Lithosphere extension

BMT provides various options for modelling lithospheric thinning;  standard McKenzie type thinning, modified McKenzie or Wernicke type thinning. The modelling can be done with or without 'depth of necking'. The necking depth is defined as the level within the crust that remains horizontal during thinning in the absence of isostatic forces. This level determines the ratio between thinning of the upper crust, where crustal material is replaced by sediments and/or water and/or air, with generally low densities, and thinning of the lower lithosphere, where crustal material is replaced by dense mantle material.

In kinematic models not specifically defining the necking depth, it is mostly implicitly set to a depth of 0 km, and crustal thinning occurs by uplift of the Moho followed by an (local or regional) isostatic response. For larger necking depths, thinning takes place not only by uplift of the Moho but also by subsidence of the surface. This has a major effect on the load acting on the lithosphere and therefore on the resulting state of flexure.

In spite of the fact that the level of necking does not have a unique meaning in terms of mechanics, we argue that kinematic models including necking depth as a modelling parameter have a significant added value compared to models that do not. The importance of incorporating appropriate necking depths in models used for estimating thinning factors becomes apparent from the figure below, which shows the relation between necking depth, effective elastic thickness (EET) and the structure of the crust, cf. Fjeldskaar et al. (2004) for more details.

The relation between necking depth, effective elastic thickness (EET) and the structure of the crust.

Fjeldskaar, W., ter Voorde, M., Johansen, H., Christiansson, H.P., Faleide, J.I. and Cloetingh, S.A.P.L., 2004. Numerical simulation of rifting in the northern Viking Graben: the mutual effect of modelling parameters. Tectonophysics 382, 189-212.