Highland Boundary Fault
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The Highland Boundary Fault is a major
fault zone In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...
that traverses Scotland from Arran and
Helensburgh Helensburgh (; gd, Baile Eilidh) is an affluent coastal town on the north side of the Firth of Clyde in Scotland, situated at the mouth of the Gareloch. Historically in Dunbartonshire, it became part of Argyll and Bute following local governm ...
on the west coast to
Stonehaven Stonehaven ( , ) is a town in Scotland. It lies on Scotland's northeast coast and had a population of 11,602 at the 2011 Census. After the demise of the town of Kincardine, which was gradually abandoned after the destruction of its royal cast ...
in the east. It separates two different geological
terrane In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its own ...
s which give rise to two distinct physiographic terrains: the
Highlands Highland is a broad term for areas of higher elevation, such as a mountain range or mountainous plateau. Highland, Highlands, or The Highlands, may also refer to: Places Albania * Dukagjin Highlands Armenia * Armenian Highlands Australia *Sou ...
and the
Lowlands Upland and lowland are conditional descriptions of a plain based on elevation above sea level. In studies of the ecology of freshwater rivers, habitats are classified as upland or lowland. Definitions Upland and lowland are portions of p ...
, and in most places it is recognisable as a change in topography. Where rivers cross the fault, they often pass through gorges, and the associated waterfalls can be a barrier to
salmon Salmon () is the common name for several list of commercially important fish species, commercially important species of euryhaline ray-finned fish from the family (biology), family Salmonidae, which are native to tributary, tributaries of the ...
migration. The fault is believed to have formed in conjunction with the Strathmore
syncline In structural geology, a syncline is a fold with younger layers closer to the center of the structure, whereas an anticline is the inverse of a syncline. A synclinorium (plural synclinoriums or synclinoria) is a large syncline with superimpose ...
to the south-east during the Acadian orogeny in a transpressive regime that caused the uplift of the Grampian block and a small sinistral movement on the Highland Boundary Fault.


Discovery

One of the earliest and most prominent references to the Highland Boundary Fault was by George Barrow in 1912 ʻOn the Geology of Lower Dee-side and the Southern Highland Borderʼ, which highlights the nature of the rocks accompanying the Highland Border and describes the mineral zones associated with metamorphism. In the same publication, Barrow also outlines the ʻHighland Faultʼ and the areas where he believes there are planes of overthrust. Barrowʼs description of the structural nature of the rocks along the Highland Border suggests that rocks along both ends of the fault plane are indistinguishable from one another, with no
breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of di ...
tion.


