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Journal of the Geological Society

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Mechanisms of crustal deformation

Thirty-sixthWilliam Smith Lecture

The energy source for crustal deformation is isotopic heating and secular cooling of the mantle. In a true solid, heat would be lost to the surface by conduction; however, solid-state creep processes allow the Earth’s solid mantle to exhibit a fluid behaviour. Thus, thermal convection can convert heating into directed motion. Variations in temperature lead to variations in densitythrough thermal expansion and contraction. Although the resultant body forces are vertical, horizontal variations in temperature lead to horizontal body forces. At an ocean ridge the variations in the thickness of the lithosphere lead to the elevation of the ridge. The result is a substantial horizontal ridge push force.This force, along with the trench pull force on the descending lithosphere is the primary force driving plate tectonics. Lithospheric thinning must occur beneath continental rifts and plateau uplifts. I suggest that lithospheric thinning is caused by the diapiric penetration of hot asthenospheric rock to the crust/mantle boundary. This penetration may also cause lithospheric stoping and plateau uplifts. Lithospheric thinning leads to large tensional stresses above the zone of thinning. These forces are generally much larger than the compressional forces generated by crustal thinning. The forces generated by lithospheric thinning are quite large and may be responsible for the propagation of continental rifts. In island arcs and Andean type orogenic belts both lithospheric thinning and crustal thickening result in large tensional forces. Probably the most complex zone of crustal deformation is the continental collision. Variations in types of continental collisions are discussed.

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