Rheologic controls on inter-rifting deformation of the Northern Volcanic Zone, Iceland

Extensional rifts are characterized by significant lateral variations in crustal rheology as lithospheric material cools and advects away from the rift axes. Nevertheless, most models used for modeling of crustal movements consist of horizontal layers with uniform properties. This paper explores the role spatial variation of rheological properties play, in modifying the style of surface deformation at rifts, by comparing finite element model predictions to inferred inter-rifting surface deformation in Iceland's Northern Volcanic Zone. Crustal deformation has been observed by satellite radar interferometry (InSAR) and GPS measurements. Extension is observed across the entire northern rift zone, whereas subsidence occurs in two distinct areas, corresponding to mapped fissure swarms, where major rifting took place in 1975–1984 and 1874–1876, respectively. Our models indicate that the observed inter-rifting plate spreading deformation field is controlled by rheological variations within the en echelon arrangement of fissure segments, and that a central ridge axes model does not apply. Uniform stretching across a plate boundary zone, where fissure swarms are weaker than the surrounding crust, and reflect the surface expression of the rift, reproduces the characteristics of the deformation field. The most realistic fissure swarm structure consists of a wedge of weak elastic material on top of a ridge, where viscoelastic material locally reaches to a shallow depth.