EFFECTIVE TENSION-SHEAR RELATIONSHIPS IN EXTENSIONAL FISSURE SWARMS, AXIAL RIFT-ZONE OF NORTHEASTERN ICELAND

The geometry of fracture systems in selected areas of the active Krafl a fissure swarm, mid-Atlantic ridge, northeastern Iceland, is analysed . Based on geodetic analysis of the present-day topography at the top of Holocene basaltic lava flows which fill the axial rift zone, the de formation of this initially horizontal surface can be reconstructed. E xtensional deformation is localised at all scales and block tilting, t hough present, remains minor. Using simple models of the surface expre ssion of normal faults, the geometrical characteristics of the topogra phic features related to active deformation during tectonic-volcanic e vents are quantitatively analysed. At crustal depths of about 1 km, no rmal faults are present and have an average 70 degrees dip. Comparison with the dip data of older normal faults observed in the uplifted and eroded shoulders of the rift zone, at palaeodepths of 1-2 km, indicat es that this dip determination is valid. Comparisons between the local case study and structural analyses of active fissure swarms on a larg er 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 shear of no rmal sense, their relative importance depending on depth. Absolute ten sion dominates in the uppermost several hundred metres of the crust, r esulting 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 inject ions. Al crustal depths of about 1 km, normal shear prevails along fau lt planes which dip 60 degrees-75 degrees. This importance of normal s hear at moderate depth, between upper and lower crustal levels where t ension prevails, is pointed out. Within the extensional context of rif ting, 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. (C) 1997 Elsevier Science Lt d.