Extent of fault

Aligned southwest to northeast from Lochranza on Arran, the Highland Boundary Fault bisects Bute and crosses the southeastern parts of the
Cowal Cowal ( gd, Còmhghall) is a peninsula in Argyll and Bute, in the west of Scotland, that extends into the Firth of Clyde. The northern part of the peninsula is covered by the Argyll Forest Park managed by Forestry and Land Scotland. The Arrochar ...
and
Rosneath Rosneath (''Ros Neimhidh'' in Gaelic) is a village in Argyll and Bute, Scotland. It sits on the western shore of the Gare Loch, northwest of the tip of the Rosneath Peninsula. It is about by road from the village of Kilcreggan, which is sited ...
peninsulas, as it passes up the
Firth of Clyde The Firth of Clyde is the mouth of the River Clyde. It is located on the west coast of Scotland and constitutes the deepest coastal waters in the British Isles (it is 164 metres deep at its deepest). The firth is sheltered from the Atlantic ...
. It comes ashore near Helensburgh, then continues through
Loch Lomond Loch Lomond (; gd, Loch Laomainn - 'Lake of the Elms'Richens, R. J. (1984) ''Elm'', Cambridge University Press.) is a freshwater Scottish loch which crosses the Highland Boundary Fault, often considered the boundary between the lowlands of Ce ...
. The loch islands of
Inchmurrin Inchmurrin ( gd, Innis Mhearain) is an island in Loch Lomond in Scotland. It is the largest fresh water island in the British Isles. Geography and geology Inchmurrin is the largest and most southerly of the islands in Loch Lomond. It reache ...
,
Creinch Creinch ( gd, Craobh-Innis, Tree Island) is an island on the Highland Boundary Fault in Loch Lomond. History Formerly Inchcroin (not to be confused with Inchcruin), Creinch lies a little north of Inchmurrin.Wilson, Rev. John ''The Gazetteer of ...
,
Torrinch Torrinch or Inchtore ( gd, Torr-Innis) is a wooded island in Loch Lomond in Scotland. The name Torremach is also recorded for it.Wilson, Rev. John ''The Gazetteer of Scotland'' (Edinburgh, 1882) Published by W. & A.K. Johnstone Geography It is ...
, and
Inchcailloch Inchcailloch ( gd, Innis na Cailleach) is an islet on Loch Lomond in Scotland. It is at its highest point. It is also known to some as Inchebroida. The name Inchcailloch means "Isle of the old woman" or "Isle of the Cowled (Hooded) Woman" i ...
all lie on the Highland Boundary Fault. From Loch Lomond the Highland Boundary Fault continues to
Aberfoyle Aberfoyle may refer to: *Aberfoyle, County Londonderry, Northern Ireland *Aberfoyle, Stirling, Scotland * Aberfoyle, Ontario, Canada *Aberfoyle, Texas Aberfoyle is an Unincorporated area, unincorporated community in Hunt County, Texas, Hunt Count ...
, then Callander,
Comrie Comrie may refer to: Places *Comrie (crater), a lunar crater *Comrie, Fife, a village in Fife, Scotland *Comrie, Perth and Kinross, a village and parish in Strathearn, Scotland People with the surname *Aaron Comrie (born 1997), Scottish footballer ...
and
Crieff Crieff (; gd, Craoibh, meaning "tree") is a Scottish market town in Perth and Kinross on the A85 road between Perth and Crianlarich, and the A822 between Greenloaning and Aberfeldy. The A822 joins the A823 to Dunfermline. Crieff has become ...
. It then forms the northern boundary of Strathmore and reaches the North Sea immediately north of Stonehaven near the ruined Chapel of St. Mary and St. Nathalan. Aeromagnetic maps of Great Britain and Northern Ireland show that the Highland Boundary Fault can be traced from
Ireland Ireland ( ; ga, Éire ; Ulster Scots dialect, Ulster-Scots: ) is an island in the Atlantic Ocean, North Atlantic Ocean, in Northwestern Europe, north-western Europe. It is separated from Great Britain to its east by the North Channel (Grea ...
to the region of
Greenock Greenock (; sco, Greenock; gd, Grianaig, ) is a town and administrative centre in the Inverclyde council areas of Scotland, council area in Scotland, United Kingdom and a former burgh of barony, burgh within the Counties of Scotland, historic ...
. In these areas, the Highland Boundary Fault is seen to be dividing a northerly low area from a southerly high area. The fault is often considered a terrane boundary: the Midland Valley terrane lies to the south whilst the Southern Highlands or Grampian terrane lies to the north In 1970, Hall and Dagley identified the Highland Boundary Fault as coincident with a regional magnetic feature dividing a string of negative anomalies in the north from positive ones in the south. On discovering this, Hall and Dagley concluded that the observed trend, which followed the length of the
Dalradian The Dalradian Supergroup (informally and traditionally the Dalradian) is a stratigraphic unit (a sequence of rock strata) in the lithostratigraphy of the Grampian Highlands of Scotland and in the north and west of Ireland. The diverse assembla ...
trough, transition from positive to negative anomalies. This linear feature of magnetic anomalies has since been referred to as the Fair Head–Clew Bay line.


Features


Tectonic controversy

At present, it is believed that the Highland Boundary Fault was active during two main orogenic events associated with the
Caledonian 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 ...
: the Grampian orogeny in the
Early 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. ...
and the Acadian orogeny in the
Middle Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, w ...
. The fault allowed the Midland Valley to descend as a major rift by up to and there was subsequent vertical movement. This earlier vertical movement was later replaced by a horizontal shear. A complementary fault, the
Southern Uplands Fault The Southern Uplands Fault (or occasionally Southern Upland Fault) is a fault in Scotland that runs from Girvan (or more specifically from the Rhins of Galloway) to Dunbar on the East coast. It marks the southern boundary of the Scottish Midland ...
, forms the southern boundary for the Central Lowlands. The age of the Highland Boundary Fault has been inferred to be between Ordovician to middle Devonian and through several generations it has been interpreted as a
graben In geology, a graben () is a depressed block of the crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German, meaning 'ditch' or 'trench'. The word was first used in the geologic contex ...
-bounding normal fault, a major sinistral
strike-slip fault In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...
, a northwest-dipping
reverse fault In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectoni ...
or
terrane In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its own ...
boundary. The reason the precise nature of the fault is still unknown is because there is little evidence of a continuous fault plane on the surface. More recently, seismic activities marking the fault line have been analysed to show that the 2003 Aberfoyle earthquake had a
hypocentre In seismology, a hypocenter or hypocentre () is the point of origin of an earthquake or a subsurface nuclear explosion. A synonym is the focus of an earthquake. Earthquakes An earthquake's hypocenter is the position where the strain energy s ...
at depth and was caused by an oblique sinistral strike-slip fault with normal movement. The fault plane was estimated to be dipping at 65° NW.


