TECTONIC BASINS AND RIFT VALLEYS

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 crusti.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. 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):538542 (Feb. 19, 1971). Peter H. Molnar