The South Iceland Seismic Zone: tectonic and sismotectonic analyses revealing the evolution from rifting to transform motion

The South Iceland Seismic Zone (SISZ) trends E-W between two major volcanic rift zones of Southern Iceland. It is 20-60 km wide and 70 km long, and ch aracterized by important seismotectonic activity. Major destructive earthqu akes in the SISZ were reported throughout the centuries, from the initial s ettlements in Iceland until now. We carried out a structural study:in this area, including analysis of aerial photographs, local observation of major faults and collection of minor fault slip data in outcrops, as well as an a nalysis of the focal mechanisms of earthquakes. At the regional scale, the main fault trends:are approximately NNE-SSW and NE-SW. ENE-WSW, NW-SE and W NW-ESE trending faults are also detected in aerial photographs and in: the field. All these faults are normal or strike-slip in character. Some of the historical major earthquake fractures are observed in the postglacial lava flows in the SISZ: most are right-lateral and trend roughly N-S. The fault trace left by the last major single earthquake in the study area, a M7 ear thquake in 1912, is mentioned as a case example. In addition to the large-s cale faults: we analyzed more than 700 minor faults at 25 sites. Most sites are located in rocks of Upper Pliocene-Pleistocene age. Inversion of fault slip data sets enabled us to reconstruct local paleostress tensors, hence to define the major tectonic regimes which have prevailed in the SISZ. Exam ples of characteristic sites are given, and the main results are presented. Two main groups of faulting mechanisms reveal two distinct stress regimes, with perpendicular directions of extension, NW-SE (primary) and NE-SW (sec ondary), Both groups, however, display inhomogeneous data sets, related to extensional and to strike-slip faulting. The primary stress regime is in ag reement with both the general behaviour of the SISZ as a left-lateral trans form zone and the opening of the rift segments. The secondary stress regime , incompatible with the primary stress regime, is interpreted in terms of s tress permutations. A population of 231 double couple focal mechanisms (M > 1 and depth > 2 km) was also analyzed in terms of stress states. The resul ts show great similarity in terms of stress directions. The present-day str ess field mainly inferred from analyses of earthquake focal mechanisms is c onsistent with the present behaviour of the SISZ as a left-lateral transfor m zone. However, the proportion of strike-slip faulting within the present- day seismic activity (71%) is significantly higher than that revealed by th e geological observation of Quaternary faults (50%). This contrast is inter preted in terms of development and evolution of the transform fault zone. T he paleostress fields identified in the Upper Pliocene-Pleistocene formatio ns of the SISZ reflect both the previous behaviour of the area, when it was located inside the rift zone, and its present behaviour as a transform zon e, thus illustrating the local evolution from rifting to transform motion. (C) 1999 Elsevier Science Ltd. All rights reserved.