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Mechanics, Dynamics and Geometry of Basin Inversion |
Bullard Laboratories, Madingley Rise, Madingley Road, Cambridge CB3 0EZ, UK
There is now general agreement that many sedimentary basins on the northwest continental shelf of Europe underwent permanent exhumation during the Tertiary. The most dramatic indicator of this process is the present-day absence of up to 3 km of anticipated post-rift subsidence in the midlands of Britain and in the East Irish Sea. Any explanation must take into account the fact that the entire shelf has very small, long wavelength, free-air gravity anomalies. This constraint is of fundamental importance and implies either that the crust has been thickened, that phase changes have occurred within the lithosphere, or that low density material has been added to the lithosphere. Tertiary epeirogenic uplift and exhumation is often attributed to horizontal shortening which is assumed to be related in a general sense to Alpine mountain building. However, the removal of 3 km of sediment from a basin, which was originally 100 km wide, requires 20–30 km of shortening. Whilst minor Tertiary shortening is observed all over the continental shelf, nowhere is it sufficient to account for the inferred amount of denudation. More significantly, exhumation is thought to have commenced in the Early Tertiary and dramatically increases from south to north. Shortening is generally younger (mid-Tertiary) and decreases in intensity from south to north. Here we argue that Tertiary uplift and denudation are a consequence of regional igneous underplating. At the beginning of the Tertiary, rifting associated with the initiation of the Iceland plume generated substantial volumes of melt. Petrological arguments and the results from inversion of rare earth element concentrations of MgO-rich igneous rocks suggest that a minimum of 2–5 km of melt were produced beneath a substantial part of the continental shelf. We infer that much of this melt was trapped within the lithosphere, presumably close to the Moho, which would have acted as a density filter. Such underplating will have caused rapid surface uplift whilst maintaining isostatic equilibrium. Simple calculations based on deep seismic reflection data and the high P-wave velocities observed beneath Scotland are consistent with the petrological arguments.
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