Horst (geology)

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In physical geography and geology, a horst is a raised fault block bounded by normal faults. [1] A horst is a raised block of the Earth's crust that has lifted, or has remained stationary, while the land on either side (graben) has subsided. [2] The word Horst in Dutch and German means heap – cognate with English "hurst".

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The Vosges Mountains in France and Black Forest in Germany are examples of horsts, as are the Table, Jura, the Dole mountains and the Rila - Rhodope Massif including the well defined horsts of Belasitsa (linear horst), Rila mountain (vaulted domed shaped horst) and Pirin mountain - a horst forming a massive anticline situated between the complex graben valleys of Struma and that of Mesta. [3] [4] [5] The word also applies to larger areas, such as the Russian Plain, Arabia, India and Central South Africa, where the continent remains stable, with horizontal table-land stratification, in distinction to folded regions such as some mountain chains of Eurasia. [2]

The Midcontinent Rift System in North America is marked by a series of horsts extending from Lake Superior to Kansas. [6]

Geomorphology

Horsts may have either symmetrical or asymmetrical cross-sections. If the normal faults to either side have similar geometry and are moving at the same rate, the horst is likely to be symmetrical and roughly flat on top. If the faults on either side have different rates of vertical motion, the top of the horst will most likely be inclined and the entire profile will be asymmetrical. Erosion also plays a significant role in how symmetrical a horst appears in cross-section.

Horsts and hydrocarbon exploration

In many rift basins around the world, the vast majority of discovered hydrocarbons are found in conventional traps associated with horsts.[ citation needed ] For example, much of the petroleum found in the Sirte Basin, Libya (of the order of tens of billions of barrels of reserves) are found on large horst blocks such as the Zelten Platform and the Dahra Platform and on smaller horsts such as the Gialo High and the Bu-Attifel Ridge.

See also

Related Research Articles

Geography of Bulgaria

Bulgaria is a country situated in Southeast Europe and occupies the eastern quarter of the Balkan peninsula, being the largest country within its geographic boundaries. It is bordering Romania to the north, Serbia and North Macedonia to the west, Greece and Turkey to the south, and the Black Sea to the east. The northern border with Romania follows the river Danube until the city of Silistra. The land area of Bulgaria is 110,994 square kilometres (42,855 sq mi), slightly larger than that of Iceland or the U.S. state of Tennessee. Considering its relatively small size, Bulgaria has a great variety of topographical features. Even within small parts of the country, the land may be divided into plains, plateaus, hills, mountains, basins, gorges, and deep river valleys. The geographic center of Bulgaria is located in Uzana.

Horst and graben

In geology, horst and graben refer to regions that lie between normal faults and are either higher or lower than the area beyond the faults. A horst represents a block pushed upward relative to the blocks on either side by the faulting, and a graben is a block generally long compared to its width that has been lowered relative to the blocks on either side due to the faulting. Horst and graben are formed when normal faults of opposite dip occur in pair with parallel strike lines. Horst and graben are always formed together. Graben are usually represented by low-lying areas such as rifts and river valleys whereas horsts represent the ridges between or on either side of these valleys.

Graben Depressed block of planetary crust bordered by parallel faults

In geology, a graben is a depressed block of the crust of a planet bordered by parallel faults.

An aulacogen is a failed arm of a triple junction. Aulacogens are a part of plate tectonics where oceanic and continental crust is continuously being created, destroyed, and rearranged on the Earth’s surface. Specifically, aulacogens are a rift zone, where new crust is formed, that is no longer active.

Rift A linear zone where the Earths crust is being pulled apart, and is an example of extensional tectonics

In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics.

Pirin

The Pirin Mountains are a mountain range in southwestern Bulgaria, with Vihren at an altitude of 2,914 m being the highest peak. One hypothesis is the mountain was named after Perun, the highest god of the Slavic pantheon and the god of thunder and lightning. Another version is that the etymology of the range derives from the Thracian word Perinthos, meaning "Rocky Mountain".

