| Abstract: | The lithosphere is the cold conductive boundary layer formed by cooling of the oceanic crust and upper mantle as it is convected away from oceanic ridges. Although its rheological properties vary continuously with depth, the lithosphere is conveniently divided into an upper elastic layer and a lower plastic layer, the latter overlying a zone of viscous flow. Chemically the lithosphere is vertically zoned with its uppermost part formed by variously hydrated oceanic crust; at M this overlies highly depleted dunite or harzburgite passing downwards over 50 km or so into garnet lherzolite. The vertical variation in density, and thus the gravitational stability of the lithosphere, is controlled by interplay of compositional variation and temperature distribution.As it enters an oceanic trench the lithosphere flexures elastically and plunges downwards at an average inclination close to 45°. During its descent it undergoes dissipative heating at its upper surface. Initially this heating drives a series of prograde metamorphic reactions in the oceanic crust ; because these are largely endothermic, the descending lithosphere heats less rapidly than previously expected, an effect which may be enhanced by percolation of the water of dehydration.Although it is commonly assumed that dehydration water is released upwards, it is not clear that this is true in the presence of the strong negative temperature gradients at the top of the slab, and water may initially be driven downwards into the slab to be released later at much greater depth. The magmatic activity which is associated with the partial melting of the uppermost part of the slab and with partial fusion of diapiric masses in the mantle above it, is critically dependent on the behaviour of the water carried down by the subduction process.The slab itself undergoes a series of phase changes during its descent some of which make a major contribution to the body force during subduction. By the time it reaches 700 km the slab has undergone significant thermal erosion, but the major compositional inhomogeneities within it are retained by the mantle into which it merges. |
