Fifty‐one new and 309 published thermochronometric ages (nine systems with closure temperatures ranging from ~450 to 70°C) from the Graubünden region of the Central Alps demonstrate that a pronounced thermal mismatch between the Austroalpine allochthon (Alpine “orogenic lid”) and the Pennine zone persisted until at least 29 Ma and, allowably, until circa 18 Ma. The observed mismatch supports previous suggestions that the famous “overthrust” between the Austroalpine allochthon and the Pennine zone, historically regarded as primarily an Eocene top‐north thrust fault, is in fact primarily an Oligocene‐Miocene normal fault that has a minimum of 60 km of displacement with top‐south or top‐southeast sense of shear. Two hallmarks of Alpine geology, deposition of the foredeep Molasse and emplacement of the Helvetic nappes, appear to be coeval, peripheral manifestations of crustal thickening via the interposition of the Pennine zone as a northward intruding wedge between the Austroalpine “lid” and the European cratonic margin, with the Helvetic system (European margin) acting as the “floor” of the wedge. We presume the Penninic wedge is driven by the buoyant rise of subducted crust no longer able to remain attached to the descending slab. If so, emplacement of the Pennine wedge could have occurred mainly after Adria was juxtaposed against cratonic Europe.
Rapid lateral and vertical velocity and thickness variations within Albian carbonates and Aptian evaporites in West African salt basins present considerable problems for seismic imaging and depth modelling. Failure to address these problems has effectively restricted sub-salt exploration success offshore southern Gabon, even with 3D PSDM data processing. Using new 3D data from Dussafu and employing a range of velocity model updating techniques, migration algorithms and geological data we have built a very detailed and accurate velocity model of the complex, stratified Albian carbonate layer. This enabled us to remove some of the artificial deformation evident in earlier processing at Base Salt and below, which resulted from inaccurate velocity models. This successful processing project relied upon close collaboration between the geology and geophysics teams within the operator and the geophysical contractor. This collaboration led to the careful selection of methods to develop a reliable earth model, producing clear seismic images which will be used as a basis for field development.
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