Thrust faults are important in mountain belts. They move deeper rocks over shallower ones and accommodate crustal shortening. They are also found in other settings where rocks have experienced horizontal shortening - including at the toes of landslides and other down-slope failures. But most famously thrusts are found on the outer margins of collision mountain belts where they form so-called foreland fold and thrust belts.

Teaching materials on thrusts and thrust belts.

Rob Butler’s thrust research pages.

Some historically important thrusts

"When a geologist finds… gneiss overlying gently inclined sheets of fossiliferous quartzite, shale and limestone, he may be excused if he begins to wonder whether he himself is not really standing on his head"
Archibald Geikie, 1884.

There is a certain amount of controversy surrounding the scientific history of thrusts. What is clear is that although arrays of small reverse faults had been described from many different parts of the world (notably in Belgian coal mines), the key recognition of large-scale sub-horizontal thrusts happened in the early 1880s. But was it in the Alps or NW Scotland? (Indeed other contenders include examples from North America and Scandinavia).

View of the Glarus Thrust at Martin's Joch.Contender number one is what is now known as the Glarus Thrust, in the Swiss Alps. The basic geology was discovered and strongly debated during the 19th century, famously leading to Arnold Escher (1807-1872) and his student Albert Heim (1849-1937) to propose a strange "double fold" structure. The contrasting double-fold of Escher and Heim.Heim’s clear descriptions in his classic 1878 text on mountain building led, in 1884, to the re-interpretation of the Glarus structure as a single, southward-derived thrust sheet by Marcel Bertrand (1847-1907) - although Bertrand had never visited the area. So although Escher and Heim had got the idea of large-scale sub-horizontal emplacement of rocks into the public arena, the appropriate interpretation of Glarus didn’t appear until 1884. Indeed, given Bertrand’s outsider status, the concept of the Glarus thrust wasn’t really accepted until the 1890s.

The classic view of the Glencoul thrust on the Aird da Loch peninsula.A similar type of chronology applies to the other pioneering area for thrust tectonics - the NW Highlands of Scotland. In the mid 18 th century there was a debate between Roderick Impey Murchison (and his student, Archibald Geikie) and James Nicol as to the stratigraphic continuity in the region. Murchison and Geikie argued for a simple stratigraphic succession while Nicol argued for fault disruption. However, the key advances were made by independently by two (arguably three) teams. First into print in 1883 was Charles Callaway (1838-1915) who provided a set of descriptions including what is now known as the Glencoul Thrust. Here metamorphic basement has been carried onto Cambrian sediments. The classic view of the Arnabol thrust.Next came Charles Lapworth (who established a similar relationship on the Scottish north coast at Ben Arnaboll. Lapworth (1842-1920) is doubly important for recognizing that the shearing along the thrusts had altered the rocks - coining the term mylonite. But the most complete descriptions come from the Geological Survey, especially the two icons of Scottish geology, Ben Peach (1842-1926) and John Horne (1848-1928). They named the major structures of the region, mapped them out, providing the first coherent description of a thrust system. We now call it the Moine Thrust Belt. Geikie got in on the act, coining the term "thrust" in a Nature paper in 1884.

In terms of straight publications, the thrust examples from NW Scotland were the first to appear with "modern" interpretation and, critically, be accepted by the local geologists. But the picture is more complex. Clearly research in the Alps was highly inspirational - especially Heim’s Untersuchungen uber den Mechanismus der Gebirgbildung (1878) - and the Scottish researchers were familiar with this work. Indeed they borrowed terms from it and other texts. So if Escher and Heim were the pioneers of sub-horizontal transport (by folding), Callaway, Lapworth, Peach and Horne made the concept of thrust tectonics work.

"By a system of reversed faults, a group of strata is made to cover a great breadth of ground and actually to overlie higher m embers of the same series. The most extraordinary dislocations, however, are those to which for distinction we have given the name of Thrust-planes. They are strictly reversed faults, but with so low a hade that the rocks on their upthrown side have been, as it were, pushed horizontally forward."
Archibald Geikie 1884, Nature.

Both Glarus and the NW Highlands are UNESCO Geoparks.

For further information on the discovery of the Glarus thrust try the classic book by Sir Edward Bailey (1935!) Tectonic Essays, manly Alpine.

The story of the Moine Thrust Belt is superbly covered by David Oldroyd’s (1990) The Highlands Controversy.

Additionally there’s a lot of information, including Peach and Horne’s diagrams, on the Assynt’s Geology website.

Rob Butler.