Fault (geology)
A fault (also jump , displacement or disturbance in the narrower sense ) is a tectonic rupture or fracture point in the rock , at which two rock areas or crustal parts are offset from one another over distances ranging from centimeters to a few dozen to a hundred kilometers . These rock areas are called broken wings , blocks or clods . The offset is called the jump height or jump width . While the umbrella term disturbance can also include different types of plastic deformation in a broader sense, only the result of a fracture deformation is referred to as a fault. The corresponding tectonics is called fracture tectonics . Faults have therefore always occurred in the uppermost part of the earth's crust. The line of intersection of a fault with the surface of the terrain or the surface of the rock is called the fault line , fault line or fault line.
Emergence
Faults are special forms of shear zones , that is, they are narrowly delimited areas of the earth's crust, in which forces that act on such a crustal area from the outside are concentrated and converted into movement. Faults only occur in the uppermost, still relatively cool areas of the earth's crust, where the rock breaks brittle when there is sufficient force . Usually these are horizontal compressive or tensile forces, which are not infrequently related to plate tectonic processes and sometimes act over a distance of hundreds or even thousands of kilometers. Other possible causes for the formation of faults are the effects of magmatism or halokinesis on the overburden , or impacts from cosmic objects .
features
On the path of movement of a fault, a pronounced separation surface often forms in the mountains , which is referred to as a fault or fault surface . Due to the friction between the rocks, smooth, sometimes mirror-polished surfaces with movement grooves, the so-called armor, can form on this.
However, the rock can also be shattered to form so-called tectonic breccias or even completely ground into rock powder (cf. → Kakirit , → Kataklasit ). In these cases the trajectory no longer has the character of a discrete separating surface, but rather that of a zone. Such areas in the mountains, known as fault zones , can cause considerable problems in civil engineering projects, especially in tunnel construction. The strength of the rock in the fault zone is significantly lower than that of the adjacent rock, which potentially facilitates driving during tunnel construction, for example by eliminating the need for blasting, but this also increases the likelihood of collapse and therefore represents a safety risk. There were considerable delays in the construction of the Gotthard base tunnel in the west tube after a collapse in a fault zone.
In sedimentary rocks with pronounced stratification, a bending of the strata is not infrequently observed near the fault area, provided that the movement occurred transversely to the stratification. This bending is known as dragging . Harnesses and dragging make it possible to determine the sense of movement at the fault area.
As a rule, there is no continuous movement on fault surfaces, but rather it takes place episodically. The movements, which last only seconds, are the cause of more or less strong earthquakes . The geometry and sense of movement of the fault (see classifications ) determine the mechanics of the hearth process or the so-called radiation characteristics of the earthquake. In the case of a fault that has not been opened up and is still active, from which an earthquake originated, the geometry and direction of thrust can be determined with the help of seismological data using the focal surface solution .
The jump height (also saigere jump height or throw ) as a size for the vertical offset, the jump width (also horizontal jump size) as a parameter for the horizontal offset (perpendicular to the jump height ) and the flat jump height as the size for the directly on the fault area can be measured Offset (vector sum of saiger jump height and jump distance).
Classifications
A: Journal of displacement
B: removal
c: deferment
The Hangendscholle during installation and removal is always the right one.
Classification according to the movement on the fault area
- Movement in the direction of strike of the fault
- Blade shifts ( transversal , horizontal or lateral shifts , English: strike-slip fault ) are caused by shear in a horizontal plane, mostly along steeply falling fault surfaces.
- Movement in the direction of fall of the fault ( dip-slip fault )
- Deportations (ger .: normal fault ), caused by crustal stretching (extensional). Sometimes the term “fault” only refers to faults, while other forms are referred to as displacements .
- Postponements (engl .: reverse fault ) caused by crustal compression or shortening (compressional)
- Thrusts (engl .: thrust fault ), also caused by compressional, wherein the fault planes in contrast to the deferment, incident at less than 45 ° sometimes even, horizontal ( söhlig lie). The latter include the cover thrusts , which can take place in the area of brittle deformation, but are not considered to be faults in the actual sense and are therefore not considered to be fractional tectonic structural elements.
In reality, the types of faults described rarely occur in their pure form. Mixed forms of upheaval and leaf displacement ( transpressive disorder) or of displacement and leaf displacement ( transtensive disorder) are the rule.
When faults with a vertical component between Hangendem (engl .: hanging wall ) and lying (engl .: footwall ) distinguished. The hanging clod (also known as hanging wing or hanging block) is the clod that is above the fault area, i.e. In other words, in the case of an imaginary borehole that penetrates both the hanging clod and the horizontal clod vertically, the hanging clod is the clod on which the upper end of the borehole is located. The hanging clod is the clod that determines the relative sense of movement: when pushed up, it is pushed up and when pushed away, it is pushed away.