Associated lithology

To the north and west of the Highland Boundary Fault lie hard Precambrian and Cambrian metamorphic rocks: marine deposits metamorphosed to
schist Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes o ...
s,
phyllite Phyllite ( ) is a type of foliated metamorphic rock created from slate that is further metamorphosed so that very fine grained white mica achieves a preferred orientation.Stephen Marshak ''Essentials of Geology'', 3rd ed. It is primarily compo ...
s and
slate Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. It is the finest grained foliated metamorphic rock. ...
s, namely the Dalradian Supergroup and the Highland Border
Ophiolite An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks. The Greek word ὄφις, ''ophis'' (''snake'') is found i ...
suite. To the south and east are
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 exte ...
conglomerates and sandstones: softer, sedimentary rocks of the Devonian and Carboniferous periods. Between these areas lie the quite different rocks of the Highland Border Complex (at one time called the Highland Boundary Complex), a weakly metamorphosed sedimentary sequence of sandstones, lavas, limestones, mudstones and conglomerates. These make up a zone which is found discontinuously along the line of the fault and which is up to in width.


Dalradian Supergroup

The Dalradian Supergroup consists of
metasediment 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 e ...
ary rocks which underwent polyphase deformation and metamorphism during the Precambrian and early Paleozoic. The oldest Dalradian rocks (the
Grampian Group The Grampian Group is an estimated 9,000 m thick sequence of metamorphosed Neoproterozoic sedimentary rocks that outcrop across the Central Highlands of Scotland, east of the Great Glen. It forms a part of the Dalradian Supergroup. The sequence t ...
) were deformed and metamorphosed around 750 Ma. The deposition of younger Dalradian sediments continued until 590 Ma, when the sediments underwent transformation to the
greenschist Greenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism, typically and 2–10 kilobars (). Greenschists commonly have an abundance of green minerals such as chlorite ...
facies during the Proterozoic and Ordovician.


Highland Border Ophiolitic Rocks

Modeling of gravity and magnetic data along the fault has confirmed the presence of an extensive
ophiolite An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks. The Greek word ὄφις, ''ophis'' (''snake'') is found i ...
suite. The Dalradian metasedimentary rocks are overlain by an obducted
ophiolite An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks. The Greek word ὄφις, ''ophis'' (''snake'') is found i ...
that is continuous for at least several kilometres on either side of the Highland Boundary Fault. The models generated from magnetic data suggest that the ophiolite is only slightly displaced vertically by the fault.


Old Red Sandstone

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 exte ...
is a magnafacies of red beds and lacustrine deposits from the
Late Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...
to the
Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...
. The NE segment of the Highland Boundary Fault is marked by an abrupt change in the dip of the Old Red Sandstone from around 20° to near-vertical and subsequently exposes the Old Red Sandstone basement.


Displacement along fault

It is currently believed that there were two main displacement events along the Highland Boundary Fault: the Acadian, and the post-Acadian. Evidence for the Acadian displacement event is based on the geochemical study of detrital garnets in the Lower Old Red Sandstone on the Northern limb of the Strathmore Basin. These garnets were linked to those in isolated Dalradian sediments in the northwest, providing evidence for post-Early Devonian (Acadian) movement to be only few tens of kilometers. In addition, the Lintrathen
ignimbrite Ignimbrite is a type of volcanic rock, consisting of hardened tuff. Ignimbrites form from the deposits of pyroclastic flows, which are a hot suspension of particles and gases flowing rapidly from a volcano, driven by being denser than the surrou ...
, which is present at the base of the Lower Devonian sequence was traced along the fault and it was found that the displacement was both short and lateral. The post-Acadian movements are highlighted in the stratigraphy of the region. The Lower Old Red Sandstone is unconformably overlain by Upper Old Red Sandstone, where the Upper Old Red Sandstone is tilted close to the Highland Boundary Fault.


Further reading

*


References

{{Reflist, 2 Geology of Scotland Silurian geology Devonian geology Seismic faults of the United Kingdom Highlands and Islands of Scotland