Journal of Conference Abstracts

Prograding Transform Zone in an Overlapping Spreading Center, New Insights from Southern Iceland

Pascal Luxey Southampton Oceanography Centre, Empress Dock, Southampton, SO14 3ZH, UK

Pascal.Luxey@soc.soton.ac.uk

Philippe Blondel Southampton Oceanography Centre, Empress Dock, Southampton, SO14 3ZH, UK

Lindsay M. Parson Southampton Oceanography Centre, Empress Dock, Southampton, SO14 3ZH, UK

Overlapping spreading center geological setting

The subaerial expression of the Mid-Atlantic Ridge comprises two parallel, and overlapping, spreading axes in southern Iceland, referred to as the West Volcanic Zone (WVZ) and the East Volcanic Zone (EVZ). The WVZ forms the northern continuation of the Reykjanes Ridge and the EVZ the southern extension of the Northern Volcanic Zone. Since 3 Ma, the EVZ has propagated southward at a rate of 5 to 10 cm/yr up to Surtsey Island. During this time, the WVZ has been actively spreading at a rate of 1 cm/yr comparable to that of the EVZ. The present-day activity of the WVZ is dominated by extensional faulting. New faults are developing and cut recent (post-glacial) lava flows. Earthquakes in the zone are often related to fault activity as the hypocenters are most of the time located on faults with surface expression. In contrast, the EVZ activity is related to volcanic processes. Most of the biggest eruptions of this century occurred here and the seismic activity is considered to reflect inflation and deflation processes in magma chambers local to the main eruption sites.

Results from our field data

The spreading rate on both volcanic zones has an important impact on the deformation style and the stress pattern orientation in the area located between the West and the East Volcanic overlapping zones. We collected data from the field to trace the evolution of the stress direction as it can be recorded along fault planes. We found four different stress phases all corresponding to a strike-slip stress pattern. The relative chronology between these different phases was not readily identified from our data except from two sites where slickenside lineation superposition shows it. From our data, however, it is possible to confirm that a N-S compression phase (1) occurred before a NE-SW compression phase (2). This phase has been followed by a SE-NW compression (3) and our last phase corresponds to E-W compression (4).

Conclusion

Numerical modelling based on the assumption that the WVZ has been permanently active since 3 Ma and that the EVZ is propagating southwards shows that the stress directions have rotated clockwise by more than 90° and the whole area between both the EVZ and the WVZ has acted as a shear zone. These results fit surprisingly well to our field analysis and so we propose that phase (1) corresponds to the beginning of the EVZ propagation, phase (2) to a location of the EVZ southern tip near Torfajökull, and phase (3) to a location near the Surtsey Islands. Phase (4) is interpreted as corresponding to a retreat of the EVZ ridge tip northwards to somewhere near Torfajökull. Since this phase, the WVZ activity has decayed and the present-day configuration may correspond to a southward retreat of the WVZ northern tip. The stress pattern in southern Iceland reflects NNE-SSW compression as is shown by earthquake focal mechanisms and numerical models