![Newark Basin Cross Section](https://upload.wikimedia.org/wikipedia/commons/2/26/Newark_Basin_Cross_Section.jpg)
A half-graben is a geological structure bounded by a
fault along one side of its boundaries, unlike a full
graben
In geology, a graben () is a depressed block of the crust of a planet or moon, bordered by parallel normal faults.
Etymology
''Graben'' is a loan word from German, meaning 'ditch' or 'trench'. The word was first used in the geologic contex ...
where a depressed block of land is bordered by parallel faults.
Rift and fault structure
![Rift segmentation](https://upload.wikimedia.org/wikipedia/commons/1/14/Rift_segmentation.png)
A rift is a region where the
lithosphere
A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion of the upper mantle (geology), mantle that behaves elastically on time sca ...
extends as two parts of the Earth's
crust pull apart.
Often a rift will form in an area of the crust that is already weakened by earlier geological activity.
Extensional fault
An extensional fault is a
fault caused by stretching of the Earth's crust. Stretching reduces the thickness and horizontally extends portions of the crust and/or lithosphere. In most cases such a fault is also a normal fault, but may create a ...
s form parallel to the axis of the rift.
An extensional fault may be seen as a crack in the crust that extends down at an angle to the vertical.
As the two sides pull apart, the hanging wall ("hanging over" the sloping fault) will move downward relative to the footwall.
The crust thins and sinks, forming a rift basin. Warm mantle material wells up, melting the crust and often causing volcanoes to emerge in the rift basin.
Extensional basins may appear to be caused by a graben, or depressed block of land, sinking between parallel normal faults that dip towards the center of the graben from both sides.
In fact, they are usually made of linked asymmetrical half-grabens. Faults with antithetic slope directions linked in to a controlling fault, or periodic changes of dip in the controlling faults, give the impression of full graben symmetry.
As the rift expands, the rift flanks lift up due to
isostatic compensation of the
lithosphere
A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion of the upper mantle (geology), mantle that behaves elastically on time sca ...
.
This creates the asymmetric topographic profile that is typical of half grabens.
The half grabens may have alternating polarities along the rift axis, dividing the rift valley into segments.
Intracontinental and marine rift basins such as the
Gulf of Suez
The Gulf of Suez ( ar, خليج السويس, khalīǧ as-suwais; formerly , ', "Sea of Calm") is a gulf at the northern end of the Red Sea, to the west of the Sinai Peninsula. Situated to the east of the Sinai Peninsula is the smaller Gulf of ...
,
East African Rift
The East African Rift (EAR) or East African Rift System (EARS) is an active continental rift zone in East Africa. The EAR began developing around the onset of the Miocene, 22–25 million years ago. In the past it was considered to be part of a ...
,
Rio Grande rift
The Rio Grande rift is a north-trending continental rift zone. It separates the Colorado Plateau in the west from the interior of the North American craton on the east. The rift extends from central Colorado in the north to the state of Chihua ...
system and the
North Sea
The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian S ...
often contain a series of half-graben sub-basins, with the polarity of the dominant fault system changing along the axis of the rift.
Often the extensional fault systems are segmented in these rifts.
Rift border faults with lengths over are separated by relay ramp structures.
The relay ramps may provide pathways for sediment to be carried into the basin.
Typically the rift is broken along its axis into segments about long.
Sedimentation
![Half-graben sedimentation](https://upload.wikimedia.org/wikipedia/commons/6/6b/Half-graben_sedimentation.png)
Four zones of sedimentation can be defined in a half-graben.
The first is "escarpment margin" sedimentation, found along the major border faults bounding the half graben, where the deepest part of the basin meets the highest rift-shoulder mountains.
Comparatively little sediment enters the half-graben across the major bounding fault, since uplift of the footwall causes the land on the footwall side to slope away from the fault.
Rivers on that side therefore carry sediment away from the rift valley.
But as the lowest part of the basin with the greatest rate of subsidence, the escarpment margin experiences the highest rates of sedimentation, which may accumulate to several kilometers in depth.
This sedimentation often includes very coarse debris such as huge blocks from rock falls, as well as fans of sediment from the basin wall.
Other material is transported across or along the basin to the deep water parts of a rift lake along the escarpment margin.
Most of the sediment will enter the half-graben down the unfaulted hanging wall side.
On the side of the basin opposite the main border fault, sedimentation occurs along the "hinged margin", which may also be called the "shoaling margin" or the "flexural margin". In this part of the basin, slopes are usually gentle and large river systems may carry sediment into the basin, which may be stored in deltas where they enter a rift valley lake. Littoral and sub-littoral carbonate deposits may well be accumulated in these conditions.
The "axial margins" at the ends of basins often include low-gradient ramps where major rivers enter the basin, building deltas and creating currents within a rift lake that can carry sediment from one end to the other.
Between adjacent half grabens there will be "accommodation zones" that may include local extension, compression or strike-slip faulting. These can create complex
morphologies within which different mechanisms affect sedimentation.
The types of sedimentation in half grabens also depends on lake levels in the rift, the climate (e.g. tropical versus temperate) in which the sediments form and the water chemistry.
Although sediments arrive primarily from the unfaulted side of the half-graben, some erosion takes place on the fault escarpment of the main border fault, and this produces characteristic
alluvial fan
An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to semiarid climates, but a ...
s where confined channels emerge from the escarpment.
Lake Baikal
Lake Baikal (, russian: Oзеро Байкал, Ozero Baykal ); mn, Байгал нуур, Baigal nuur) is a rift lake in Russia. It is situated in southern Siberia, between the federal subjects of Irkutsk Oblast to the northwest and the Repu ...
is an unusually large and deep example of half-graben evolution.
The lake is , with a maximum depth of .
Sediment in the depression may be up to in depth. The system also includes some small
Quaternary
The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...
volcanoes.
In this lake, at first a series of half-grabens were linked in a linear chain. As the rift valley aged, extensive deformation developed on both sides of the lake, converting them into asymmetric full grabens.
Examples of half-grabens
References
Citations
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{{Structural geology
Structural geology