The Highland Boundary Fault of Scotland

Scott Jones

Spring 2008

Global Tectonics   *    Emporia State University


Introduction Tectonic History Geology Conclusion References

Image 1 (left)-Scottish highlands in Autumn (permission to use; Ross, 2008).

Image 2 (right)-A view of the Midland Valley taken from the summit of Distinkhorn where one of the few granite intrusions of Midland Valley is exposed (copyrighted image, permission to use; Brown, 2008).

Introduction

In todayís society, plate tectonics is studied throughout the world, but many of the earliest studies focused on the tectonic past of Scotland. A journey from southern Scotland to the Central Highlands reveals a drastic change in landscape marked by the Highland Boundary Fault. The landscape south of the fault consists of lowlands and rolling hills, but the landscape north of the fault is mountainous and rugged. This puzzled early geologists, and it took them some time to realize that Scotland was created by terranes that had accreted to one another. Those terranes were separated by faults, and the Highland Boundary Fault marked the largest landscape difference of the four major faults found in Scotland (see Figure 1).

Figure 1-Four major faults of Scotland (permission to use; Rogie, 2004).

The landscape of Scotland north of the Highland Boundary Fault is categorized as highlands and islands, but in between the Highland Boundary Fault and the Great Glen Fault the area is named the Grampian or Central Highlands. The land south of the fault, between the Highland Boundary Fault and Southern Upland Fault, is called the Midland Valley or central lowlands. The southern most land in Scotland is referred to as the Southern Upland. This webpage examines the tectonic history and geology of the Highland Boundary Fault Zone.

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Tectonic History

Scotland is a collection of terranes that were formed at different times and locations. The Highland Boundary Fault was formed approximately 500 million years during the Caledonian Orogeny, and it marks the boundary between the Midland Valley region and the Highland region. Prior to the Caledonian Orogeny, the crustal foundation of northern Scotland made up part of the southern edge of Laurentia. During that time, Scotlandís Midland Valley crustal foundation was a volcanic island chain in the Iapetus Ocean off the coast of Laurentia. Once the Caledonian Orogeny began, crustal foundations of England and Scandinavia moved closer to the Midland Valley and northern Scotland. The foundations finally converged, and the Iapetus Ocean was closed. The pressure created by the collision of the terranes, led to major folding events in the Highlands of Scotland.

By the start of the Devonian, 400 million years ago, the tectonic forces that had brought Scotlandís foundations together had ceased (Scottish Geology, 2008). Major faults arose where the foundations were sutured. The Highland Boundary Fault marked the boundary of Northern Scotland and the Central Highlands to the Midland Valley. The southern edge of the Midland Valley was marked by the Southern Upland Fault. The Midland Valley then began to rift as tensional forces pulled the Highlands to the north and the Uplands to the south. The Midland Valley became a graben, but a view of the basement rocks presented a different picture. Those crustal rocks showed a horst-like formation caused by compressional forces (Craig, 2003). The reason for this horst found in the deeper rocks may be due to the specific location of the rocks studied in the Midland Valley. As a whole the area between the Highland Boundary Fault and the Southern Upland Fault is a graben or a valley; therefore, it is called Midland Valley.

The landmass containing Scotland continued to moved north during the Devonian, Carboniferous, and Permian. It wasnít until Jurassic times that America and Europe split, which pushed Scotland further north. At the start of the Tertiary, 60 million years ago, the formation of the North Atlantic caused major fractures in Scotlandís crust. This led to massive volcanic eruptions on the western edge of Scotland. During the Quaternary ice ages, Midland Valley experienced glacial erosion and deposition that deepened valleys and moulded hills and streams (Gordon et al., 2002).

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Geology

Scotland is made up of many different types of rocks formed at different time periods and locations. The areas located on opposite sides of the Highland Boundary Fault are excellent sites to locate and study the contrasting rocks of Scotland.

Highlands

The majority of rocks that form the Central Highlands or Grampian Highlands are 800 to 600 million years old and layers of them are 25 km thick. They are named the Dalradian Supergroup. They initially were laid down as deep marine deposits. During the sedimentation process an ice age occurred on land that led to glacial debris being deposited along with the marine deposits. These rocks were eventually metmorphisized during the Caledonian Orogeny. The majority of them are now schists, phyllites, and slates, and with the help of erosion, some of these rocks are now exposed on the surface. In some places there is an intrusion of granite that formed deep within the crust where the pressure and heat was greatest during the collision. Figure 2 shows the major rock types found in Grampian Highlands north of the fault.

