Graben

Last updated
Fault-Horst-Graben.svg
Infrared-enhanced satellite image of a graben in the Afar Depression Graben Afar ASTER 20020327.jpg
Infrared-enhanced satellite image of a graben in the Afar Depression

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

Contents

Etymology

Graben is German for ditch or trench. The plural form is either graben [1] or grabens. [2] The German plural is Gräben.

Formation

A graben is a valley with a distinct escarpment on each side caused by the displacement of a block of land downward. Graben often occur side-by-side with horsts. Horst and graben structures indicate tensional forces and crustal stretching.

Graben are produced from parallel normal faults, where the displacement of the hanging wall is downward, while that of the footwall is upward. The faults typically dip toward the center of the graben from both sides. Horsts are parallel blocks that remain between graben; the bounding faults of a horst typically dip away from the center line of the horst. Single or multiple graben can produce a rift valley.

Half-graben

The Newark Basin, an early Mesozoic half-graben Newark Basin Cross Section.jpg
The Newark Basin, an early Mesozoic half-graben

In many rifts, the graben are asymmetric, with a major fault along only one of the boundaries, and these are known as half-graben. The polarity (throw direction) of the main bounding faults typically alternates along the length of the rift. The asymmetry of a half-graben strongly affects syntectonic deposition. Comparatively little sediment enters the half-graben across the main bounding fault because of footwall uplift on the drainage systems. The exception is at any major offset in the bounding fault, where a relay ramp may provide an important sediment input point. Most of the sediment will enter the half-graben down the unfaulted hanging wall side (e.g. Lake Baikal). [3]

Rima Ariadaeus on the Moon is thought to be a graben. The lack of erosion on the Moon makes its structure with two parallel faults and the sunken block in between particularly obvious. Rima Ariadaeus-1.jpg
Rima Ariadaeus on the Moon is thought to be a graben. The lack of erosion on the Moon makes its structure with two parallel faults and the sunken block in between particularly obvious.

Examples

Antarctica

Africa

Asia

Europe

North America

Canada

Guatemala

United States

Multi-national

Oceania

South America

See also

Notes

  1. Schlumberger Oilfield Glossary
  2. "horst and graben". Encyclopædia Britannica . Retrieved 2012-11-15.
  3. Hans Nelson, C.; Karabanov, Evgeny B.; Colman, Steven M.; Escutia, Carlota (1999). "Tectonic and sediment supply control of deep rift lake turbidite systems: Lake Baikal, Russia". Geology. 27 (2): 163–166. doi:10.1130/0091-7613(1999)027<0163:TASSCO>2.3.CO;2.
  4. Sprigg, R.C. (1961). "The Oil and Gas Prospects of the St. Vincents Gulf Graben". The APPEA Journal. 1 (1): 71–88. doi:10.1071/AJ60011.

Related Research Articles

Thrust fault a type of reverse fault that has a dip of 45 degrees or less

A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks.

Fault (geology) Fracture or discontinuity in rock across which there has been displacement

In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movement. Large faults within the Earth's crust result from the action of plate tectonic forces, with the largest forming the boundaries between the plates, such as subduction zones or transform faults. Energy release associated with rapid movement on active faults is the cause of most earthquakes.

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.

Horst (geology) A raised fault block bounded by normal faults

In physical geography and geology, a horst is a raised fault block bounded by normal faults. 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. The word Horst in Dutch and German means heap – cognate with English "hurst".

Rift valley Linear lowland created by a tectonic rift or fault

A rift valley is a linear shaped lowland between several highlands or mountain ranges created by the action of a geologic rift or fault. A rift valley is formed on a divergent plate boundary, a crustal extension or spreading apart of the surface, which is subsequently further deepened by the forces of erosion. When the tensional forces are strong enough to cause the plate to split apart, a center block drops between the two blocks at its flanks, forming a graben. The drop of the center creates the nearly parallel steeply dipping walls of a rift valley when it is new. That feature is the beginning of the rift valley, but as the process continues, the valley widens, until it becomes a large basin that fills with sediment from the rift walls and the surrounding area. One of the best known examples of this process is the East African Rift. On Earth, rifts can occur at all elevations, from the sea floor to plateaus and mountain ranges in continental crust or in oceanic crust. They are often associated with a number of adjoining subsidiary or co-extensive valleys, which are typically considered part of the principal rift valley geologically.

