The Sgurr Beag Thrust is an important

tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents ...

structure within the

Neoproterozoic The Neoproterozoic Era is the unit of geologic time from 1 billion to 538.8 million years ago. It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran periods. It is ...

metasedimentary rock In geology, metasedimentary rock is a type of metamorphic rock. Such a rock was first formed through the deposition and solidification of sediment Sediment is a naturally occurring material that is broken down by processes of weathering and ...

sequences of the

Scottish Highlands The Highlands ( sco, the Hielands; gd, a’ Ghàidhealtachd , 'the place of the Gaels') is a historical region of Scotland. Culturally, the Highlands and the Lowlands diverged from the Late Middle Ages into the modern period, when Lowland ...

. The

thrust Thrust is a reaction force In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can al ...

, or similar structures correlated with it, form the boundary between rocks of the Glennfinnan Group and the underlying Morar Group. It divides the Wester Ross Supergroup from the

Loch Ness Supergroup The Loch Ness Supergroup is one of the subdivisions of the Neoproterozoic sequence of sedimentary rocks (or their metamorphic equivalents) in the Scottish Highlands. It is found everywhere in tectonic contact above the older Wester Ross Supergrou ...

. The history of this structure remains poorly understood although it is thought to be at least partly of Caledonian age.

Extent

The Sgurr Beag Thrust was identified as a zone of high

strain Strain may refer to: Science and technology * Strain (biology), variants of plants, viruses or bacteria; or an inbred animal used for experimental purposes * Strain (chemistry), a chemical stress of a molecule * Strain (injury), an injury to a mu ...

in the western part of the Highlands and was originally called the "Sgurr Beag Slide". It is currently interpreted to extend from the

Morvern Morvern, historically also spelt Morven, is a peninsula and traditional district in the Highlands, on the west coast of Scotland. It lies south of the districts of Ardgour and Sunart, and is bounded on the north by Loch Sunart and Glen Tarbe ...

peninsula in the southwest (with a possible extension onto the Isle of Mull), to the

Dornoch Firth The Dornoch Firth ( gd, Caolas Dhòrnaich, ) is a firth on the east coast of Highland, in northern Scotland. It forms part of the boundary between Ross and Cromarty, to the south, and Sutherland, to the north. The firth is designated as a nationa ...

where it disappears beneath the

Old Red Sandstone The Old Red Sandstone is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the northeastern seaboard of North America. It also ext ...

. It is often marked by the presence of

inlier An inlier is an area of older rocks surrounded by younger rocks. Inliers are typically formed by the erosion of overlying younger rocks to reveal a limited exposure of the older underlying rocks. Faulting or folding may also contribute to the obs ...

s of Lewisian

basement A basement or cellar is one or more Storey, floors of a building that are completely or partly below the storey, ground floor. It generally is used as a utility space for a building, where such items as the Furnace (house heating), furnace, ...

, which are found in its hanging-wall (structurally above the fault plane). In eastern Sutherland there are several thrust structures that could correlate with the Sgurr Beag Thrust, particularly the Swordly Thrust or the Skinsdale Thrust.

Age

The Loch Ness Supergroup has a depositional age range extending to about 850 Ma (million years ago).There are two main tectonic/metamorphoic events that have affected this sequence since its deposition, the Knoydartian Orogeny (a series of events 820–725 Ma) and the Caledonian Orogeny (consisting of the

Grampian Orogeny The Caledonian orogeny was a mountain-building era recorded in the northern parts of the British Isles, the Scandinavian Mountains, Svalbard, eastern Greenland and parts of north-central Europe. The Caledonian orogeny encompasses events that occ ...

and the Scandian Phase, mid

Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. ...

early Devonian The Early Devonian is the first of three epochs comprising the Devonian period, corresponding to the Lower Devonian series. It lasted from and began with the Lochkovian Stage , which was followed by the Pragian from and then by the Emsian, whi ...

). The age of peak metamorphism within the rocks above the thrust is dated to 840–720 Ma, the timing of the Knoydartian event(s). Some mineral growth is of Scandian age, but it remains unclear as to when the Sgurr Beag Thrust moved, although there is evidence of two phases of movement, possibly of Knoydartian and Scandian age. A contrast between the pre-Caledonian metamorphic histories of the Morar Group and the Loch Ness Group above the Sgurr Beag Thrust suggest that the final large-scale movement on the thrust was of Caledonian age.

References

{{Reflist Geology of Scotland Thrust faults