In
geology
Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ear ...
, a nappe or thrust sheet is a large sheetlike body of
rock
Rock most often refers to:
* Rock (geology), a naturally occurring solid aggregate of minerals or mineraloids
* Rock music, a genre of popular music
Rock or Rocks may also refer to:
Places United Kingdom
* Rock, Caerphilly, a location in Wales ...
that has been moved more than or above a
thrust fault
A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks.
Thrust geometry and nomenclature
Reverse faults
A thrust fault is a type of reverse fault that has a dip of 45 degrees or less.
If ...
from its original position. Nappes form in compressional tectonic settings like
continental collision
In geology, continental collision is a phenomenon of plate tectonics that occurs at convergent boundaries. Continental collision is a variation on the fundamental process of subduction, whereby the subduction zone is destroyed, mountains prod ...
zones or on the overriding plate in active
subduction
Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...
zones. Nappes form when a mass of rock is forced (or
"thrust") over another rock mass, typically on a low angle fault plane. The resulting structure may include large-scale recumbent
folds, shearing along the fault plane,
[Twiss, Robert J. and Eldridge M. Moores, ''Structural Geology,'' W. H. Freeman, 1992, p. 236 ] imbricate thrust stacks,
fensters and
klippe
350px, Schematic overview of a thrust system. The shaded material is called a window_(geology).html"_;"title="nappe._The_erosional_hole_is_called_a__window_(geology)">window_or_fenster._The_klippe_is_the_isolated_block_of_the_nappe_overlying_aut ...
s.
The term stems from the French word for ''
tablecloth
A tablecloth is a cloth used to cover a table. Some are mainly ornamental coverings, which may also help protect the table from scratches and stains. Other tablecloths are designed to be spread on a dining table before laying out tableware and ...
'' in allusion to a rumpled tablecloth being pushed across a table.
[
]
History
Nappes or nappe belts are a major feature of the Europe
Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...
an Alps
The Alps () ; german: Alpen ; it, Alpi ; rm, Alps ; sl, Alpe . are the highest and most extensive mountain range system that lies entirely in Europe, stretching approximately across seven Alpine countries (from west to east): France, Sw ...
, Dinarides
The Dinaric Alps (), also Dinarides, are a mountain range in Southern and Southcentral Europe, separating the continental Balkan Peninsula from the Adriatic Sea. They stretch from Italy in the northwest through Slovenia, Croatia, Bosnia and Herz ...
, Carpathians
The Carpathian Mountains or Carpathians () are a range of mountains forming an arc across Central Europe. Roughly long, it is the third-longest European mountain range after the Ural Mountains, Urals at and the Scandinavian Mountains at . The ...
and Balkans
The Balkans ( ), also known as the Balkan Peninsula, is a geographical area in southeastern Europe with various geographical and historical definitions. The region takes its name from the Balkan Mountains that stretch throughout the who ...
. Since the 19th century many geologists have uncovered areas with large-scale overthrusts. Some of these were substantiated with paleontological
Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (geology), epoch (roughly 11,700 years before present). It includes t ...
evidence. The concept was developed by Marcel Alexandre Bertrand
Marcel Alexandre Bertrand (2 July 1847 – 13 February 1907) was a French geologist born in Paris. He was the son of mathematician Joseph Louis François Bertrand (1822–1900), and son-in-law to physicist Éleuthère Mascart (1837-1908).
He st ...
, who unraveled the complex 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 k ...
history of the Alps and identified the feature as ''nappe de charriage''. He reinterpreted earlier studies by Arnold Escher von der Linth
Arnold Escher von der Linth (8 June 1807 in Zürich12 July 1872) was a Swiss geologist, the son of Hans Conrad Escher von der Linth (1767–1823).
He made the first ascent of the Lauteraarhorn on 8 August 1842 together with Pierre Jean Édouard D ...
and Albert Heim
Albert Heim (12 April 184931 August 1937) was a Swiss geologist, noted for his three-volume ''Geologie der Schweiz''.
