Tension-shear deformation in the Krafla fissure swarm: the Leirhnjúkur area (work carried out with Dauteuil and Villemin)

$\\$The geometry of the fracture pattern of a small graben (studied area: 280 m long and 150 m wide) in the Krafla fissure swarm was analyzed in detail. Based on geodetic analysis of the present-day topography at the top of Holocene basaltic lava flows which fill the axial rift zone, the deformation of this initially horizontal surface can be reconstructed. Extensional deformation is localized at all scales and block tilting, albeit present, remains minor.

Using simple models of the surface expression of normal faults, the geometrical characteristics of the topographic features related to active deformation during tectonic-volcanic events are quantitatively analyzed. At crustal depths of about one kilometer, normal faults are present and have an average 70$^{\circ }$ dip. Comparison with the dip distribution of older normal faults observed in the uplifted and eroded shoulders of the rift zone, at paleodepths of 1-2 km, indicates that this dip determination is valid. Comparisons between the local case study and structural analyses of active fissure swarms on a larger scale suggest that normal faulting plays a major role in the middle section of the thin, newly formed brittle crust of the rift zone. In the axial oceanic rift zone of NE-Iceland, the extensional deformation in the upper crust is dominated by horizontal tension and normal shear, their relative importance depending on depth. Absolute tension dominates in the uppermost several hundred meters of the crust, resulting in the development of fissure swarms. Effective tension plays an important role at a deeper level (2-5 km), because of the presence of magmatic fluid pressure from magma chambers which feed dyke injections. At crustal depths of about 1 km, normal shear prevails along fault planes which dip 60$^{\circ }$-75$^{\circ }$. This importance of normal shear at moderate depth, between upper and lower crustal levels where tension prevails, is pointed out. Within the extensional context of rifting, these variations of tectonic behaviour with depth are controlled by both the lithostatic pressure and the effective tension induced by the presence of magmatic fluid pressure.