tectonic basins and rift valleys

▪ landform

Introduction

      landforms characterized by relatively steep, mountainous sides and flat floors. The steep sides are created by displacement on faults such that the valley floor moves down relative to the surrounding margins, or, conversely, the margins move up relative to the floor. Differences in the elevations of valley floors and surrounding mountains or plateaus range from only several hundred metres to more than 2,000 metres in major rift valleys. The widths of tectonic valleys and basins vary from as little as 10 kilometres to more than 100 kilometres. Their lengths typically are hundreds of kilometres, but range from a few tens to thousands of kilometres.

      The vast majority of tectonic basins and valleys is produced by an extension of the Earth's crust and the subsequent dropping of a block of crust into the space created by the divergence of large crustal blocks or lithospheric plates. The extension of the brittle crust causes it to fracture, and as the adjoining crustal blocks or plates move apart, a smaller block slides down into the resulting gap. The down-dropping of this block between the surrounding fault blocks, which commonly rise during an episode of crustal extension, creates a rift valley or tectonic basin. The geologic term for this type of tectonic depression is “graben,” the German word for “ditch” or “trough.”

      Tectonic depressions also can be produced by horizontal compression of the crust—i.e., by crustal shortening. Two types of compressional tectonic valleys and basins can be recognized: ramp valleys and foreland basins. A ramp valley is analogous to a rift valley but is formed by the margins of the valley being pushed over its floor. A foreland basin, on the other hand, results from a gentle downward bending or flexing of the entire lithosphere.

Rift valleys (rift valley)

 In the simplest case, a rift valley forms when a block of crust, tens of kilometres wide and hundreds of kilometres long, drops down between two diverging lithospheric plates, much as the keystone in an arch will fall if the walls of the arch move apart. This process is responsible for the relatively symmetrical cross sections of most parts of the East African Rift System, where the valley floor lies 1,000 metres or more below the higher plateaus of Ethiopia and Kenya. In some places, the sides of the rift valley make single, steep walls as high as 1,000 metres. In others, the edges of the valleys consist of steps or tiers with each small inner block dropping with respect to its neighbouring outer block. Thus the deepest part of the rift valley is not always at its centre.

      Volcanoes (volcano) mark the axes of some, but by no means all, rift valleys. Where the lithospheric plates separate and the crust is thinned, the underlying parts of the lithosphere in the mantle also must diverge, allowing hot material from the asthenosphere to rise to shallow depths. Some such material from the asthenosphere has erupted at volcanoes within the eastern rift of the East African Rift System in Ethiopia and Kenya and within a small section of the western rift in Congo (Kinshasa). Most of the western rift, which extends from Uganda through Lake Tanganyika and Lake Nyasa (Malaŵi), however, has no volcanoes.

      Many rift valleys are asymmetrical with one steep wall and one gentle side. The steep wall is formed by slip on one or two major faults; however, unlike the simple grabens described above, no major fault bounds the other side of the rift valley. Instead, the other side is formed by a flexing of the lithosphere and by a tilting of the surface. Small faults are common, but over all there is a relatively gentle slope into the rift valley. Death Valley, in California, has a very steep eastern margin and a gentler western edge. The floor of Death Valley is moving down along a fault along its eastern margin and is rotating about an axis west of the valley. Thus, the most rapid sinking is along the valley's eastern edge, where the lowest point in the Western Hemisphere, Badwater, lies 86 metres below sea level. Similarly, the Baikal Rift in Siberia, which contains the deepest lake in the world, Lake Baikal, has a very steep northwestern edge and a gentler southeastern margin.

      Within some rift valleys are narrow ridges (10 to 20 kilometres wide) that are bounded by steep sides, separating the ridges from neighbouring parts of the valleys. A ridge of this kind is called a horst (horst and graben), a block of crust bounded by faults such that the flanks of the range have dropped with respect to it. A horst is the opposite of a graben. The third highest mountain in Africa, Margherita Peak of the Ruwenzori Range (located along the border of Uganda and Congo) marks the highest point on a horst within the western rift of the East African Rift System.

      Horsts can be found in most rift valleys, but unlike the Ruwenzori, they rarely dominate the landscape. The floors of most rift valleys have dropped relative to the surrounding landscape, but the tops of horsts rarely stand higher than the surface outside the valleys. Thus most horsts are merely blocks that have remained at nearly the same height as the unbroken crust outside of the rift valleys. Most horsts exist because rift valleys formed adjacent to them, not because they were elevated.

      Some rift valleys, such as the East African Rift Valley in Ethiopia and Kenya, have formed over large domes. Upwelling of hot material within the underlying asthenosphere not only pushes the overlying lithosphere up but heats it as well, causing it to expand. To some extent the upward bulging of the lithosphere causes it to stretch, and this stretching manifests itself as a rift valley. Rift valleys that have formed in this way are commonly associated with extensive volcanism.

      Certain rift valleys seem to be created by distant forces acting upon the lithosphere. These valleys cannot be associated with large domes, and in general volcanism is rare or absent. The Baikal Rift, for example, seems to be associated with the same forces that are pushing India into the rest of Eurasia. Moreover, though the elevations of the flanks are high (more than 3,000 metres in some places), the overall elevation decreases rapidly to only a couple of hundred metres at distances of just 50 to 100 kilometres northwest of Lake Baikal. Thus, a broad dome is not present.