Born in Zürich, he was educated at Zürich and Berlin universities. Very early in life he became interested in the physical fe ...
in the Glarus Alps
The Glarus Alps (german: Glarner Alpen) are a mountain range in central Switzerland. They are bordered by the Uri Alps and the Schwyz Alps to the west, the Lepontine Alps to the south, the Appenzell Alps to the northeast. The eastern part of th ...
. His work in Switzerland
). Swiss law does not designate a ''capital'' as such, but the federal parliament and government are installed in Bern, while other federal institutions, such as the federal courts, are in other cities (Bellinzona, Lausanne, Luzern, Neuchâtel ...
influenced Escher and Maurice Lugeon
Maurice Lugeon FRS(For) HFRSE FGS (10 July 1870 – 23 October 1953) was a Swiss geologist, and the pioneer of nappe tectonics. He was a pupil of Eugène Renevier. Named for Maurice Lugeon, the lugeon is a measure of transmissivity in rocks ...
. Several years later, nappe structure was investigated in northwestern Scotland
Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to the ...
by Charles Lapworth
Charles Lapworth FRS FGS (20 September 1842 – 13 March 1920) was a headteacher and an English geologist who pioneered faunal analysis using index fossils and identified the Ordovician period.
Biography
Charles Lapworth was born at Faring ...
. Lugeon later transferred the ideas of nappes to the Carpathians
The Carpathian Mountains or Carpathians () are a range of mountains forming an arc across Central Europe. Roughly long, it is the third-longest European mountain range after the Ural Mountains, Urals at and the Scandinavian Mountains at . The ...
.
Structure
''Nappe'' can be qualified in a number of ways to indicate various features of a formation. The frontal part in the direction of movement, is called ''the leading edge'' of a nappe; numerous folds and secondary thrusts and duplexes
A duplex house plan has two living units attached to each other, either next to each other as townhouses, condominiums or above each other like apartments. By contrast, a building comprising two attached units on two distinct properties is ...
are common features here and are sometimes called ''digitations''. The surface of a thrust fault
A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks.
Thrust geometry and nomenclature
Reverse faults
A thrust fault is a type of reverse fault that has a dip of 45 degrees or less.
If ...
which caused movement of a nappe is called a '' decollement'', ''detachment plane'' or sole of thrust. The ''root area'' is an area where the nappe is completely separated from its substratum. It is often compressed and reduced, even underthrust below the surrounding tectonic units, resulting in a specific structure called a '' suture''. A nappe whose root area is unknown, is called a ''rootless nappe''.
Areas with a nappe structure often contain two types of geological features:
* A ''nappe outlier'' or ''klippe
350px, Schematic overview of a thrust system. The shaded material is called a window_(geology).html"_;"title="nappe._The_erosional_hole_is_called_a__window_(geology)">window_or_fenster._The_klippe_is_the_isolated_block_of_the_nappe_overlying_aut ...
'' is a small area isolated from the main body of the nappe by erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is distin ...
that lies on the autochthonous base; the summit of Veľký Rozsutec in the Western Carpathians
The Western Carpathians are a mountain range and geomorphological province that forms the western part of the Carpathian Mountains.
The mountain belt stretches from the Low Beskids range of the Eastern Carpathians along the border of Poland wi ...
is a typical example.
* A ''fault inlier'', ''fenster'', or window
A window is an opening in a wall, door, roof, or vehicle that allows the exchange of light and may also allow the passage of sound and sometimes air. Modern windows are usually glazed or covered in some other transparent or translucent materia ...
is an area of the autochthonous basement uncovered by erosion, but entirely surrounded by the body of the nappe; the Hohe Tauern window
The Hohe Tauern window is a geological structure in the Austrian Central Eastern Alps. It is a window (in German ''fenster'') in the Austroalpine nappes where high-grade metamorphic rocks of the underlying Penninic nappes crop out. The struct ...
in the Alps is a typical example.
Classification
According to petrographical composition, two basic types of nappes are known:
* Basement nappes are composed generally of crystalline basement
In geology, basement and crystalline basement are crystalline rocks lying above the mantle and beneath all other rocks and sediments. They are sometimes exposed at the surface, but often they are buried under miles of rock and sediment. The baseme ...
rocks (but may contain basement sedimentary cover), forming so-called thick-skinned style. Nappes of this type usually reach a large thickness and form independent superunits such as Penninic nappes
The Penninic nappes or the Penninicum, commonly abbreviated as Penninic, are one of three nappe stacks and geological zones in which the Alps can be divided. In the western Alps the Penninic nappes are more obviously present than in the eastern ...
