Tectonic stress regimes, rift extension and transform motion: the South Iceland Seismic Zone

The South Iceland Seismic Zone (SISZ) is located at the junction of three r ift segments in southwestern Iceland. The presence of different types of fa ulting and of differently orientated subgroups in Upper Pliocene to Holocen e formations indicate polyphase tectonism. We measured 736 minor faults at 25 sites. Two types of relationships between stress regimes are represented . The first type, named IDS (inhomogeneous data set), is characterized by t he presence of two types of fault mechanisms, normal and strike-slip, consi stent with a single direction of extension. The second type, named OSR (opp osite stress regimes), is characterized by the presence of perpendicular di rections of extensions for a single type (normal or strike-slip) of faultin g. Because of contradictory chronological criteria, we infer that the OSR a lternated during the brittle tectonic activity of the SISZ. Two stress regi mes, primary and secondary, are characterized by directions of extension NW -SE and NE-SW, respectively. The general fracture pattern characterized for the primary stress regime in the SISZ includes NNE-SSW trending right-late ral strike-slip faults, conjugate ENE-WSW trending left-lateral faults and NE-SW normal faults. This distribution is quite consistent with a Riedel- t ype model of fault pattern in a left-lateral sheer zone. The stress states characterized based on analysis of both the earthquake focal mechanisms and the recent faulting show great similarity in terms of stress directions. T he main difference is the larger ratio of strike-slip motions representing 71% of the total population in the case of earthquake focal mechanisms; whe reas for the whole set of faults the proportion of strike-slip faulting was 50%. We explain that a temporal evolution of the tectonic regime in the SI SZ region, accompanied by a gradual change in stress field, starts with rif t-type pure extension and progressively leads to development of preferentia lly strike-slip structures in the kinematic context of left-lateral transfo rm motion. (C) Elsevier, Paris.