PLATE DYNAMICS NEAR DIVERGENT BOUNDARIES - GEOPHYSICAL IMPLICATIONS OF POSTRIFTING CRUSTAL DEFORMATION IN NE ICELAND

The bulk of a tectonic plate is thought to move continously at a rate consistent with the geologic average. On the other hand, movements are highly episodic at plate boundaries. We study the plate dynamics that relate to these two different modes by modelling the displacements ob served using the global positioning system in Northeast Iceland 1987-1 990. These observations were made about 10 years after an episodic div ergent movement between the North American and Eurasian plates 1975-19 81. The horizontal displacement field fits well a two-dimensional mode l of postrupture stress relaxation assuming a thin elastic layer overl ying a layer of Newtonian viscosity. This analysis indicates values of about 10 m2/s for the stress diffusivity and 0.3-2 X 10(18) Pa s for the Newtonian viscosity of the lower layer. However, no significant co rrelation exists between the observed and modeled vertical displacemen ts probably because of the relative inaccuracy of the vertical compone nt observations. Assuming that contemporary plate motion is the sum of many displacements that have diffused from boundaries where episodic displacements occur periodically, we simulate the spatial transition f rom episodic to continuous plate movements. The plate ''boundary zone' ' where movements are episodic or quasi-episodic is of the order of 10 0 km wide, depending on the stress diffusivity (which may be fairly un iform throughout the world) and the frequency of episodic movements.