Basins and ranges

      Some areas, such as the Basin and Range Province of the western United States (Utah, Nevada, and California), contain an extensive network of relatively small tectonic depressions closely akin to rift valleys. The topography consists of basins 10 to 30 kilometres wide and 50 to 200 kilometres long, separated by ranges of similar dimensions. The basins contain young sediment derived from the neighbouring ranges and are quite flat. The sides of the basins can be steep or gentle. Where a major fault separates a basin from a range, the edge of the basin is often steep. Where the edge of the basin is produced by the tilting of the basin down and of the range up, the flank is gentle, with average slopes of from a few to 15°. These tilted, gently dipping slopes are particularly apparent wherever lavas, resistant to erosion in dry climates, had flowed onto the surfaces before they were tilted. Such tilted lava-capped surfaces are known as louderbacks. In sum, the tectonic basins of the Basin and Range Province are similar to rift valleys, but their dimensions are smaller, and the ranges are tilted blocks or horsts.

      Networks of basins and ranges exist in several other high plateaus. Northerly trending basins lace the Tibetan Plateau; however, unlike those of the western United States, they are more widely spaced, occurring hundreds of kilometres apart. Moreover, a single northerly trending range in Tibet does not in general separate neighbouring basins from one another. The development of a basin and range morphology in Tibet is at a much earlier geologic stage than that of the western United States. The landscape of western Turkey likewise is cut by easterly trending basins and neighbouring ranges that were formed by crustal extension in its north–south dimension. This morphology of basins and ranges extends westward beneath the Aegean Sea. Many of the islands in the Aegean are ranges between basins that stand high enough to poke above sea level. Thus, whereas the dominant feature in a rift valley is the deep wide valley itself, the ranges and valleys are of comparable importance in basin and range topography.

Pull-apart basins

      Some tectonic valleys are rectangular or rhomb-shaped basins, bounded by as many as four steep sides. The Dead Sea, the lowest place on Earth, lies 396 metres below sea level at the bottom of just such a basin. Another is the Imperial Valley of southern California, most of which also lies below sea level. These tectonic valleys are closely related to major strike-slip faults—nearly vertical faults along which material on one side moves horizontally with respect to that on the other.

      In regions such as the Dead Sea or southern California, nearly parallel strike-slip faults bound two sides of the tectonic valley and end at the valley. Slip on the overlapping segments of the strike-slip faults results in crustal extension in the region between the two faults. Thus two sides of the tectonic valley are bounded by faults with primarily horizontal displacement, and the other two sides are bounded by faults with vertical components of slip. These basins are called pull-apart basins because the crust is literally pulled apart in the section between the two strike-slip faults.

Ramp valleys

      As previously noted, these depressions are similar to rift valleys, but they have been formed by the opposite process—crustal shortening. A ramp valley develops when blocks of crust are thrust toward one another and up onto an intervening crustal block. The latter is forced down by the weight of this material, resulting in the formation of the valley. The thrusting of the material onto the intervening crustal block creates high mountains adjacent to the valley.

      Ramp valleys are characterized by steep sides tens of kilometres apart, and flat floors, which contain debris eroded from the neighbouring mountains. Escarpments on the edges of ramp valleys are not as sharply defined as for simple rift valleys, but the surrounding mountains can be higher than those that bound the latter. To a casual observer, the landscapes of ramp and rift valleys are very similar. In fact, early theories for rift valleys incorrectly attributed their origin to that of ramp valleys.

      The most spectacular example of a ramp valley is the Turfan Depression, the second lowest place on Earth (154 metres below sea level), which lies within the Tien Shan of western China and along the northern margin of the Gobi. In general, the rapid filling of ramp valleys in all but the most arid climates makes them ephemeral features; however, small, young ramp valleys can be found in the South Island of New Zealand east of the Southern Alps, and remnants of ramp valleys lie within the Rocky Mountains of the western United States.

Foreland basins

      These lie in front of major mountain ranges—e.g., south of the Himalayas, north of the Alps, and east of the Canadian Rocky Mountains. Most basins of this kind are subsurface features, filled with sediment eroded from the adjacent mountain ranges; thus, they are not easily recognized in the flat landscape that is visible. Foreland basins are formed because the overthrusting of the mountains onto a neighbouring lithospheric plate places a heavy load on the plate and flexes it down, much as a diving board is flexed down by the weight of the diver. Foreland basins are deepest and young sediments are thickest next to the mountain range, and the thickness of material decreases gradually and smoothly away from the mountains. The rapid deposition of sediment from the mountains makes a nearly flat surface, such as the Indo-Gangetic Plain of northern Pakistan and India where the Indus and Ganges rivers flow south of the Himalayas. Foreland basins can be important sites of oil and gas reserves.

Peter H. Molnar

Additional Reading

B.H. Baker, P.A. Mohr, and L.A.J. Williams, Geology of the Eastern Rift System of Africa (1972), provides a summary of the eastern branch of the East African Rift System. More specific treatment is found in B.H. Baker and J. Wohlenberg, “Structure and Evolution of the Kenya Rift Valley,” Nature, 229(5286):538–542 (Feb. 19, 1971). Also, a useful series of reviews can be found in G. Pàlmason (ed.), Continental and Oceanic Rifts (1982).Peter H. Molnar

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