Nappe

Nappe

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In geology
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...

, a nappe is a large sheetlike body of rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

 that has been moved more than 2 km (1.2 mi) or 5 km from its original position. Nappes form during continental plate collisions, when folds are sheared so much that they fold back over on themselves and break apart. The resulting structure is a large-scale recumbent fold
Fold (geology)
The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...

. The term stems from the French word for tablecloth
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 food....

.

History


Nappes or nappe belts are a major feature of the Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

an Alps
Geology of the Alps
The Alps form part of a Tertiary orogenic belt of mountain chains, called the Alpide belt, that stretches through southern Europe and Asia from the Atlantic all the way to the Himalayas. This belt of mountain chains was formed during the Alpine orogeny. A gap in these mountain chains in central...

, Carpathians and Balkans. Since the 19th century many geologists have uncovered areas with large-scale overthrusts. Some of these were substantiated with paleontological
Paleontology
Paleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...

 evidence. The concept was developed by M.A. Bertrand
Marcel Alexandre Bertrand
Marcel Alexandre Bertrand was a French geologist who was born in Paris. He was a student at the École Polytechnique, and beginning in 1869 he attended the Ecole des Mines de Paris. Beginning in 1877 he performed geological mapping studies of Provence, Jura Mountains and the Alps...

, who unraveled the complex tectonic history of the Alps and identified the feature as nappe de charriage. He reinterpreted earlier studies by Escher
Arnold Escher von der Linth
Arnold Escher von der Linth was a Swiss geologist, the son of Hans Conrad Escher von der Linth ....

 and Heim
Albert Heim
Albert Heim was a Swiss geologist, noted for his three-volume Geologie der Schweiz.Born at Zürich, he was educated at Zürich and Berlin universities. Very early in life he became interested in the physical features of the Alps, and at the age of sixteen he made a model of the Tödi group...

 in the Glarus Alps
Glarus Alps
The Glarus Alps are a mountain range in central Switzerland. They extend from the Oberalp Pass to the Klausen Pass, and are bordered by the Urner Alps to the west, the Lepontine Alps to the south and the Appenzell Alps to the northeast...

. His work in Switzerland
Switzerland
Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....

 influenced A. Escher von der Linth
Arnold Escher von der Linth
Arnold Escher von der Linth was a Swiss geologist, the son of Hans Conrad Escher von der Linth ....

 and M. Lugeon
Maurice Lugeon
Maurice Lugeon 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, determined by pressurized injection of water through a bore hole driven through the rock...

. Several years later, nappe structure was investigated in northwestern Scotland
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...

 by Ch. Lapworth
Charles Lapworth
Charles Lapworth was an English geologist.-Biography:He was born at Faringdon in Berkshire and educated as a teacher at the Culham Diocesan Training College near Abingdon, Oxfordshire. He moved to the Scottish border region, where he investigated the previously little-known fossil fauna of the area...

. Lugeon later transferred the ideas of nappes to the Carpathians.

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
Fold (geology)
The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...

 and secondary thrusts
Thrust fault
A thrust fault is a type of fault, or break in the Earth's crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger...

 and duplexes are common features here and are sometimes called digitations. The surface of a thrust fault
Thrust fault
A thrust fault is a type of fault, or break in the Earth's crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger...

 which caused movement of a nappe is called a decollement
Decollement
Décollement is a gliding plane between two rock masses. In French, "décoller" means "to detach from" or "to rip off" and was first used by geologists studying the structure of the Swiss Jura Mountains, but is also known as a detachment zone. This is a structure of strata owing to deformation,...

, 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
Suture (geology)
In structural geology, a suture is a major fault zone through an orogen or mountain range. Sutures separate terranes, tectonic units that have different plate tectonic, metamorphic and paleogeographic histories...

