Extension and subsidence of the continental lithosphere
The uniform stretching model successfully accounts for the general features of many extensional sedimentary basins. However, the amount of extension measured across normal faults in the upper crust is often thought to be significantly less than that calculated from subsidence analysis and crustal thinning. At present, more complicated models, which incorporate two-layer stretching, multiple stretching phases and flexural rigidity, are used to explain this extension discrepancy. The principal aim of this dissertation is to show that the extension discrepancy can be resolved in the northern North Sea without abandoning the uniform stretching model. Other observations are also explained by minor changes to the model. Basin evolution is addressed both on a small and on a large scale. A kinematic model for hanging wall deformation, which is assumed to occur by arbitrarily inclined simple shear and by differential compaction, is proposed. Fault geometries can be calculated from sedimentary horizons within hanging walls using an inversion scheme based on this model. Results suggest that hanging wall shear is inclined towards the main fault. This implies that the amount of extension across a fault is considerably greater than the apparent horizontal displacement. Syn-rift footwall uplift is explained by combining the simple domino-style fault model with the uniform stretching model. The 'steer's head' cross-sectional geometry of sedimentary basins is usually explained either by fluctuations in sea-level or by increasing flexural rigidity of the continental lithosphere during post-rift cooling. Here, a two-layer stretching model is proposed, where the lithospheric mantle is stretched over a fractionally wider region than is the crust. This accounts for the observed extent of post-rift stratigraphic onlap in the North Sea and does not alter conclusions concerning the extension discrepancy. The geometrical and thermal consequences of lithospheric simple shear are investigated using a numerical model. Results predict that, as for the uniform stretching model, crustal thinning is symmetrical about the basin. Maximum thinning is also coincident with maximum subsidence. However, the magnitude of post-rift subsidence varies across the basin, allowing the uniform stretching model and the lithospheric simple shear model to be distinguished. The different models described here have been applied to regional seismic reflection profiles and well-log information from the northern North Sea. On the best constrained profile, the extension measured across normal faults agrees well with that calculated by subsidence analysis. The major observations are thus consistent with the predictions of the uniform stretching model.