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 second 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 – a small crustal block that witnessed the formation and breakup of two supercontinents, by Alex G. Neches

The second oldest of Scotland’s crustal blocks is bounded by the Moine Thrust Belt to the northwest and the Great Glen Fault to the southeast, and extends as far north as the archipelagos of Orkney and Shetland. For over a hundred years this area has fascinated geologists, who turned it into a natural, open-air laboratory for studying the relationships between basement rocks and sediment covers, and the intricate systems of folds and faults.

The Northern Highlands Terrane has a very long and complex history for its small area; understanding it requires a glimpse into the Earth’s geography of the end of the Precambrian.

Continents and oceans have not always had their present configuration. Tectonic plates drift on Earth’s upper mantle – an almost solid environment with fluid-like properties – and are, therefore, mobile. Individual continents along with a variable number of crustal fragments that detach and re-attach periodically can assemble into supercontinents that later break apart, opening new oceans in the process and closing old ones. This recurring cycle has been recorded throughout most of Earth’s existence.

Around 1000 million years ago, a new supercontinent called Rodinia began to form by gradually encompassing all the existing landmasses at the time. The Northern Highlands Terrane was but a small piece in a vast jigsaw puzzle; it occupied a central position, surrounded by much larger continental masses: Laurentia (most of nowadays North America, Greenland and parts of Scotland), Baltica (the Scandinavian peninsula and eastern Europe) and Amazonia (most of the current Amazon drainage basin). The collision of these blocks marked the final assemblage of Rodinia and generated a process of mountain formation known as the Grenville Orogen, evidence of which is well preserved.
Like all scientists, geologists propose hypotheses and rely on evidence to explain mysteries. Finding answers is often very challenging, but also rewarding. One of the riddles that puzzled them regards the origin of the Northern Highlands Terrane. Had it broken apart from Laurentia, or had it detached from one of the other landmasses? The origin of a crustal block is usually determined by studying basement rocks and finding connections with larger continents. The process is difficult as most rocks have suffered intense physical and chemical changes after prolonged and sometimes repeated episodes of burying, heating, melting and faulting.

By studying the basement gneisses, whose parent rocks of igneous origin are as old as 2900 million years, geologists were able to determine that the Northern Highlands Terrane is indeed related to Laurentia. The local gneiss shares many similarities with its more famous northwestern cousin, the Lewisian Gneiss of the neighbouring Hebridean Terrane; fragments of it have even been affected by the same geological events.

This ancient basement is overlaid by more recent rocks, known under the collective name of Moine Supergroup – a thick cover of sandstone, siltstone and mudstone with minor igneous intrusions, most of which were deposited during a hundred million year period in the final stages of the Grenville Orogen. The sedimentary rocks were later deformed during multiple episodes of metamorphism.

[Image 1] Simplified map of the Northern Highlands Terrane (Orkney and Shetland not represented). The igneous intrusions pictured are not only those mentioned herein, but all that occurred from Neoproterozoic until a more recent geological period, Paleogene Image info: created using QGIS 3.4 software; contains British Geological survey materials (BGS Geology 1:625000) © UKRI [2020]

A second question that challenged geologists regards the deposition environment of the Moine sediments. Had this happened in a continental basin or an oceanic basin? The sequence of rocks is devoid of fossils, quite monotonous and suffered intense strain and folding. Moreover, siltstone and mudstone are very easily recrystallized and altered under heat and pressure, so geologists could only hope to perform analyses on sandstone.

Tireless field investigations revealed a few small outcrops of undisturbed sediments with an abundance of features imprinted in sandstone: channels, dunes and ripple marks, which allowed scientists to make valuable observations. Some sediments appear banded, indicating a rapid succession of strata, while others are cross-bedded, indicating that the original bedding plane was not straight, but inclined. Traces of longshore currents provided vital clues that sediments came from Laurentia.

The conclusion was that a network of braided streams and rivers with abundant flows descended from the Grenville mountains carrying the sediments that would form the Moine Supergroup over hundreds of kilometres and depositing them on the shallow continental shelf and farther away on the deep ocean basin. The Grenville mountains were eroded at very fast rates. Their geographical position favoured monsoon-like precipitation that coupled with a complete lack of vegetation cover. Such an environment is hard to envision and may require an effort of imagination, as similar conditions are nowhere to be found these days.

Soon after deposition, younger parts of the Moine sedimentary rocks were intruded by magma in the shape of dykes and sills. The first filled existing fractures in rock. The latter filled spaces between layers of rock. Magma solidified into granite, a rock with crystals large enough to be visible. The Moine sedimentary rocks and the granite intrusions were affected by three metamorphic episodes.