. 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
    Klippe
    thumb|right|350px|Schematic overview of a thrust system. The shaded material is called a [[nappe]]. The erosional hole is called a [[window |window or fenster]]. The klippe is the isolated block of the nappe overlying autochthonous material....

    is a small area isolated from the main body of the nappe by erosion
    Erosion
    Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

     that lies on the autochthonous base; the summit of Veľký Rozsutec
    Velký Rozsutec
    Veľký Rozsutec is a mountain situated in the Malá Fatra mountain range in the Žilina Region, Slovakia. The peak is situated in the north part of Malá Fatra called Krivánska Malá Fatra and is part of the Malá Fatra National Park and Rozsutec National Nature Reserve .Veľký Rozsutec and the...

     in the Western Carpathians
    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 with Slovakia toward the Moravian region of the Czech...

     is a typical example.
  • A Fault inlier, fenster, or window
    Window (geology)
    thumb|right|350px|Schematic overview of a thrust system. The [[fault |hanging wall block]] is called a [[nappe]]. If an [[erosion]]al hole is created in the nappe that is called a window. A [[klippe]] is a solitary outcrop of the nappe in the middle of autochthonous material.A tectonic window...

    is an area of the autochthonous basement uncovered by erosion, but continuously surrounded by the body of the nappe; the Hohe Tauern window
    Hohe Tauern window
    The Hohe Tauern window is a geological structure in the Austrian Central Eastern Alps. It is a window in the Austroalpine nappes where high-grade metamorphic rocks of the underlying Penninic nappes crop out...

     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
    Basement (geology)
    In geology, the terms basement and crystalline basement are used to define the rocks below a sedimentary platform or cover, or more generally any rock below sedimentary rocks or sedimentary basins that are metamorphic or igneous in origin...

     rocks, forming so-called thick-skinned style
    Thick-skinned deformation
    Thick-skinned deformation is a geological term which refers to crustal shortening that involves basement rocks and deep-seated faults. Crustal shortening occurs when the region is undergoing horizontal compression. This occurs in orogenesis, or mountain building, during which the crust is shortened...

    . Nappes of this type usually reach a large thickness and form independent superunits such as Penninic nappes
    Penninic nappes
    The Penninic nappes or the Penninicum 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 Alps , where they crop out as a narrow band...

    .
  • Cover nappes or so called Superficial nappes are composed generally of sedimentary rock
    Sedimentary rock
    Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....

    s that form the upper part of crust, forming so-called thin-skinned style
    Thin-skinned deformation
    Thin-skinned deformation is a style of deformation in plate tectonics at a convergent boundary which occurs with shallow thrust faults that only involves cover rocks , and not deeper basement rocks....

    . Therefore nappes of this type form smaller units, such as the Hallstatt nappe in the Austroalpine nappes
    Austroalpine nappes
    The Austroalpine nappes are a geological nappe stack in the European Alps. The Alps contain three such stacks, of which the Austroalpine nappes are structurally on top of the other two...

     of the Alps.

Mechanisms of emplacement



Nappes are generally considered as compressional
Thrust tectonics
Thrust tectonics or contractional tectonics is concerned with the structures formed, and the tectonic processes associated with, the shortening and thickening of the crust or lithosphere.-Deformation styles:...

 structures, however some exceptions could be found especially among the gravitational slides along low angle faults. Gravitational forces could be even important in certain cases during emplacement of compressional thrusts. It is assumed that the ability to move huge masses of rock may be influenced by several factors that could act together or change one into another.

At a shallower depths, low pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

s and temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

s can't cause plastic
Plastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...

 and viscous behavior of solid rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

 which is 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
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

ey rocks or evaporites, which can then act as a tectonic lubricant
Lubricant
A lubricant is a substance introduced to reduce friction between moving surfaces. It may also have the function of transporting foreign particles and of distributing heat...

s. The process, which significantly reduces the frictional resistance is the fluid overpressure, which acts against the normal pressure, reducing high lithostatic pressures and allows fracturation, cataclasis and formation of tectonic breccia
Fault breccia
Fault breccia, or tectonic breccia, is a breccia that was formed by tectonic forces....

 or fault gouge
Fault gouge
Fault gouge is an unconsolidated tectonite with a very small grain size. Fault gouge has no cohesion, it is normally an unconsolidated rock type, unless cementation took place at a later stage...

 that could act as decollement
Decollement
Décollement is a gliding plane between two rock masses. In French, "décoller" means "to detach from" or "to rip off" and was first used by geologists studying the structure of the Swiss Jura Mountains, but is also known as a detachment zone. This is a structure of strata owing to deformation,...

 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 fault
A thrust fault is a type of fault, or break in the Earth's crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger...

 sheets is currently explained on the model of the orogenic wedge, 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. Gravitational spreading, possibly accompanied by an initial phase of diapirism is generated by large heat flow that causes detachment in a hinterland. Other mechanisms as push from behind, action of tangential compressive forces, shortening of the basement are essentially variations of the previous mechanisms.