POSTRIFTING ANELASTIC DEFORMATION AROUND THE SPREADING PLATE BOUNDARY, NORTH ICELAND .2. IMPLICATIONS OF THE MODEL DERIVED FROM THE 1987-1992 DEFORMATION FIELD

A decade-long rifting episode that began in the Krafla volcanic system , north Iceland, in 1975 caused substantial, regional, postevent anela stic deformation. This was modeled as viscous relaxation in an elastic /viscoelastic structure by Hofton and Foulger [this issue]. The result s from modeling the deformation detected in north Iceland have far-rea ching implications both for local and regional processes and for the f undamental behavior of deformation around spreading plate boundaries i n general. Tilt in the vicinity of the Krafla volcano fits the model w ell after 1988/1989 which suggests that the volcano magma chamber stop ped inflating/deflating in 1988. A viscosity of 0.8 x 10(18) Pa s was required to match the local tilt data, less than that predicted for no rth Iceland as a whole. Vertical motion measured using the Global Posi tioning System (GPS) 1987-1992 around the ice cap Vatnajokull is incon sistent with isostatic uplift. Using the elastic-viscoelastic model to predict motion in other regions of Iceland suggests that the deformat ion effects of the Krafla episode are significant in many parts of Ice land and should be taken into account when modeling deformation there. Though not a realistic plate boundary model, interesting complexities of the elastic-viscoelastic model are highlighted by deformation mode ling of an infinitely long dike. This predicts that the amount of hori zontal displacement close to the dike may exceed the amount of initial dike opening early in the spreading cycle. A more realistic approxima tion to the plate boundary in north Iceland, involving five overlappin g segments that experience dike emplacements at discrete intervals, su ggests that the width of the zone within which transient, time-depende nt deformation occurs may be several hundred kilometers wide, consider ably wider than the neovolcanic zone. A kinematic approach to describi ng plate motions is not appropriate close to spreading plate boundarie s and elsewhere where the viscosity of the Earth is low.