Mountain formation The geological processes that underlie the formation of mountains

Mountain formation refers to the geological processes that underlie the formation of mountains. These processes are associated with large-scale movements of the Earth's crust. Folding, faulting, volcanic activity, igneous intrusion and metamorphism can all be parts of the orogenic process of mountain building. The formation of mountains is not necessarily related to the geological structures found on it.

Eurotas (river)

The Eurotas or Evrotas is the main river of Laconia and one of the major rivers of the Peloponnese, in Greece. The river's springs are located just northwest of the border between Laconia and Arcadia, at Skortsinos. The river is also fed by underwater springs at Pellana and by tributaries coursing down from Mt. Taygetos and Mt. Parnon, which flank the Eurotas valley to the west and east, respectively. The river is 82 kilometres (51 mi) long, flowing in a north-south direction and emptying into the Laconian Gulf.

Fault block

Fault blocks are very large blocks of rock, sometimes hundreds of kilometres in extent, created by tectonic and localized stresses in Earth's crust. Large areas of bedrock are broken up into blocks by faults. Blocks are characterized by relatively uniform lithology. The largest of these fault blocks are called crustal blocks. Large crustal blocks broken off from tectonic plates are called terranes. Those terranes which are the full thickness of the lithosphere are called microplates. Continent-sized blocks are called variously microcontinents, continental ribbons, H-blocks, extensional allochthons and outer highs.

Extensional tectonics is concerned with the structures formed by, and the tectonic processes associated with, the stretching of a planetary body's crust or lithosphere.

Basin and range topography

Basin and range topography is an alternating landscape of parallel mountain ranges and valleys. It is a result of crustal extension/stretching of the lithosphere due to mantle upwelling, gravitational collapse, crustal thickening, or relaxation of confining stresses. Crustal extension causes the thinning and deformation of the upper crust in an orientation perpendicular to the direction of extension. As the plates pull apart, they thin allowing the hot mantle to rise close to the surface. When the crust is extended it fractures along a fault plane, creating a series of long parallel normal faults. Between these normal faults are blocks, which subside, get uplifted or tilted. This is known as block faulting. Basins are formed due to subsidence of a block, while the blocks adjacent to the subsidence gets uplifted creating ranges. Normal faults are on both sides of the blocks; creating alternating elevated or subsided blocks, otherwise known as horst and graben. Basins and ranges can also be formed by blocks that are tilted causing one side to subside while the other side gets uplifted. These only have one side with a normal fault, this is known as tilted block faulting. Extension causes the plate to stretch, fracture and thin. Mountains rise and valleys drop, over a long period of time creating what we see as basin and range topography.

Sirte Basin

The Sirte Basin is a late Mesozoic and Cenozoic triple junction continental rift along northern Africa that was initiated during the late Jurassic Period. It borders a relatively stable Paleozoic craton and cratonic sag basins along its southern margins. The province extends offshore into the Mediterranean Sea, with the northern boundary drawn at the 2,000 meter (m) bathymetric contour. It borders in the north on the Gulf of Sidra and extends south into northern Chad.

Half-graben

A half-graben is a geological structure bounded by a fault along one side of its boundaries, unlike a full graben where a depressed block of land is bordered by parallel faults.

Tilted block faulting

Tilted block faulting, also called rotational block faulting, is a mode of structural evolution in extensional tectonic events, a result of tectonic plates stretching apart. When the upper lithospheric crust experiences extensional pressures, the brittle crust fractures, creating detachment faults. These normal faults express themselves on a regional scale; upper crust fractures into tilted fault blocks, and ductile lower crust ascends. This results in uplift, cooling, and exhumation of ductily deformed deeper crust. The large unit of tilted blocks and associated crust can help form an integral part of metamorphic core complexes and can occur on both continental and oceanic crust.