The earliest episode, which might in fact have been a series of discrete events, occurred more than 800 million years ago. It is called Knoydartian and has only been recorded on a local scale. The sedimentary rocks were buried, melted, recrystallized and folded under medium heat and pressure conditions.

The Northern Highlands Terrane with its now metamorphosed Moine Supergroup witnessed a series of events that straddled the boundary between Precambrian and Cambrian. The impending break-up of Rodinia was foreshadowed by magmatic intrusions that solidified into pegmatite – an igneous rock whose striking feature are its very large crystals. The scattered fragments of Rodinia soon reassembled to form a new, but very short-lived supercontinent, Pannotia, whose break-up was also preceded by igneous intrusions, which solidified into porphyritic granites. The break-up of Pannotia caused the opening of the Iapetus Ocean, whose subsequent closure was accompanied by the Caledonian orogen. The first Caledonian phase, called the Grampian, was caused by collision of a volcanic arc with Laurentia. The second Caledonian phase, called the Scandian, was caused by the collision of Baltica and Laurentia. The Moine Supergroup, already metamorphosed following the Knoydartian event, underwent additional deformation during both Caledonian phases.

The Caledonian orogen, without a doubt one of the most spectacular mountain building events to have ever occurred, is characterized by specific geological structures: recumbent folds and nappes; it is perhaps not a great exaggeration to consider these as signature features of the Caledonian in Scotland.

[Image 2] Hand specimens of (a) Granite, (b) Pegmatite and (c) Porphyritic granite. Granite and Pegmatite are both examples of igneous rocks. Crystal size indicates how slow (large crystals) or fast (small crystals) magma cooled. Porphyry does not describe a rock, but a texture of a rock that is given by large crystals caught into a finer mass. This indicates that slow cooling was followed by rapid cooling. The images are purely illustrative and specimens do not originate from Scotland. Image source: Specimen (a) is from Norway. Copyright of Siim Sepp (https://www.sandatlas.org/granite/). Specimen (b) is from Finland. Copyright of Siim Sepp (www.sandatlas.org/pegmatite/). Specimen (c) is from England. Copyright of the University of Oxford (www.earth.ox.ac.uk)

We often think of rocks as very hard, compact materials. It is difficult to imagine that when certain conditions are met, rocks undergo elastic deformation. They flow like liquids and yet break like solids. When compressional forces – from opposite directions – are intense, layers of rock not only bend and fold like strings, but the folds are overturned and laid down horizontally. These are called ‘recumbent folds’. When compressional forces are very intense and prolonged, a mass of rocks may end up being thrust over another mass of rocks along a low-angle fault plane. This results in a series of ‘nappes’, which are large sheets of rock displaced over many kilometres. If a section were cut through a nappe, older rocks would appear emplaced over younger ones, while fragments of basement would be seen as small inliers surrounded by more recent rocks. When compressional forces are exceptional, rocks will eventually break.

The Northern Highlands Terrane abounds in such features. The Moine Supergroup, following the Grampian and Scandian events, was re-arranged into a major series of nappes. The older nappes carry the youngest sedimentary rocks that were deposited further away downstream from the Grenville mountains. These are the Glenfinnan, Loch Eil and East Sutherland groups. The youngest nappe carries the oldest sediments that were deposited first, closer to the Grenville source. This is the Morar group. The situation is even more complicated, as minor nappes also exist and all nappes are severely faulted. This makes geologists’ work particularly demanding, but no less intriguing. Despite revolutionary modern techniques and state-of-the-art field and laboratory equipment, the exact ages and relationships between different bodies of rock are still riddles to be solved.

The legacy of the Caledonian orogen had profound implications for the geological evolution of Scotland, not only of the Northern Highlands Terrane, but all other crustal blocks southeast of it: Grampian, Midland Valley and Southern Uplands. Their stories are tales for another time.

Alex G. Neches

[Image 3] The hill of Beinn Aird da Loch rises 110 m above Loch Glencoul in Highland. The Glencoul Thrust (red line) is one of the many thrust faults that make up the Moine Thrust Belt. The rocks above the red line and below the orange line are basement gneisses. The rocks between the red and orange lines are Cambrian sediments deposited during the existence of the Iapetus Ocean. The force of the Caledonian orogen fractured and ripped part of the gneisses and brought them on top of the much younger rocks, which naturally rest on them. Image source: Andrew (www.flickr.com/photos/arg_flickr/14365057502/ and www.flickr.com/people/arg_flickr/)

[Image 4] Timeline illustrating major events in the geological history of the Northern Highlands Terrane (dates are approximate): Gneiss p. max. Age of the oldest parent rocks of the basement gneisses; G. Granite intrusions; P. Pegmatite intrusions; P.G. Porphyritic granite intrusions; Gr. Grampian orogenic event; S. Scandian orogenic event.

Based on information from the following sources:

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