Alternatively, the pushed-off or not pushed-off floe is referred to as high or horst floe and the pushed-off or not pushed-off floe as deep or trench floe. This designation is particularly recommended if the character of the fault is not known due to a lack of outcrop.
Division according to incidence for stratification
Furthermore, faults differ in their angle to the collapse of the rock stratification :
- Antithetical faults collapse against the stratification.
- Synthetic or homothetic faults dip in the same direction as the stratification.
Classification by stroking for stratification
Depending on the coating of a fault plane or -line in proportion to the strike of the layers or of the fold axes in a particular region is referred to as longitudinal , transverse or diagonal apertures and -verwerfungen or disorders . Diagonal breaks are obsolete also known as pike-corner faults .
Other names
Cover cleft is an outdated mining term for a shallow dipping fault that is not linked to a specific direction of movement or a specific formation process.
Active and inactive faults
Faults are determined according to whether the tectonic tension regime in which they arose still exists, i.e. That is, whether there is still (intermittent) movements at the fault surface, a distinction is made between recent (still active) faults and fossil (inactive, old) faults. Many active faults can be found in young fold mountains and in the crust areas in front of these fold mountains or in active rift valleys . So-called earthquake lines , narrow zones on the earth's surface in which earthquakes are frequently registered , often run along active fault systems. One of the best-known faults in this regard is the San Andreas Fault in the US state of California, which is also a plate boundary and therefore only actually has a fault character in the upper part of the earth's crust. Active faults do not necessarily have to be created (created) in the existing stress regime, but can also be "old weak zones", i.e. That is, to meanwhile inactive faults that have been reactivated in the current stress regime . Many of the faults on which the German low mountain range were raised, for example the Franconian Line or the Harz North Rim Fault , are old faults that were reactivated in the course of the formation of the Alps.
If active faults extend below relatively densely populated areas with sensitive infrastructure, it makes sense within the framework of disaster control to monitor these faults using geotechnical and geodesy methods ( GPS , leveling ). For example, in the run-up to the construction of the Karawanken tunnel, the periadriatic seam was the target of such monitoring measures.
Rejection systems
Several, almost parallel, unidirectional faults at a short lateral distance are referred to as relay fractures . Faults falling in opposite directions form clumps or ditches , in which the fault areas tend to move away from each other or towards each other towards the depth. Clods that are tilted on parallel faults in the same direction and falling in the same direction are called half-nests or half-trenches . Such fault or fault systems can, at a width of several tens to hundreds of kilometers, have a longitudinal extension of hundreds or thousands of kilometers and extend over entire continents. Well-known examples are the East African Rift Valley , which extends from Lake Tanganyika to the Dead Sea, the Mediterranean-Mjosen Zone , which extends from the mouth of the Rhône to the Oslo area , and the Basin and Range Province , which extends over several states extends in the western United States to northwestern Mexico. The main faults in these systems typically extend deep into the continental lithosphere and merge at depth into ductile shear zones. However, the largest contiguous fault system on earth is that of the mid-ocean ridges .
Continental crust is not subducted and is therefore often geologically relatively old. It is also relatively easy to access for geological research. It is therefore known that the character of many fault systems has changed several times in the course of the earth's history, depending on the prevailing large tectonic (plate tectonic) conditions, from extensive to compressive to extensive again, etc. The sense of movement at individual faults can also change, so that In the course of many millions of years at one and the same fault, sometimes predominantly displacement, sometimes predominantly displacement and sometimes predominantly leaf displacement has taken place (see also → Active and inactive faults ).
Geomorphology, hydrology, magmatism, raw material geology
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A fault is often difficult to recognize in the terrain, because it is usually located under a weathered blanket or ground and the original jump height has been more or less leveled out by erosion and / or sedimentation . However, the geomorphology can indicate the presence of faults in the subsurface. If there is significantly more weather-resistant rock on a floe than on the neighboring floe, the rock of the first-mentioned floe forms a terrain step or the floe is at least topographically higher. The topographically higher lying clod is not necessarily the tectonically higher clod, because the corresponding more resistant rock on the neighboring, actually tectonically higher clod may already have been completely eroded (→ relief reversal ).
At faults, the rock is often shattered by the movements and therefore less resistant to erosion than the adjacent rock. In addition, surface water can easily penetrate into the subsoil at faults and trigger weathering processes. Both of these factors mean that the rock in the immediate vicinity of faults is cleared more easily than the adjacent rock, which is why terrain cuts in high areas of humid climates often follow the course of fault lines. The long valley furrows in many young fold mountains are an example of this .
