DIKES, MINOR FAULTS AND MINERAL VEINS ASSOCIATED WITH A TRANSFORM-FAULT IN NORTH ICELAND

The Tjornes fracture zone is a transform fault connecting the axial ri ft zone in North Iceland with that of the Kolbeinsey ridge off the nor th coast of Iceland. On the north coast of the Flateyjarskagi peninsul a there is a 3-5 km wide zone of intense deformation, which we interpr et as the main on-land part of this transform fault. This transform zo ne strikes N64-degrees-W and is characterized by unusually steeply dip ping lavas (25-44-degrees-WNW), extensive secondary mineralization, nu merous slickensided surfaces, normal faults (1-20 m throw), dikes that strike parallel to the zone, and occasional 3-20 m wide subzones of c rushed rocks striking parallel to the transform fault. In a 17 km long continuous coastal profile south of the transform zone there is a 30- degrees clockwise change (from south to north) in the lava strike. The dike strike also changes clockwise north along this profile, but only by 4-degrees. This indicates that the clockwise change in the lava st rike is original and not due to subsequent tectonic rotation. When the axial rift zones of North Iceland and the Kolbeinsey ridge are modell ed as single offset mode I cracks, finite-element results show that th e Tjornes fracture zone developed between these mode I cracks in a reg ion of maximum shear stress. The underlap between these axial rift zon es has, apparently, decreased during the past several million years. T he model predicts that as this underlap decreased, the angle between t he Husavik-Flatey fault and that of the associated axial rift zones in creased and the activity of the Husavik-Flatey fault decreased. Both t hese predictions are borne out by the available data.