Rift and fault structure
Rift extension. Top: Full graben between two faults, each sloping towards center of rift. Bottom: half graben, more common
Block view of a rift formed of three segments, showing the location of the accommodation zones between them at changes in fault location or polarity (dip direction)
A rift is a region where the lithosphere 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 faults 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.
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, East African Rift, Rio Grande rift system and the North Sea 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 10 kilometres (6.2 mi) 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 50 to 150 kilometres (31 to 93 mi) long.