Scotland. Peter Friend

Чтение книги онлайн.

FIG 25. Diagram showing major plate-scale ocean closings and openings, with compressive events on the plate margins that generated events during the Caledonian and later Variscan mountain building. Ma (mega-annum) = million years ago.

      The dominant bedrock of both these supergroups is metamorphic. In other words, the bedrock has been altered but not melted, during the growth of new minerals under the high temperatures and/or pressures generated by compressive movements and thickening of the crust. The original rocks of the Moine and Dalradian were mostly formed as sediments, mainly muds and sands but also occasionally lime-rich sediments. These sediments have now been transformed into schists (also called pelites; originally mudstones) and psammites (originally sandstones).

      Knowledge of the age of the original rocks and the age of their alteration depends on sophisticated analysis of the decay of radioactive mineral components. 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, СКАЧАТЬ