Implications of fracturing mechanisms and fluid pressure on earthquakes and fault slip data in the east Iceland rift zone Romain Plateaux a, ⁎, Françoise Bergerat b , Nicole Béthoux a , Thierry Villemin c , Muriel Gerbault a a GeoAzur (UMR 6526), CNRS-UNS-OCA-IRD-UPMC, Observatoire Océanologique de Villefranche, B.P.48, La Darse, 06235 Villefranche-sur-Mer, France b ISTeP (UMR 7193), CNRS-UPMC, boîte 117, 4 place Jussieu, 75252 Paris cedex 05, France c EDYTEM (UMR 5204), CNRS-Université de Savoie, Campus scientifique, 73376 Le Bourget du lac cedex, France abstract article info Article history: Received 15 December 2010 Received in revised form 26 December 2011 Accepted 11 January 2012 Available online 20 January 2012 Keywords: Icelandic rift Stress permutation Volcano-tectonics Paleostresses Fluid pore pressure Fracturing mechanisms Comparison of the eroded off-rift zone left inactive by plate motion with the inner active seismic rift zone allows us to constrain the fracturing mechanisms. In eastern off-rift zone, we measured 423 fault slips (including normal and strike-slip faults). Inversion of fault slip data reveals the parallelism of the minimum stress (σ 3 ) computed for the normal and for the strike-slip faulting, and consistency with the direction of plate divergence. North of the Vatnajökull, in the active rift zone, we analysed 14,250 earthquakes recorded by the Icelandic Meteorological Office (IMO) between 2004 and 2009, especially northeast of the Askja volcano. Two main average focal depths were determined at 5 km (Herðubreið table mountain) and 15 km (Upptyppingar hyaloclastite ridge). The dou- ble couple focal mechanisms determined by IMO revealed that more than half of the mechanisms are strike-slip. Faulting type both in active and off-rift zone reveals an unusual importance of strike-slip regime in such an ex- tensional tectonic context. This can be explained by stress permutations (σ 1 /σ 2 ). Similarities in terms of stress orientations and type of faulting are observed both in the old and present-day rift zones. We assume that the seismic events may be generated by rapid deep magma intrusion, also associated with shallower hydrothermal activities. We thus propose that the presence of fluids, shear failure (double couple focal mechanism) and stress permutations in both active and off-rift zones are closely linked. Assuming a Drucker–Prager failure criterion, we evaluated analytically that a state near of lithostatic pore pressure is a necessary condition for shear failure at shallow (5 km) and deep (15 km) depth, in a simple context of crustal extension that allows for stress permuta- tions. However, processes favouring stress permutations cannot be further constrained from our observations since the dynamics of fluid, materiel heterogeneity and post-glacial rebound can also play a significant role. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The existence of Iceland results from the exceptional superimposi- tion of the Mid-Atlantic Ridge on the Icelandic hotspot. The on-land spreading ridge axis comprises three active rift segments, themselves characterised by “en échelon” systems composed by central volca- noes, eruptive fissures, normal faults and extensional fissures, con- necting the offshore Reykjanes and Kolbeinsey ridges (Fig. 1a). Volcanic, plutonic and tectonic processes closely interact in the active rift zone, involving complex arrangements of fracturing mechanisms both in space and time. In contrast, off-rift volcanism is uncommon and not directly related to rift extension; no active tectonic processes occur in this zone. Iceland provides the opportunity to study the geometry and the mechanisms of fracturing in a segmented rift system by using a wide range of investigations. (1) Field surveys exhibit good quality outcrops of various type of structures (e.g.: striated fault planes, ex- tension fractures, dykes, hyaloclastite ridges) revealing the recent deformation (e.g.: Bergerat et al., 1998; Garcia et al., 2003; Gudmundsson et al., 1996; Saemundsson, 1979). (2) The South Ice- land Lowlands seismic network (SIL) run by the Icelandic Meteoro- logical Office (IMO) through 59 permanent seismic stations in the entire Iceland, provides a record of earthquakes down to magnitude M w =−1. Seismotectonic analysis is suitable on earthquake se- quences (e.g.: Angelier et al., 2008; Plateaux et al., 2010; Stefánsson et al., 2008). (3) A network of continuous geodetic GPS stations (CGPS) is operated by IMO to monitor crustal deformation (e.g.: Jouanne et al., 2006; LaFemina et al.;, 2005; Sigmundsson et al., 2006; Sturkell et al., 2003). The combination of all these fields of study allows us to characterise in detail the deformation in the active rift together with the off-rift zone, left inactive by plate divergence. The studied area is located at the junction between the East Volca- nic Zone (EVZ) and the North Volcanic Zone (NVZ) in east Iceland (Fig. 1a). It extends from the northeastern edges of the Vatnajökull glacier, including the active volcanic system Kverkhnjükar with its central volcano and fissure swarm, to the eastern coast of Iceland. Tectonophysics 581 (2012) 19–34 ⁎ Corresponding author. Tel.: + 33 0493763756. E-mail address: plateaux@geoazur.obs-vlfr.fr (R. Plateaux). 0040-1951/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2012.01.013 Contents lists available at SciVerse ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto