ABSTRACT
Over the past 20 years much work has gone into understanding the mechanisms of lithospheric extension, and producing models to explain the geometry and evolution of rift basins. These models have tended to simplify the deformation occurring within the upper crust by assuming that extension occurs either by pure shear (such as the uniform extension model) or exclusively along a few large basin-bounding faults (such as the flexural cantilever model). However, the stratigraphic geometry of many basins cannot be explained by these simple models; for example, the common occurrence of thick sequences on the footwalls of basin-bounding faults. This requires the generation of accommodation space by subsidence. However, the flexural cantilever model predicts uplift, while the uniform stretching model cannot model the effects of faults at all. This paper looks at the St. George’s Channel Basin, which has been mapped in detail using an extensive seismic dataset, and attempts to explain the structure and stratigraphic geometry of the basin by applying forward modelling techniques to four cross sections taken from different areas of the basin. This has allowed the determination of the detailed mechanisms by which extension has occurred within the upper crust, and of variations in these both spatially and through time. Results from the modelling indicate that a combination of extension on basin bounding faults and pure shear distributed across the basin is required to explain the geometry of the basin, although the pure shear component of extension probably represents extension on numerous sub-seismic scale faults across the basin. The relative influence of these different mechanisms of extension varies considerably across the basin, and throughout its history. During the Triassic, for example, up to 80 % of total extension in the brittle upper crust occurred by pure shear. During the Middle and Upper Jurassic, however, extension in the upper crust occurred mainly on a single fault (the St. George's Fault) in the basin centre, but on the edges of the basin, where the fault died out, pure shear extension was dominant.
