2.3.5. Lithosphere deformation behaviour

The flow properties of mantle rocks control the thickness and strength of the lithospheric plates, the degree of coupling between moving lithospheric plates and the pattern and rate of asthenospheric convection, and the rate of melt extraction at mid-ocean ridges. To be able to understand the dynamic behaviour of the outer parts of the solid Earth, notably the dynamics of lithospheric extension and associated rifting and sedimentary basin development, a detailed knowledge of the rheology of the upper mantle (30-410 km depth) and between the 410 and 670 transition zones is essential. At present, these flow properties are surprisingly poorly known. Experimental work has yielded constitutive equations describing various types of flow in mantle rocks, but it is not clearly established to what extent the experimentally observed flow mechanisms are relevant for natural crust and mantle conditions. A second problem is that trace amounts of water and melt can cause drastic weakening effects of mantle rocks. Such fluid-related weakening effects are widely recognized as important, for example in controlling the strength of trans-lithospheric faults underlying developing sedimentary basins. However, only limited data exist on such effects, and a quantitative, mechanistic understanding suitable for extrapolation to nature is lacking.

These problems can be addressed by means of experimental studies, scanning and transmission electron microscopy (SEM, TEM) and field studies of exposed upper mantle rocks. These approaches will be integrated with the aim of arriving at quantitative, mechanism-based descriptions of mantle rheology suitable for use in modelling the dynamics of the upper mantle and transition zone. Field-based studies involving structural geological and EM work on upper mantle rocks deformed in a variety of geological environments will be used to identify the flow mechanisms actually occurring in the upper mantle. Special attention will be paid to upper mantle rocks showing possible asthenosphere flow structures produced when the rocks contained some fluid or partial melts. Work will also continue on upper mantle shear zone rocks since such zones may play a major role in controlling the lithospheric strength during extension.