Mechanisms of crustal deformation

Abstract

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.

A heat pipe mechanism for volcanism and tectonics on Venus

“…Regio is about 80 m. The geoid anomaly AN associated with Airy compensation is given by [Turcotte andSchubert, 1982, p. 2251. where G is the gravitational constant and hco is the reference crustal thickness corresponding to the reference geoid.…”

“…Turcotte (1983) has proposed an alternative mechanism for delamination, lithospheric stoping in which the lithosphere fails along pre-existing zones of weakness and blocks of lithosphere break away and sink into the mantle. [1983] have suggested that delamination occurs at island arcs.…”

Lithospheric scale gravitational flow: the impact of body forces on orogenic processes from Archaean to Phanerozoic

“…Assuming local isostatic compensation, and disregarding flexural stresses and shear tractions at the base of the lithosphere, the gravitational stress is the depth average of (s zz 2 s yy ). It is the difference between the averaged lithostatic stress under the deformed lithosphere and the averaged lithostatic stress determined in the lowland region down to the same depth, i.e., to the base of the deforming lithosphere (Artyushkov 1973;Turcotte 1983;Molnar & Lyon-Caen 1988). The depth-averaged strength at the imposed nominal strain rate is js zz 2 s xx j, the effective differential stressdriving convergence.…”

“…The Argand Ratio (AR) can be defined as the local ratio of the gravitational stress (arising from lateral variation of gravitational potential energy and thus of density) to the averaged lithospheric strength at a nominal strain rate (i.e., the stress-driving deformation). The ratio AR may be compared with the Argand Number (Ar) of England & McKenzie (1982, 1983), a global parameter by which one can quantify the overall regional balance between gravitational stress and viscous stress in an indentation problem (see also Houseman & England 1986;Buck & Soukoutis 1994;Schmalholz et al 2002Schmalholz et al , 2005. AR is obtained from Ar by replacing the nominal buoyant stress factor in Ar with the locally variable buoyant stress (relative to a reference column), and similarly replacing the nominal viscous stress scale factor in Ar with the locally variable strength of the lithosphere.…”

“…Under this tectonic driving force, thin sheet deformation (England & McKenzie 1982, 1983 occurs, and the tectonic stress balance changes with the thickness of the deforming lithosphere. Disregarding erosion and sedimentation, the lithospheric column changes under the action of (1) a triaxial state of stress, (2) local isostasy, and (3) thermal relaxation.…”