The development of divergent margins: Insights from the North Volcanic Zone, Iceland

Abstract Iceland is a natural laboratory where plate separation and mid-ocean ridge formation can be studied directly in the field. In this setting, it is not fully understood how magma interacts with faulting and fissuring, and how rift zones propagate. In order to improve the understanding of how these processes operate, we conducted a detailed field study about the kinematics and propagation of 33 Holocene faults along the N–S Theistareykir rift (North Volcanic Zone) and mapped all tension fractures. This rift hosts the Holocene Theistareykir central volcano, for which ground deformation data (InSAR and GPS) indicate an inflating shallow magma chamber throughout 2006–2008. Analyses of the cumulative fault slip distribution at 696 sites show two opposite directions of fault/rift propagation: The first, more developed, is directed northwards and affects the area located north of Theistareykir volcano, whereas south of the volcano, the rift propagates southwards. Tension fractures increase in frequency, with respect to faults, outwards from the volcano, along a N–S to NNE–SSW direction. The presence of aligned vents, dykes and topographic bulging along some of the N–S structures, and the active magma chamber below Theistareykir volcano, suggest that faults and tension fractures propagate following repeated dyke intrusions from the magma chamber outward along the plate margin. The fact that the rift is more developed north of the volcano than south of it, is interpreted as the effect of buttressing from another, coterminous magma chamber sitting below a neighbouring central volcano. Other causes for the general observed pattern such as tectonic stresses, mechanical interaction of faults, and changes in the rheological characteristics of rocks, are discussed as well.