A third Global Positioning System (GPS)
survey of a regional network surrounding
the Krafla volcanic system, north Iceland, was
conducted in 1992 following a major crustal spreading
episode which began in this system in 1975. Differencing
the 1992 results with those from 1987 and 1990 reveals a
regional deformation field with a maximum, rift-normal
expansion rate of 4.5 cm/year near the rift, decreasing to
3 cm/year at large distances. The time-averaged spreading
rate in north Iceland, 1.8 cm/year, cannot account for this
deformation. The vertical deformation field reveals
regional uplift throughout the network area at its
maximum closest to the rift and decreasing with distance.
Three different models are applied to study the postdike
injection ground deformation: (1) stress redistribution
in an elastic layer over a viscoelastic half-space, (2)
stress redistribution in an elastic-viscous layered
medium, and (3) continued opening at depth on the dike
plane in an elastic half-space. Using model 1, the
effects of historical episodes in the region are
subtracted from the observed displacement fields and
the remaining motion is modeled as relaxation following the
recent Krafla rifting episode. The best fit model
involves a half-space viscosity of 1.1 x 10
Pa s,
a relaxation time of 1.7 years, and an elastic layer
thickness for northeast Iceland of 10 km. The vertical
field indicates that the Krafla dike complex rifted
the entire elastic layer. Using model 2, the motion
1987-1990 and 1990-1992 can be simulated adequately
given the survey errors, but the 1987-1992 deformation
is poorly fitted, suggesting that a more realistic
geophysical model is required. Using model 3, a
range of dikes will fit the deformation field.
AGU Index Terms: 1206 Crustal Movements-interplate; Mineralogy, Petrology, and Rock Chemistry; 8150 Plate boundary-general; 8158 Plate motions-present and recent; 0000 Mineralogy, Petrology, and Rock Chemistry
Keywords/Free Terms: Postrifting deformation, surface motions, viscoelastic modeling, North Iceland.
JGR-Solid Earth 96JB02466
Vol. 101
, No. B11
, p. 25,403