Abstract. We show that a new subduction zone can initiate at a preexisting dipping fault zone with reasonable plate forces, consistent with what is observed in the western Pacific. The dynamics of subduction initiation within a viscoelastic medium has been systematically explored with the finite element method. We investigate the compression of oceanic lithosphere with both force and velocity boundary conditions and track the thermal structure in which heat is transported by both advection and diffusion. The viscosity of the medium is non-Newtonian and temperaturedependent. We also examine the influence of a perfectly plastic yield stress. A new method to model the preexisting weak zone as a graded fault with remeshing is introduced. The first 400 km of plate convergence is examined during which time there are profound changes in plate boundary dynamics. Initially, topography across the plate boundary is characterized by over 1000 m of uplift close to the trench. Subsequently, a bathymetric depression develops on the overriding plate with an initial subsidence rate of 125 m/Myr for a slab subducting at 2 cm/yr. Tectonic subsidence of the overriding plate is primarily dependent upon the depth of slab penetration. For a plate force of 4x10 •2 N/m, subduction initiates even with a fault shear stress of 3 MPa, the resistance most consistent with seismological observations. However, subduction will not initiate when this fault shear stress is >5 MPa. Our results are consistent with plate reconstructions, which predict the initiation of subduction across preexisting weak zones within oceanic basins.

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Forward and reverse 2D flexural modelling of syn-rift and post-rift stratigraphy, has been used to investigate departures from McKenzie post-rift subsidence for the Cretaceous-Tertiary following Late Jurassic rifting in the northern North Sea Basin. The analysis shows uplift in the Palaeocene of 375–525 m relative to calibrated McKenzie subsidence curves, followed by a rapid decrease in uplift in the Early Eocene of 160–310 m. Both Palaeocene uplift and Eocene subsidence increase northwards. The timing, distribution and magnitude of regional Palaeocene uplift is consistent with an origin relating to the development of a mantle plume between 65 and 55 Ma. Transient regional Palaeocene uplift in the northern North Sea Basin was produced by long wavelength dynamic uplift generated by the flow field of a mantle plume. Heating of the asthenosphere or lithosphere beneath the North Sea by hot plume material, or magmatic underplating are not believed to have contributed to Palaeocene uplift in the northern North Sea Basin. Rapid Early Eocene subsidence was generated by a decrease in dynamic uplift caused by a reduction in plume activity. The plume responsible for North Sea Palaeocene uplift may have been the early Iceland plume or a distinct British Tertiary Igneous Province plume related to the same mantle convection event.

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