Kutai Basin

The Kutai sedimentary basin extends from the central highlands of Borneo, across the eastern coast of the island and into the Makassar Strait. With an area of 60,000 km2, and depths up to 15 km, the Kutai is the largest and deepest Tertiary age basin in Indonesia. Plate tectonic evolution in the Indonesian region of SE Asia has produced a diverse array of basins in the Cenozoic. The Kutai is an extensional basin in a general foreland setting. Its geologic evolution begins in the mid Eocene and involves phases of extension and rifting, thermal sag, and isostatic subsidence. Rapid, high volume, sedimentation related to uplift and inversion began in the Early Miocene. The different stages of Kutai basin evolution can be roughly correlated to regional and local tectonic events. It is also likely that regional climate, namely the onset of the equatorial ever wet monsoon in early Miocene, has affected the geologic evolution of Borneo and the Kutai basin through the present day. Basin fill is ongoing in the lower Kutai basin, as the modern Mahakam River delta progrades east across the continental shelf of Borneo.

The geology of Ukraine is the regional study of rocks, minerals, tectonics, natural resources and groundwater in the country. The oldest rocks in the region are part of the Ukrainian Shield and formed more than 2.5 billion years ago in the Archean eon of the Precambrian. Extensive tectonic evolution and numerous orogeny mountain building events fractured the crust into numerous block, horsts, grabens and depressions and Ukraine was intermittently flooded as the crust downwarped during much of the Paleozoic, Mesozoic and early Cenozoic, before the formation of the Alps and Carpathians defined much of its current topography and tectonics. Ukraine was impacted by the Pleistocene glaciations within the last several hundred thousand years. The country has numerous metal deposits as well as minerals, building stone and high-quality industrial sands.

The geology of Nevada began to form in the Proterozoic at the western margin of North America. Terranes accreted to the continent as a marine environment dominated the area through the Paleozoic and Mesozoic periods. Intense volcanism, the horst and graben landscape of the Basin and Range Province originating from the Farallon Plate, and both glaciers and valley lakes have played important roles in the region throughout the past 66 million years.

The geology of Romania is structurally complex, with evidence of past crustal movements and the incorporation of different blocks or platforms to the edge of Europe, driving recent mountain building of the Carpathian Mountains. Romania is a country located at the crossroads of Central, Eastern, and Southeastern Europe. It borders the Black Sea to the southeast, Bulgaria to the south, Ukraine to the north, Hungary to the west, Serbia to the southwest, and Moldova to the east.

The geology of Denmark includes 12 kilometers of unmetamorphosed sediments lie atop the Precambrian Fennoscandian Shield, the Norwegian-Scottish Caledonides and buried North German-Polish Caledonides. The stable Fennoscandian Shield formed from 1.45 billion years ago to 850 million years ago in the Proterozoic. The Fennoscandian Border Zone is a large fault, bounding the deep basement rock of the Danish Basin—a trough between the Border Zone and the Ringkobing-Fyn High. The Sorgenfrei-Tornquist Zone is a fault-bounded area displaying Cretaceous-Cenozoic inversion.

The geology of the Norwegian Sea began to form 60 million years ago in the early Cenozoic, as rifting led to the eruption of mafic oceanic crust, separating Scandinavia and Greenland. Together with the North Sea the Norwegian Sea has become highly researched since the 1960s with the discovery of oil and natural gas in thick offshore sediments on top of the Norwegian continental shelf.

References

  1. Fossen H. (2010-07-15). Structural Geology. Cambridge University Press. p. 154. ISBN   9781139488617.
  2. 1 2 Wikisource-logo.svg One or more of the preceding sentences incorporates text from a publication now in the public domain : Chisholm, Hugh, ed. (1911). "Horst". Encyclopædia Britannica . 13 (11th ed.). Cambridge University Press. p. 740.
  3. Geographic Dictionary of Bulgaria 1980 , p. 368
  4. Dimitrova & al 2004 , p. 53
  5. Donchev & Karakashev 2004 , pp. 128–129
  6. United States Geological Survey (1989). "Clastic Rocks Associated with the Midcontinent Rift System in Iowa" (PDF). U.S. GEOLOGICAL SURVEY BULLETIN 1989–I: 11. Retrieved Sep 3, 2018.