Tectonic analysis of the Husavik-Flatey Fault (northern Iceland) and mechanisms of an oceanic transform zone, the Tjornes Fracture Zone.

The inversion of similar to 1000 fault slip data collected in the Flateyjar skagi Peninsula allows reconstruction of four main tectonic regimes. These include normal and strike-slip faulting modes and are related to the genera l behavior of the Husavik-Flatey Fault (HFF), a major structure of the Tjor nes Fracture Zone connecting the Kolbeinsey Ridge and the North Icelandic R ift. The two most important regimes (E-W and NE-SW extensions), consistent with the right-lateral motion along the Husavik-Flatey Fault, constitute th e main tectonic group. The two others (NW-SE and N-S extensions), forming t he subordinate tectonic group, are incompatible and result from drastic str ess permutations. The relationships between these stress regimes imply not only sigma (1)/sigma (2) and sigma (2)/sigma (3) stress permutations but al so sigma (1)/sigma (3) reversals. A critical review of other data available , such as lava bedding, dike, and major fault attitudes, allows us to compl ete the structural pattern of the Flateyjarskagi peninsula and to highlight the mechanism of the transform zone. The complex pattern of dikes and faul ts in the northern part of Flateyjarskagi can be explained by the superposi tion of several processes: (1) a transform-perpendicular extension (E-W to ESE-WNW trends), (2) a simple shear (NNE-SSW to NE-SW trends), and (3) a st ress perturbation due to the transform motion (NW-SE trends). An important factor controlling the transform mechanism is the variation of coupling alo ng the HFF. The obliquity between the direction of transform motion and the trend of extension for the two main regimes may vary between 20 degrees an d 90 degrees, reflecting repeated changes of the coefficient of friction al ong the HFF. Such change from very low mechanical coupling (weak fault) to moderate friction may occur very rapidly since it takes place several times in a few years, as shown by focal mechanisms of earthquake analysis.