A groundwater leak can also indicate the presence of a fault. A so-called rejection source - a special form artesian sources - if an arises aquifer (aquifer) is sealed off on a fault by a water damming layer (Aquiclud) and thereby the water is accumulated in the aquifer up to the ground surface.
In a similar way, faults can form trap structures for crude oil and natural gas : If a highly porous , permeable rock, in which fossil hydrocarbons can migrate towards the earth's surface, is bounded by a fault at the top by a low-porosity, impermeable rock, then crude oil and natural gas can get below this impermeable rock accumulate in eligible amounts.
Faults can serve as an ascent path for magma , which then emerges on the surface as lava and covers large areas, builds up individual volcanoes or penetrates into near-surface sediment layers and forms laccolites . Remaining in the routes to the surface magma solidifies to dykes (ger .: dykes ). Instead of magma, however, hydrothermal solutions can also rise from the depths, from which minerals are precipitated on the fracture surfaces , right up to the complete filling of the fracture points in the form of mineral veins. Are adequately ore precipitated minerale, economically exploitable can vein deposits occur.
Distortions in the map
In geological maps , faults or rather the intersection lines ("outcrops") of faults with the terrain surface ("fault lines") are usually shown as thick black or red lines - in contrast to layer contacts, which are marked with significantly thinner black lines. The relative sense of movement of faults with a vertical component (upheavals, overflows and faults) can easily be recognized in regions with largely horizontal (bottom) layers on the map based on the age of the rocks of neighboring clods: clods on which younger rock is present are has been lowered relative to clods on which older rock stands. Occasionally, certain signatures on the fault lines indicate the sense of movement at the corresponding fault area. A fault line shown in the form of a comb or rake symbolizes a displacement, triangular symbols along a fault line symbolize an upward or overturning. The tines or triangles always point in the direction of the hanging clod. Half arrows on a fault line symbolize a leaf displacement.
Remote sensing and planetology
Faults can be identified on satellite images of the earth and other celestial bodies based on certain features. Mostly these are linear structures ("fault lines"), which are clearly recognizable on the earth, especially in areas with hardly any vegetation, especially when the rock on either side of such a linear structure differs significantly or when there are abnormalities in the Relief connected.
On the moon, linear structures, so-called grooves (rimae) or rupes, were first examined by Hieronymus Schröter at the beginning of the 19th century. The first comprehensive tectonic map of the Earth's moon with several 1000 faults was created by the United States Geological Survey in the 1950s. Extensive fault systems are also known from Mars (prime example: Valles Marineris ) and the moons of the gas giants of the outer solar system (e.g. Europe ).
literature
- Gerhard H. Eisbacher: Introduction to Tectonics . 1st edition. Ferdinand Enke Verlag, Stuttgart 1991, ISBN 3-432-99251-3 , pp. 69-81 .
- Rudolf Hohl (ed.): The history of the development of the earth . 6th edition. Werner Dausien Verlag, Hanau 1985, ISBN 3-7684-6526-8 , p. 201-213 .
- Dieter Richter: General Geology . 3. Edition. de Gruyter Verlag, Berlin / New York 1985, ISBN 3-11-010416-4 , p. 218-233 .
Web links
Individual evidence
- ^ A b c Hans Murawski, Wilhelm Meyer: Geological dictionary. 12th edition. Spektrum Verlag, Heidelberg 2010, ISBN 978-3-8274-1810-4 , p. 181 f.
- ↑ Achrain Tunnel. ( Memento from November 29, 2014 in the Internet Archive ) Brief information about the corresponding construction project on the Jäger Bau GmbH website, accessed on November 14, 2014.
- ^ Geological fault zone in the west tube. Article on baublatt.ch from March 11, 2010, accessed on November 14, 2014.
- ↑ H. Schmid, K. Peters: Report on surveying investigations on the question of the north-south Alpine border in connection with recent crustal movements (periadriatic seam / Karawanken profile). Communications from the Austrian Geological Society. Vol. 71/72 (year 1978/79), 1980, pp. 307-316 ( PDF 900 kB).
- ↑ Stephen White, Harald Stollhofen, Ian G. Stanistreet, Volker Lorenz: Pleistocene to Recent rejuvenation of the Hebron Fault, SW Namibia. Geological Society, London, Special Publications. Vol. 316, 2009, pp. 293-317, doi: 10.1144 / SP316.18 (alternative full text access : ResearchGate )