Lithospheric necking and regional isostasy at extensional basins 2. Stress‐induced vertical motions and relative sea level changes

Abstract

We investigate long‐wavelength differential vertical motions at extensional basins which are the result of temporal changes in the intraplate stress field in the lithosphere. These stress‐induced vertical motions are intimately coupled to the state of flexure (flexural curvature) of the lithosphere. We compare a previous model of flexure under the sediment load only (FSL model) and a recent more consistent model, which incorporates regional compensation of other vertical loads induced by lithospheric extension as well. These additional loads are controlled by the depth at which the necking of the lithosphere occurs (DON model). Tlie FSL model and models assuming a shallow level of necking (≤15 km) are very similar, they predict tilting of the basin with the hinge point landward of the shelf edge. Compression results in uplift of the landward pan of the basin, which generally corresponds to the coastal plain or inner to middle shelf, and downwarping of the outer shelf and slope. The differential vertical motions correspond to relative sea level cycles which are out of phase in different parts of the basin. Internal architecture of systems tracts can be different from predictions by eustasy. By contrast, deeper levels of necking (>20 km) predict a uniform sense of motion landward of the shelf edge. Type I sequence boundaries can be very easily induced for these conditions, and sequence stratigraphie features are probably hard to distinguish from predictions by eustasy. These results are for a pure elastic plate rheology. Stress‐induced vertical motions for a more realistic depth‐dependent rheology are more complicated because of the additional effect of stress‐induced reduction of flexural rigidity. For these conditions, gradual buildup of tension causes a complete relative sea level cycle. For high tensional stress levels, total vertical motions become rather similar to vertical motions for compression. This may be of importance for the issue of global synchroneity of relative sea level changes.