.
* Cover nappe
Cover or covers may refer to:
Packaging
* Another name for a lid
* Cover (philately), generic term for envelope or package
* Album cover, the front of the packaging
* Book cover or magazine cover
** Book design
** Back cover copy, part of c ...
s or so called ''superficial nappes'' are composed generally of sedimentary rock
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
s that form the upper part of crust, forming so-called thin-skinned style. Therefore, nappes of this type form smaller units, such as the Hallstatt nappe
Hallstatt ( , , ) is a small town in the district of Gmunden, in the Austrian state of Upper Austria. Situated between the southwestern shore of Hallstätter See and the steep slopes of the Dachstein massif, the town lies in the Salzkammergut ...
in the Austroalpine nappes of the Alps.
Mechanisms of emplacement
Nappes are generally considered as compressional structures, however some exceptions could be found especially among the gravitational slides along low angle faults. Gravitational forces could even be important in certain cases during emplacement of compressional thrusts. The movement of huge masses of rock may be influenced by several forces, forces that may act together or sequentially. These forces frequently result in high temperature and pressure metamorphism and strong deformation of nappe rocks.
At shallower depths, low pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
s and temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied o ...
s can't cause the plastic
Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adaptab ...
and viscous
The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water.
Viscosity quantifies the inter ...
behavior of solid rock
Rock most often refers to:
* Rock (geology), a naturally occurring solid aggregate of minerals or mineraloids
* Rock music, a genre of popular music
Rock or Rocks may also refer to:
Places United Kingdom
* Rock, Caerphilly, a location in Wales ...
necessary to move along low angle faults. It is considered that such characteristics may be achieved at significantly less extreme conditions in the clay
Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4).
Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...
ey rocks or evaporites
An evaporite () is a water-soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as ocean ...
, which can then act as tectonic lubricant
A lubricant (sometimes shortened to lube) is a substance that helps to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move. It may also have the function of transmitting forces, t ...
s. The process, which significantly reduces the friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction:
*Dry friction is a force that opposes the relative lateral motion of t ...
al resistance, is the fluid overpressure, which acts against the normal pressure, thereby reducing high lithostatic pressures and allowing fracturation, cataclasis and formation of tectonic breccia or fault gouge
Fault gouge is a type of fault rock best defined by its grain size. It is found as incohesive fault rock (rock which can be broken into its component granules at the present outcrop, only aided with fingers/pen-knife), with less than 30% clasts ...
that could act as a decollement plane. Evaporites are also often related the decollement and thrust planes. Evaporites are strongly prone to shear deformation and therefore preferred planes of detachment.
Behavior of thrust
Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that syst ...
sheets is currently explained on the model of the orogenic wedge
Orogeny is a mountain building process. An orogeny is an event that takes place at a convergent plate margin when plate motion compresses the margin. An ''orogenic belt'' or ''orogen'' develops as the compressed plate crumples and is uplifted t ...
, which is dependent on the internal wedge taper θ. Gravitational sliding is movement generated by the movement down an inclined plane under the action of gravity
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
. Gravitational spreading, possibly accompanied by an initial phase of diapirism
A diapir (; , ) is a type of igneous intrusion in which a more mobile and ductily deformable material is forced into brittle overlying rocks. Depending on the tectonic environment, diapirs can range from idealized mushroom-shaped Rayleigh–T ...
, is generated by large heat flow that causes detachment in a hinterland.[Price, N.J., McClay, K.R., 1981]
''Introduction.''
p. 1-5 in Price, N.J., McClay, K.R. (Eds.), ''Thrust and Nappe Tectonics.'' Geological Society, Special Publications vol. 9, London, 528 p. Other mechanisms, such as push from behind, action of tangential compressive forces, and shortening of the basement, are essentially variations of the previous mechanisms.
References
{{Authority control
Plate tectonics
Structural geology