Crustal blocks are sometimes raised or lowered between roughly parallel faults without being subjected to tilting. The resulting features are called rift valleys and horsts. A rift valley or graben (after the German word for a ditch) is a long and narrow valley formed by subsidence between two parallel faults (Figure 6.18a). Rift valleys are not true valleys (p. 220) and they are not all associated with linear depressions. Many rift valleys lie in zones of tension in the Earth’s crust, as in the Great Rift Valley of East Africa, the Red Sea, and the Levant, which is the largest graben in the world. Grabens may be very deep, some in northern Arabia holding at least 10 km of alluvial fill. Rift valleys are commonly associated with volcanic activity and earthquakes. They form where the Earth’s crust is being extended or stretched horizontally, causing steep faults to develop. Some rift valleys, such as the Rhine graben in Germany, are isolated, while others lie in graben fields and form many, nearly parallel structures, as in the Aegean extensional province of Greece. A half-graben is bounded by a major fault only on one side (Figure 6.18b). This is called a listric (spoon-shaped) fault. The secondary or antithetic fault on the other side is normally a product of local strain on the hanging wall block. Examples are Death Valley in the Basin and Range Province of the USA, and the Menderes Valley, Turkey.
A horst is a long and fairly narrow upland raised by upthrust between two faults (Figure 6.19a). Examples of horsts are the Vosges Mountains, which lie west of the Rhine graben in Germany, and the Black Forest Plateau, which lies to the east of it. Tilted or monoclinal blocks are formed where a section of crust between two faults is tilted (Figure 6.19b). The tilting may produce mount ains and intervening basins. In the Basin and Range Province of the western USA, these are called tilt-block mountains and tilt-block basins where they are the direct result of faulting (Plate 6.9).