autochthonous basement uncovered by erosion, but entirely surrounded by the body of the nappe; the Hohe Tauern window 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 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. Cover nappes or so called superficial nappes are composed generally of sedimentary rocks 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 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 pressures and temperatures can't cause the plastic and viscous behavior of solid rock necessary to move along low angle faults. It is considered that such characteristics may be achieved at significantly less extreme conditions in the clayey rocks or evaporites, which can then act as tectonic lubricants. The process, which significantly reduces the frictional 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 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 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, such as push from behind, action of tangential compressive forces, and shortening of the basement, are essentially variations of the previous mechanisms. References Plate tectonics Structural geology