graben

USGS image

Infrared-enhanced satellite image of a graben in the Afar Depression.

A graben is a depressed block of land bordered by parallel faults. Graben is German for ditch.

A graben is the result of a block of land being downthrown producing a valley with a distinct scarp on each side. Grabens 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 grabens, the bounding faults of a horst typically dip away from the center line of the horst.

A single graben or multiple grabens can produce a rift valley.

Half-grabens

In many rifts the grabens are asymmetric, with a major fault along only one of the boundaries, and these are known as half-grabens. 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^[1]).

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.

Famous grabens

• The Basin and Range Province of southwestern North America is an example of multiple horst/graben structures, including Death Valley.
• The Rhine valley to the north of Basel, Switzerland
• The Oslo graben around Oslo, Norway
• The East African Rift Valley
• The Saguenay River Valley, 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 Grabens in Antarctica
• Gulf of Saint Vincent in South Australia, Australia
• The Guanabara Bay in Rio de Janeiro , Brazil
• The Central Lowlands (Midland Valley) of Scotland
• Lake Baikal, Siberia, Russia

References

• McKnight, Tom L; Hess, Darrel (2000). "The Internal Processes: Graben". Physical Geography: A Landscape Appreciation. Upper Saddle River, NJ: Prentice Hall. pp. 417. ISBN 0-13-020263-0. 

1. ^ Nelson,C.H., Karabanov,E.B., Colman,S. and Escutia,C. 1999, Tectonic and sediment supply control of deep rift lake turbidite systems: Lake Baikal, Russia. Geology, 27, 163-166