Tectonic significance of the South Iceland Seismic Transform Zone

The subaerial expression of the Mid-Atlantic Ridge on Iceland comprises two overlapping spreading axes, referred to as the West Volcanic Zone (WVZ) and the East Volcanic Zone (EVZ), respectively. The way the spreading rate is distributed on both volcanic zones has an important impact on the stress pattern in the overlap area. Our field data from the area trace the evolution of the stress direction as recorded by slip motion on fault planes. We found four different strike-slip stress phases. An early N-S compression phase (A) preceded a NE-SW compression phase (B). This phase was followed by a SE-NW compression phase (C). However, we cannot date an E-W compression, phase (D), relative to the other phases. Numerical modeling based on the assumption that the WVZ has been permanently active during the last 3 Myr and that the EVZ is propagating southward confirms that stress directions have rotated clockwise by more than 140°. These results fit perfectly with our field analysis, and we propose that phase A corresponds to initial EVZ ridge-tip propagation, phase B to emplacement of the EVZ southern tip near Torfajökull, and phase C to an extreme southern location near the Surtsey Islands of the EVZ southern tip. Phase D could correspond to an intermediate stage between phases B and C. We suggest that this sequence of tectonics, recorded in a regional overlapping ridge-tip setting, is directly analogous to smaller-scale and more common phenomena at second-order ridge discontinuities throughout the global mid-ocean ridge system.