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.

Depression (geology) Landform sunken or depressed below the surrounding area

In geology, a depression is a landform sunken or depressed below the surrounding area. Depressions form by various mechanisms.

Fault block very large block of rock

Fault blocks are very large blocks of rock, sometimes hundreds of kilometres in extent, created by tectonic and localized stresses in the 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.

The Newark Supergroup, also known as the Newark Group, is an assemblage of Upper Triassic and Lower Jurassic sedimentary rocks which outcrop intermittently along the United States East Coast. They were deposited in a series of Triassic basins approximately 190 million years ago as North America separated from Africa. The exposures extend from Nova Scotia to North Carolina. Related basins are also found underwater in the Bay of Fundy. The group is named for the city of Newark, New Jersey.

Newark Basin A sediment-filled rift basin in northern New Jersey, south-eastern Pennsylvania and southern New York

The Newark Basin is a sediment-filled rift basin located mainly in northern New Jersey but also stretching into south-eastern Pennsylvania and southern New York. It is part of the system of Eastern North America Rift Basins.

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 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.

Baikal Rift Zone divergent boundary

The Baikal Rift Zone is a series of continental rifts centered beneath Lake Baikal in southeastern Russia. Current strain in the rifts tends to be extending with some shear movement. A series of basins form along the zone for more than 2,000 kilometres (1,200 mi), creating a rift valley. The rifts form between the Eurasian Plate to the west and the Amur Plate to the east.

The Cheshire Basin is a late Palaeozoic and Mesozoic sedimentary basin extending under most of the county of Cheshire in northwest England. It extends northwards into the Manchester area and south into Shropshire. The basin possesses something of the character of a half-graben as its deepest extent is along its eastern and southeastern margins, where it is well defined by a series of sub-parallel faults, most important of which is the Red Rock Fault. These faults divide the basin from the older Carboniferous rocks of the Peak District and the North Staffordshire Coalfield.

Geology of Russia regional geology of Russia

The geology of Russia, the world's largest country, which extends over much of northern Eurasia, consists of several stable cratons and sedimentary platforms bounded by orogenic (mountain) belts.

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.

A transfer zone in geology is an area where deformational strain is transferred from one structural element to another typically from fault to fault in rift systems. Therefore, listric faults and monoclinal folds in the hanging wall are typical structures linked by transfer zones; however, complexities do exist. The terms interbasin and intrabasin transfer zones have been proposed to delineate the magnitude of the transfer zone. Transfer zones can be described according to the fault dip directions; synthetic or conjugate and according to their deformation style; convergent or divergent. Transfer zones can be farther identified by its maturity or ; whether the major fault relationship is approaching, overlapping, collateral or collinear. Since transfer zones are normally found in extensional settings many studies have been done within the East African rift system and the Gulf of Suez rift system. Transfer zones have also played a role in hydrocarbon exploration and extraction within the Albertine graben.

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.

Growth fault

Growth faults are syndepositional or syn-sedimentary extensional faults that initiate and evolve at the margins of continental plates. They extend parallel to passive margins that have high sediment supply. Their fault plane dips mostly toward the basin and has long-term continuous displacement. Figure one shows a growth fault with a concave upward fault plane that has high updip angle and flattened at its base into zone of detachment or décollement. This angle is continuously changing from nearly vertical in the updip area to nearly horizontal in the downdip area.

Northern North Sea basin

The North Sea is part of the Atlantic Ocean in northern Europe. It is located between Norway and Denmark in the east, Scotland and England in the west, Germany, the Netherlands, Belgium and France in the south.

References