STRUCTURAL-ANALYSIS OF A TRANSFORM-FAULT RIFT-ZONE JUNCTION IN NORTH ICELAND

On the north coast of Iceland, the rift zone in North Iceland is shift ed about 120 km to the west where it meets with, and joins, the mid-oc ean Kolbeinsey ridge. This shift occurs along the Tjornes fracture zon e, an 80-km-wide zone of high seismicity, which is an oblique (non-per pendicular) transform fault. There are two main seismic lineaments wit hin the Tjornes fracture zone, one of which continues on land as a 25- km-long WNW-trending strike-slip fault. This fault. referred to as the Husavik fault, meets with, and joins, north-trending normal faults of the Theistareykir fissure swarm in the axial rift zone. The most clea r-cut of these junctions occurs in a basaltic pahoehoe lava flow, of H olocene age, where the Husavik fault joins a large normal fault called Gudfinnugja. At this junction, the Husavik fault strikes N55-degrees- W, whereas Gudfinnugja strikes N5-degrees-E, so that they meet at an a ngle of 60-degrees. The direction of the spreading vector in North Ice land is about N73-degrees-W, which is neither parallel with the strike of the Husavik fault nor perpendicular to the strike of the Gudfinnug ja fault. During rifting episodes there is thus a slight opening on th e Husavik fault as well as a considerable dextral strike-slip movement along the Gudfinnugja fault. Consequently, in the Holocene lava flow, there are tension fractures, collapse structures and pressure ridges along the Husavik fault, and pressure ridges and dextral pull-apart st ructures subparallel with the Gudfinnugja fault. The 60-degrees angle between the Husavik strike-slip fault and the Gudfinnugja normal fault is the same as the angle between the Tjornes fracture zone transform fault and the adjacent axial rift zones of North Iceland and the Kolbe insey ridge. The junction between the faults of Husavik and Gudfinnugj a may thus be viewed as a smaller-scale analogy to the junction betwee n this transform fault and the nearby ridge segments. Using the result s of photoelastic and finite-element studies, a model is provided for the tectonic development of these junctions. The model is based on an analogy between two offset cuts (mode I fractures) loaded in tension a nd segments of the axial rift zones (or parts thereof in the case of t he Husavik fault). The results indicate that the Tjornes fracture zone in general, and the Husavik fault in particular, developed along zone s of maximum shear stress. Furthermore, the model suggests that, as th e ridge-segments propagate towards a zero-underlapping configuration, the angle between them and the associated major strike-slip faults gra dually increases. This conclusion is supported by the trends of the ma in seismic lineaments of the Tjornes fracture zone.