Immature and mature transform zones near a hot spot: The South Iceland Seismic Zone and the Tjornes Fracture Zone (Iceland)

We describe and compare the two transform zones that connect the Icelandic rift segments and the mid-Atlantic Ridge close to the Icelandic hot spot, in terms of geometry of faulting and stress fields. The E-W trending South Iceland Seismic Zone is a diffuse shear zone with a Riedel fault pattern including N0 degrees-N20 degrees E trending right-lateral and N60 degrees-N70 degrees E trending left-lateral faults. The dominant stress field in this zone is characterised by NW-SE extension, in general agreement with left-lateral transform motion. The Tjornes Fracture Zone includes three major lineaments at different stages of development. The most developed, the Husavik-Flatey Fault, presents a relatively simple geometry with a major fault that trends ESE-WW. The stress pattern is however complex, with two dominant directions of extension, E-W and NE-SW on average. Both these extensions are compatible with the right-lateral transform motion and reveal different behaviours in terms of coupling. Transform motion has unambiguous fault expression along a mature zone, a situation close to that of the Tjornes Fracture Zone. In contrast, transform motion along the immature South Iceland Seismic Zone is expressed through a more complicate structural pattern. At the early stage of the transform process, relatively simple stress patterns prevail, with a single dominant stress field, whereas, when the transform zone is mature, moderate and low coupling situations may alternate, as a function of volcanic-tectonic crises and induce changes in stress orientation. (C) 2007 Published by Elsevier B.V.