K. Heki et al., PLATE DYNAMICS NEAR DIVERGENT BOUNDARIES - GEOPHYSICAL IMPLICATIONS OF POSTRIFTING CRUSTAL DEFORMATION IN NE ICELAND, J GEO R-SOL, 98(B8), 1993, pp. 14279-14297
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.