Scotland. Peter Friend
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The Moine Supergroup appears to have been deposited in the Neoproterozoic (about 1000 – 900 million years ago), so it was being formed at the same time as part of the Torridonian succession, although horizontal movements have brought them closer since they formed. Today, the Moine contains evidence of at least three different episodes of mineral alteration, the first around 850 million years ago (Knoydartian), the second 470 million years ago (Grampian; mid-Ordovician) and the last roughly 430 million years ago (Scandian; mid-Silurian), each resulting from phases of movement in the Earth’s crust where the rocks were moved, folded and fractured (Fig. 21). The Grampian and Scandian episodes are usefully distinguished as important phases in building the core of the Caledonian mountain belt. A further phase, the Acadian (mid-Devonian, 400 million years ago), is more clearly seen in other areas, showing that the movement pattern along the mountain belt involved many distinct continental fragments with different movement histories (Fig. 25). Much later, in the Mesozoic and Cenozoic, this belt was split by the plate divergence that formed the Atlantic Ocean, explaining why today there are other fragments of the Caledonian belt in Canada, Greenland and Scandinavia.
The Dalradian Supergroup was originally a succession of sediments more varied in type than the Moine. This has allowed the mapping of distinctive rock types across the country, revealing a complex pattern of folds (some upright, others over-folded) and fracture surfaces, themselves often folded after their original formation. These were formed by complex, multi-phase movements which occurred during a general convergence of the crust in a northwest/southeast direction. Radioactive dating indicates that much of this movement took place 470 million years ago, in the same Grampian episode that also deformed the Moine. It is estimated that the crustal rocks of the northern part of the Grampian Highland terrane were uplifted by some 25–35 km during this event, creating a major mountain range. Note that, despite such large amounts of uplift being indicated by research on the pressures that cause the metamorphism, mountains themselves never reach heights above sea level of this magnitude. The present height of Mount Everest is about 9 km, and this is thought to be some indication of the maximum height to which mountains can be lifted, given the powers of erosion that can be generated in present-day steep and high mountain belts. The mountains being measured in planets and moons may be bigger because of the different gravitational forces present.
Igneous intrusions were also formed during the Caledonian episodes, as heat from the compression produced molten magma that rose in the deforming crust, cooled and solidified, most commonly forming granites. These igneous volumes were emplaced both during and after the various phases of Caledonian movement. Where they have been exposed by erosion, they have given rise to differences in the material properties of the bedrock that have locally influenced the present-day landscapes.
The Great Glen Fault is one of the most obvious features of the landscape when Scotland is viewed from a satellite in space. Unlike the complex forms of the coastline and the river valleys, it represents a simple, straight or perhaps very slightly curved, vertical fracture cutting the crust (Figs 19, 20, 22). This major feature separating the Northern Highland and Grampian Highland terranes, and bisecting the Caledonian core, is now thought to have been part of a system of fractures that formed first in the Scandian phase (mid-Silurian, 430 million years ago) due to compressive continental movements that involved a strong enough oblique component to produce sliding parallel to the bedrock fabric of folds and faults generated by the general compression. A recent estimate of the amount of strike-slip sliding between Laurentia and Baltica (Fig. 25) during this phase is that it was about 1200 km, although this total movement was distributed between numerous faults. In the simple analysis of fault mechanics in Chapter 3 (Fig. 17), a clear distinction was drawn between reverse faulting, resulting from convergence or compression, and strike-slip faulting, resulting from shearing. The present belief is that the Great Glen, and other similar faults, formed as a result of a combination of compression and shearing, sometimes referred to as oblique-slip, or transpression.
Episode 5: formation of the Lower Palaeozoic of the Southern Uplands terrane
Strongly folded, fractured and altered Ordovician and Silurian bedrock predominates in the Southern Uplands terrane. The commonest material is mudstone, often altered to slate. Altered sandstones are also common, with lesser amounts of altered limestone and volcanic material (Fig. 19). In the present landscapes, much of this material has been weathered and covered to some degree with Ice Age deposits, so good exposures of the sediments are rare and the hills of the Southern Uplands are generally more rounded and less rocky than those of the Highlands.
FIG 26. Sketch sections of an accretionary prism forming during subduction: (a–c) the development of thrusting (reverse faulting) within an accretionary prism; (d) age relations within the thrust stack. Thrust sheets get younger towards the southeast (i.e. 1 is older than 3) but, within each sheet, beds get younger towards the northwest. The beds are often very tightly folded and dip steeply.
It is thought that these sediments first formed as an accretionary prism, created when ocean crust in the southeast was subducted (see Chapter 3) beneath the deforming continent to the northwest, now represented by the Highlands. As subduction continued, the newly deposited sediments were folded and scraped up into a number of slices that were made of younger and younger ocean floor sediment as the movement continued (Fig. 26). How much of the Southern Uplands formed as one of these accretionary prisms is uncertain, but it is clear that the setting was marginal to the main Caledonian mountains that lay to the north. The oceanic crust was subducted along a line (locally called the Iapetus Suture: see Fig. 20) that lay to the southeast of the Southern Uplands, roughly along the present Scotland–England border.
Episode 6: formation of the Lower Old Red Sandstone
Old Red Sandstone is the name commonly given to the red sandstones, mudstones and conglomerates that underlie rocks of Carboniferous age. The Old Red rests unconformably on older rocks in all of the Scottish terranes except the Hebridean, where it is absent (Figs 19, 20). Successions of this bedrock have been classified as Lower, Middle and Upper Old Red Sandstone, depending on their fossil content and spatial relationships. Episode 6 concerns only the deposition of the Lower Old Red Sandstone.
Although fossil evidence for dating the Lower Old Red Sandstone is not common, the primitive fish and plant fossils that do occur indicate that it was deposited during the late Silurian and early Devonian, about 420 – 400 million years ago (Fig. 21). The weathering properties of these rocks are such that, in their present-day erosional landscapes, the conglomerates (with their associated lavas) have generally resisted erosion, tending to produce distinct ridges and steep slopes.
The processes of surface modification that deposited the Lower Old Red Sandstone took place largely on land, in rivers and lakes, with small amounts of sediment transported locally by the wind. Great thicknesses of lava are also important, particularly in the Midland Valley, Grampian Highlands and the Cheviot area of the Southern Uplands. The andesitic composition of these lavas suggests they were formed by internal Earth movements related to the plate subduction associated with Episode 5, and they are the earliest Scottish rocks to have yielded reliable measurements of their magnetism at the time of their formation. This information has been used to show that