escape tectonics The term ‘escape tectonics’ refers to the lateral extrusion of fault-bounded geological blocks as a result of compression. This extrusion can occur on all scales, ranging from small-scale faults, with only centimetres or metres of displacement, to large-scale crustal faults, with hundreds of kilometres of displacement, such as the Red River Fault in Asia (Fig. 1). The classic example of escape tectonics has long been held to be the eastward extrusion of parts of south-east Asia as a response to the collision and northward motion of India into Asia. Initial collision of India with Asia occurred about 50 million years ago; geophysical evidence, derived from the pattern of magnetic stripes in the Indian Ocean, indicates that the northward motion of India, with respect to Asia, slowed from about 100 mm a year to about 50 mm a year at this time. This lower rate continues to the present day. As a result, almost 2500 km of convergence between the two has occurred since collision, shortening and thickening the crust to produce the Himalayan mountain chain and the Tibetan Plateau.
Estimates of the amount of crustal mass beneath the Tibetan Plateau are, however, about 30 per cent too little to account for all the shortening that must have taken place. In addition, many of the large earthquakes that affect the region occur, not at the boundary between the Indian and Asian plates, but within the Asian plate. In the 1970s a number of workers drew on these lines of evidence, and on the recognition of large-scale strike-slip faults (hundreds of kilometres in length) in south-east Asia from then newly available satellite imagery. As a result, they proposed that the balance of the missing material from the India–Asia collision must have been extruded or ‘escaped’ eastwards (Fig. 1). The escape of this material is thought to have been accommodated by movements on the large strike-slip faults that cut across south-east Asia, and is thought to be responsible for much of the Cenozoic deformation of the region. Supporting evidence for this extrusion model was obtained from a series of laboratory experiments using a rigid indentor (representing India) and plasticine (representing Asia) which produced a series of ‘slip lines’ of material motion, generated by the indentor penetrating into the Asian plate, in a pattern strikingly similar to the pattern of faults observed in south-east Asia.
More recent work, however, has raised some concerns about the validity of applying the concept of‘escape tectonics’ to the large-scale Cenozoic evolution of south-east Asia. The extrusion of south-east Asia requires that the continental fragments behave and deform as rigid (or semi-rigid) plates. While such an assumption may be valid over short timescales, over the longer timescale of the India–Asia collision and the Cenozoic deformation of south-east Asia continents are thought to behave in a more viscous manner and ‘flow’ rather than act as rigid blocks. Computer models of India–Asia collision, which treat Asia as being viscous rather than rigid, indicate that there is very little effect on the tectonics of the region beyond the length of the collision zone. Studies of slip rates on active faults indicate that continental blocks within Tibet are indeed moving eastward at rates of some 10–30 mm per year, as predicted by the extrusion model. These measurements, however, provide only a snapshot of the rates at a particular time, and few of the measured faults have been continuously active since the time of collision. The cumulative motions measured on the larger faults in the area, from the offsets of key well-dated rock units, while large, fall short of the total predicted by a model of ‘pure’ escape tectonics.
A definitive test of such large-scale models of escape tectonics is not currently possible, given that much more information needs to be gathered on the motion histories of the large crustal-scale faults. It can be stated, however, that some extrusion does indeed appear to occur. It may well be that escape tectonics on a short timescale may be part of a longer-term and larger-scale process of viscous flow of continental material.
Conall Mac Niocaill
Bibliography
Royden, L. H. and and Burchfiel, B. C. (1997) The Tibetan Plateau and surrounding regions. In van der Pluijm, B. A. and Marshak, S. (eds) Earth structure: an introduction to structural geology and tectonics, pp. 416–23. WCB/McGraw-Hill, New York.
Tapponier, P.,, Peltzer, G.,, Le Bain, A. Y.,, Armijo, R.,, and and Cobbold, P. (1982) Propagating extrusion tectonics in Asia. New insight from simple experiments with plasticine. Geology, 10, 611–16.