Figure 2-Major rock types of the Grabian Highlands (copyrighted image, permission to use; Scottish Geology, 2008).

Along the Highland Boundary Fault there is also an exposed suite of rocks known as the Highland Border Complex. Pillow lavas, shales, and jasper are common in some areas where this complex is exposed, but other lithologies are present at different sites. This complex represents marine sediments that were obducted during the Caledonian Orogeny, and they then were overthrusted by the Dalradian rock during the Silurian (Scottish Geology, 2008).

Midland Valley

The Midland Valley is made up of rocks that range from 470 to 350 million years old. During this time period lavas were erupted to form the foundation of the Midland Valley. These lava rocks are able to be found alongside the Highland Boundary Fault, yet many of them were covered by eroded sand, silt, and mud from the Highland areas during the Devonian and Carboniferous times (400-300 million years ago). It was during these times that Scotland was located near the equator. The climate was hot and dry, but there were seasonal rainfalls. The rain led to floods and rivers that laid down red sandstone, and this time period was marked as the Old Red Sandstone. The tropical climate also gave rise to reefs and swampy forests that led to coal formations. Volcanoes, such as Arthurís Seat in Edinburgh, were also major features of the Midland Valley. Those volcanoes produced resistant rock types that have withstood erosion better than some of the sedimentary rocks later deposited. Those volcanic rocks, basalt and granite, along with a few igneous intrusions have led to the formation of some of the hills found in Midland Valley (Gordon et al., 2002).

Figure 3-Major rock types of the Midland Valley (copyrighted image, permission to use;Scottish Geology, 2008).

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Conclusion

The Highland Boundary Fault is a scar deep in the Earth that has been around hundreds of millions of years, yet the processes on Earth have allowed this scar to be visible at many places including Loch Lomond (Image 3) and Stonehaven (Image 4). The fault has cut right through the center of these two locations making them places of geologic wonder. The fault has also created breathtaking views of landscapes so different from one another. It poses no significant earthquake threat becasue on average, Britain has only about seven earthquakes a year that exceed a magnitude of 2.5 (McKerrow, 1983). Overall, the area of the Highland Boundary Fault has given scientists a map to the past, and that map should be around for many more millions of years.

Image 3-Loch Lomond (copyrighted image, permission to use; Grewe, 2004)

Image 4-Craigeven Bay in Stonehaven, Scotland. Center of image and sea stacks mark the position of the Highland Boundary Fault bounded on the right by Dalradian rocks and on the left there is the Highland Border Complex. (permission to use; Scottish Geology: Mitchell 2008)

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References

Brown, G. 2008. Geograph.
http://www.geograph.org.uk/photo/114122 [retrieved on 23, Apr. 2008].

Craig, G.Y. 1983. Geology of Scotland, 2nd edition. John Wiley & Sons, New York, 1-22, 77-104.

Grewe, A. 2004. The Armin Grewe homepage. http://www.armin-grewe.com/holiday/scotland2004winter/vorlich.htm [retrieved on 1, May 2008).

Gordon, J.E., Lees, R.G., Leys, K.F., MacFadyen, C.J., Puri, G., Threadgould, R., and Kirkbride, V. 2002. Natural heritage zones: Earth sciences. http://www.snh.org.uk/futures/Data/pdfdocs/EARTH_Heritage.pdf [retrieved on 23, Apr. 2008].

McKerrow, W.S. 1983. The chronology of Caledonian folding in the British Isles. Geology 48, p. 1905-1913.

Rogie, A. last update 6/17/2004. Geology of southwest Scotland. http://www.virtual-geology.info/vft/2004-andy/page2.htm [retrieved on 22, Apr. 2008].

Ross, R. unknown. Holiday breaks in Scotland and Wester Ross.
http://www.camusnagaul.com/scottishhighlandsautumnbreak.htm [retrieved on 23, Apr. 2008].

Scottish Geology. last update 3/7/2008. http://www.scottishgeology.com/ [retrieved on 23, Apr. 2008].

Woodcock, N. 1994. Geology and environment in Britain and Ireland. UCL Press, London, 1-157.
This webpage was designed for Global Tectonics
Instructor: Dr. James S. Aber of Emporia State University
For questions or comments contact Scott Jones
Created on April 25, 2008