morphotectonics

morphotectonics Morphotectonics combines shape and tectonic activity to explain landforms mainly at continental and sub-continental scales, whereas structural geomorphology considers the passive influence of geologic structure on landforms. Morphotectonics studies the direct effect of Earth movements on landform evolution. It includes the study of mountains, erosion surfaces, drainage patterns, and sediments on land and offshore.

Before the 1960s, morphotectonics concentrated on, for example, the influence of faulting on landforms, and uplift as a mechanism for raising large areas, which then eroded to form erosion surfaces. These have now been joined by studies of the relationships between global tectonics and geomorphic evolution. The morphotectonic development of landforms takes place on the same timescale as plate tectonics. C. D. Ollier has proposed the term ‘evolutionary geomorphology’ for this part of Earth science.

Morphotectonics requires input from at least three branches of Earth science: geophysics, geology, and geomorphology. Until the 1980s, tectonics was largely dependent on the first two, and the directly observable evidence of landforms was neglected. Landforms are, however, much easier and cheaper to study than the deeper parts, as observations are direct and not made by interpretation of distant geophysical signals.

The contrast between active and passive plate margins is one way to subdivide the study of morphotectonics. Along many active margins, plates converge, leading to subduction, uplift, and formation of mountains such as the Andes and the Himalayas. Passive margins are where continents or cratons have rifted to form rift valleys and then trailing edges. They are also often the location of highlands, such as the Appalachians in the North America, the Drakensberg in Africa, and the Eastern Highlands of Australia.

The orogenic (mountain-building) processes that thrust and fold rocks are very different from the vertical uplift that provides the elevation for erosional development of mountains. When large areas are uplifted high enough, plateaux or mountains are formed; for example, the Andes, often cited as an example of mountains formed by subduction and associated folding. However, the Andes appear to have formed by vertical uplift and faulting followed by intense erosion, on both sides, during the latter half of the Tertiary. Vertical uplift and warping of large areas are also features of the Himalayas, demonstrated by convex river terraces along the major rivers. The fact that the same major rivers rise on the Tibetan Plateau and flow through the highest mountain range on Earth is important morphotectonic evidence that the rivers existed before uplift of the Himalayas.

Figure 1 gives an example of major morphotectonic features of a passive margin and their interpretation. Inland areas are maintained as a palaeoplain that was formed before continental break-up. The palaeoplain, which is little eroded and retains ancient drainage lines, is equated with the basal unconformity. Erosion of the landmass is concentrated in the near-coastal zone, which is backed by a great escarpment (see escarpments). The great escarpment is embayed by major valleys, and a belt of mountains lies between it and the coastal plain. The mountains do not rise above the level of the projected palacoplain. Outlying plateaux and coastal facets are remnants of the palaeoplain. A warp axis in the zone of maximum uplift forms a drainage divide, and may be revealed by drainage modification. In this diagram, drainage before formation of the passive margin was in the direction from sea to land. The great escarpment retreats and may reach or pass the warp axis.

Exotic terranes have important morphotectonic implications, because they bring together continental fragments that have very different geomorphic histories. If this is the case, as has been suggested for New Guinea, it may be useless to try to match erosion surfaces or river patterns, even in neighbouring valley sides, if they have evolved in quite different ways.

Morphotectonics can make an important contribution to theories of continental evolution by placing constraints of age and processes on models developed using geology and geophysics.

Colin F. Pain

Bibliography

Ollier, C. D. (1981) Tectonics and landforms. Longman, London.