Interaction and linkage of extension fractures and normal faults: examplesfrom the rift zone of Iceland

Field and photogeological studies were made of 90 zones of interacting frac ture segments along the rift zone of Iceland. Each zone consists of a pair of extension fractures or a pair of normal faults, with lengths from metres to kilometres. These zones evolve from an underlapping stage, through an o verlapping stage (the most common configuration) and, finally, to a linkage stage. Of all the zones, only 7% are underlapping, whereas 93% are overlap ping, with hook-shaped fracture pairs. The length/width ratios of the overl apping zones have a mean value of 3.5. The preferred geometry of the overla pping zones depends upon the initial configuration of the interacting fract ures (length, overstep) and the development of the interaction. In the over lapping zones, most fracture pairs show moderate shear (strike-slip) compon ents, related to local variations in the extension (opening) directions. Ve rtical displacements on normal faults decrease as the overstep and length o f overlapping zones increase; both, in turn, are proportional to the total lengths of the faults forming the pair. The geometrical and kinematic featu res of overlapping spreading centres at mid-ocean ridges show close similar ities to those reported here. These similarities indicate that the architec ture and evolution of overlapping zones are scale independent. (C) 2000 Els evier Science Ltd. All rights reserved.