An aulacogen is a tectonic trough on a craton which is formed as the failed arm of a triple-rift junction.

J. William Kerr has written:

'Structural framework of Lancaster Aulacogen, Arctic Canada' -- subject(s): Structural Geology

A failed rift valley is known as an aulacogen. It is a three-way continental plate junction in which two of the ridges continue spreading while the third stops. Examples of failed rift valleys are the Rio Grande Rift and the East African Rift.

See http://en.wikipedia.org/wiki/Rift_valley A rift valley is a linear-shaped lowland between highlands or mountain ranges created by the action of a geologic rift or fault. This action is manifest as crustal extension, a spreading apart of the surface which is subsequently further deepened by the forces of erosion. Rifts can occur at all elevations, from the seafloor to plateaus and mountain ranges. They can occur in continental crust or in oceanic crust. Rift valleys are often associated with a number of adjoining subsidiary or co-extensive valleys which are typically considered geologically part of the principal rift valley. The most extensive rift valley is located along the crest of the mid-ocean ridge system and is the result of seafloor spreading. Examples of this type of rift include the Mid-Atlantic Ridge and the East Pacific Rise. Many existing continental rift valleys are the result of a failed arm (aulacogen) of a triple junction, although there are two, the East African Rift and the Baikal Rift Zone, which are currently active, as well as a third which may be, the West Antarctic Rift. In these instances, not only the crust, but also entire tectonic plates are in the process of breaking apart to create new plates. If they continue, continental rifts will eventually become oceanic rifts. Other rift valleys are the result of bends or discontinuities in horizontally-moving (strike-slip) faults. When these bends or discontinuitues are in the same direction as the relative motions along the fault, extension occurs. For example, for a right lateral-moving fault, a bend to the right will result in stretching and consequent subsidence in the area of the irregularity. In the view of many geologists today, the Dead Sea lies in a rift which results from a leftward discontinuity in the left lateral-moving Dead Sea Transform fault. Where a fault breaks into two strands, or two faults run close to each other, crustal extension may also occur between them as a result of differences in their motions. Both types of fault-caused extension commonly occur on a small scale, producing such features as sag ponds or landslides.

See http://en.wikipedia.org/wiki/Rift_valley A rift valley is a linear-shaped lowland between highlands or mountain ranges created by the action of a geologic rift or fault. This action is manifest as crustal extension, a spreading apart of the surface which is subsequently further deepened by the forces of erosion. Rifts can occur at all elevations, from the seafloor to plateaus and mountain ranges. They can occur in continental crust or in oceanic crust. Rift valleys are often associated with a number of adjoining subsidiary or co-extensive valleys which are typically considered geologically part of the principal rift valley. The most extensive rift valley is located along the crest of the mid-ocean ridge system and is the result of seafloor spreading. Examples of this type of rift include the Mid-Atlantic Ridge and the East Pacific Rise. Many existing continental rift valleys are the result of a failed arm (aulacogen) of a triple junction, although there are two, the East African Rift and the Baikal Rift Zone, which are currently active, as well as a third which may be, the West Antarctic Rift. In these instances, not only the crust, but also entire tectonic plates are in the process of breaking apart to create new plates. If they continue, continental rifts will eventually become oceanic rifts. Other rift valleys are the result of bends or discontinuities in horizontally-moving (strike-slip) faults. When these bends or discontinuitues are in the same direction as the relative motions along the fault, extension occurs. For example, for a right lateral-moving fault, a bend to the right will result in stretching and consequent subsidence in the area of the irregularity. In the view of many geologists today, the Dead Sea lies in a rift which results from a leftward discontinuity in the left lateral-moving Dead Sea Transform fault. Where a fault breaks into two strands, or two faults run close to each other, crustal extension may also occur between them as a result of differences in their motions. Both types of fault-caused extension commonly occur on a small scale, producing such features as sag ponds or landslides.