geological structure of horst and graben. (Photo credit: Wikipedia)

A graben is a depressed block of land bordered by parallel faults. Graben is German for ditch or trench. A graben is the result of a block of land being downthrown producing a valley with a distinct scarp on each side. Graben often occur side-by-side with horsts. Horst and graben structures are indicative of tensional forces and crustal stretching.

Graben are produced from parallel normal faults, where the hanging wall is downthrown and the footwall is upthrown. 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.

One of the world’s deepest graben with over 1000 metres of downthrow is the Mount Unzen volcanic complex in southern Japan.
The Newark Basin, an early Mesozoic half-graben

Infrared-enhanced satellite image of a graben in the Afar Depression, Ethiopia. (Photo credit: Wikipedia)

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.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 alternate 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, due to the effects 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).

Some Famous Grabens
The Basin and Range Province of southwestern North America is an example of multiple horst/graben structures, including Death Valley, with Salt Lake Valley being the easternmost and Owens Valley being the westernmost.
The Rio Grande Rift Valley in Colorado/New Mexico/Texas of the United States
The Rhine valley to the north of Basel, Switzerland
The Oslo graben around Oslo, Norway
The East African Rift Valley
The Saguenay Graben, Quebec, Canada
The Narmada River valley in central India
The lower Godavari River valley in southern India
The Ottawa-Bonnechere Graben in Ontario and Quebec, Canada
The Lambert Graben in Antarctica
Gulf St Vincent in South Australia, Australia
The Guanabara Bay in Rio de Janeiro, Brazil
The Central Lowlands (Midland Valley) of Scotland
Baikal Rift Zone, Siberia, Russia
Lake Tahoe, California and Nevada, U.S.A.
The Euan G.C. Smith Graben, Seatoun, New Zealand.
The Republic Graben in Republic, Washington.

Links and Sources:

Wikipedia