VISCOSITY STRUCTURE BENEATH NORTHEAST ICELAND

The dynamics of crustal rifting in Iceland depend strongly on the lowe r crustal rheology, which controls the intensity of upper crustal stre ss concentration and scale time of heat diffusion from the underlying mantle plume. While magnetotelluric surveys suggest the presence of a pervasive hot and highly ductile lowermost crust with possibly high fr action of partial melt, observations of low seismic attenuation and st rong shear wave transmission suggest a much cooler lower crust and upp er mantle. Since viscosity is also sensitive to the degree of partial melt present, viscosity estimates for these regions could shed light o n the factors responsible for these observations. In this study we uti lize horizontal and vertical displacement vectors determined in GPS ca mpaigns in northeast Iceland since 1986. These are modeled in terms of steady state tectonic loading plus postseismic/postdiking relaxation following the 1975-1984 Krafla rifting episode, as first proposed by F oulger and others. With the elastic part of the model fixed by externa l constraints, these data have a high sensitivity to the viscosity str ucture beneath Iceland. Lower crust and upper mantle viscosities of ab out 3x10(19) Pa s and 3x10(18) Pa s, respectively, yield the closest a greement with the data. Our lower crustal viscosity estimate is consis tent with the low attenuation and low (subsolidus) temperature for the lower crust inferred in recent studies. Inversions for fissure openin g during the Krafla rifting episode yield about 7 m of opening centere d on the Krafla rift, as is observed. Allowing for contemporaneous dee p rifting on vertical faults along the Askja segment partially account s for the observed increase in separation across the rift during 1987- 1992 but does not account for large displacements in the southeastern part of the network or the large relative subsidence around the Askja rift during 1987-1990. Recent deep normal faulting beneath the Askja r ift and further south might explain all of these remaining features.