Nature 358, 488 - 490 (06 August 1992); doi:10.1038/358488a0

Post-rifting stress relaxation at the divergent plate boundary in Northeast Iceland

G. R. Foulger^*, C.-H. Jahn^†, G. Seeber^†, P. Einarsson^‡, B. R. Julian^§ & K. Heki^*

^* Department of Geological Sciences, University of Durham, Science Laboratories, South Road, Durham DH13LE, UK
^†Institut fur Erdmessung, Universitat Hannover, D-3000 Hannover 1, Neinburger Strasse 5, Germany
^‡Science Institute, University of Iceland, Dunhaga 3, Reykjavik, Iceland
^§U.S. Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, California 94025, USA
On leave from the Kashima Space Research Centre, Communications Research Laboratory, Japan.

INTERACTION of the elastic lithosphere with the underlying anelastic asthenosphere causes strain to propagate along the Earth's surface in a diffusion-like manner following tectonism at plate boundaries. This process transfers stress between adjacent tectonic segments and influences the temporal tectonic pattern along a plate boundary. Observations of such strain transients have been rare, and have hitherto been confined to strike-slip and underthrusting plate boundaries¹. Here we report the observation of a strain transient at the divergent (spreading) plate boundary in Iceland. A Global Positioning System survey undertaken a decade after an episode of dyke intrusion accompanying several metres of crustal spreading reveals a spatially varying strain field with the expected diffusion-pulse shape and an amplitude three times greater than the 5.7 cm that would be expected from the average spreading rate². A simple one-dimensional model with a thin elastic layer overlying a viscous layer fits the data well and yields a stress diffusivity of 1.1 á0.3 m²s^-1. Combined with struc-tural information from magnetotelluric measurements, this implies a viscosity of 0.3-2 10¹⁹ Pa s—a value comparable to that derived for Iceland from post-glacial rebound²³, but low compared with estimates for mantle viscosity obtained elsewhere³.

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