved with a simple explanation. The rest of the puzzle will engage geologists for another while. The formation of Scotland lasted from the dawn of Planet Earth, 3200 million years ago, to the Middle Paleozoic era, 400 million years ago. The following period was not without geological events, but was somewhat more settled. The Quaternary glaciation, commonly known as ‘the last ice age’ is perhaps the most recent major event that left a visible mark across Scotland’s ancient landscapes, which continue to be shaped today. Geological processes tend to be slow, but they don’t stop.Alex G. Neches
Based on information from the following sources:
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