SummaryThe pattern of crustal warping in northern Spitsbergen as a result of widespread deglaciation is plotted. This is facilitated by the occurrence on ancient strandlines of pumice fragments at 4 major horizons14C dated at 6500 (A), 6200 (B), 4100 (C) and 2200 yearsb.p.(D). Trace element analyses indicate an origin for the pumice in the Jan Mayen area. At least 2 pumice horizons are thought to be contemporaneous with eruptions at 6500 and 4100 yearsb.p.

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SUMMAR YA high-resolution global tomographic inversion of P-wave traveltimes has been undertaken utilizing an a priori model. The results image cylindrical slow velocities in the upper and lower mantle beneath many current hotspot locations. Locations at which such plume-like features are imaged passing from the uppermost lower mantle to the upper mantle include, Afar, Society Islands, Crozet, Kerguelen, Iceland, Hawaii, East Africa, Cape Verde and the Canary Islands. The validity of these images has been investigated with synthetic recovery tests. These images suggest that these plumes could be from the lower mantle and therefore are not hindered in crossing the upper/lower mantle boundary.

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[1] Seismic studies of the deep mantle suffer from the fact that the probing seismic waves must traverse the highly heterogeneous and poorly resolved shallow structure. One potential way forward is to develop highresolution models of the crust and upper mantle using other information. Here we describe the construction of a geodynamic a priori model of some aspects of upper mantle seismic velocity heterogeneity. It is based on an equal area tomographic grid and it has been produced at two scales, a 1°Â 1°resolution at the equator, (i.e., each cell has an approximate dimension of 100 km by 100 km), and a 5°Â 5°resolution at the equator. Both have a constant layer thickness of 100 km. Currently, the model accommodates the subducting lithosphere and global variation in continental crustal thickness and age of oceanic lithosphere. The shape of subducting oceanic lithosphere was derived from profiles through seismicity. The shape was combined with estimates of plate velocities and age of subducting lithosphere using an analytic solution of the thermal field to define the slab thermal anomaly. The temperature perturbation was converted to a slowness (1/velocity) perturbation. For oceanic lithosphere, a plate-cooling model was used to convert lithosphere age to slowness perturbation via a temperature perturbation. The variation in the thickness of continental crust, around a global average, formed the third element of the slowness perturbation model. This model has already been applied in a high-resolution mantle tomographic study of lower mantle heterogeneity.