Chapter 4 - Plate boundaries and driving mechanisms

Abstract

A classification of the boundary zones between pairs of plates is based on the changes in surface area that occur as the plates diverge from, converge with, or move parallel to, the plate boundary zone. Plate boundaries may thus be termed constructive, destructive or conservative. In continents, constructive plate boundaries produce rifts and, eventually, extended continental margins, whereas mid-ocean ridges act at divergent oceanic plate boundaries. The detailed and regional structures of rifts and ridges depend on the balance and distribution of normal faulting and magmatism, the two means by which newly created area is accommodated. Destructive plate boundaries involving oceanic lithosphere can maintain narrow surface trenches by virtue of being able to accommodate lost area downwards into the mantle by subduction. In continents, destructive plate boundaries form broad high elevation zones of high complexity across strike where the inability to subduct large amounts of continental crust means a slab cannot retain any locational relationship to a surface fault. Conservative plate boundaries form relatively simple troughs in the absence of preexisting structure; the presence of preexisting structures tends however to give rise to considerable along-strike complexity. Plate motion is ultimately a consequence of convection in the mantle, but it does not simply serve to horizontally transport mantle material between sites of convective upwelling and downwelling. Instead, the balance of forces driving plates seems to consist of gravitational responses to the thicknesses and density structures of plates on and in the uppermost mantle and viscous shear forces raised at transform plate margins and the bases of the plates. The pattern and significance of basal shear may be modulated by the distribution of upwelling sites in the lower mantle, to which plate motion contributes via the long-term distribution of subduction zones.