Dikes, minor faults and mineral veins associated with a transform fault in North Iceland

Abstract

The Tjörnes fracture zone is a transform fault connecting the axial rift zone in North Iceland with that of the Kolbeinsey ridge off the north coast of Iceland. On the north coast of the Flatcyjarskagi peninsula there is a 3–5 km wide zone of intense deformation, which we interpret as the main on-land part of this transform fault. This transform zone strikes N64°W and is characterized by unusually steeply dipping lavas (25–44°WNW), extensive secondary mineralization, numerous slickensided surfaces, normal faults (1–20 m throw), dikes that strike parallel to the zone, and occasional 3–20 m wide subzones of crushed rocks striking parallel to the transform fault. In a 17 km long continuous coastal profile south of the transform zone there is a 30° clockwise change (from south to north) in the lava strike. The dike strike also changes clockwise north along this profile, but only by 4°. This indicates that the clockwise change in the lava strike is original and not due to subsequent tectonic rotation. When the axial rift zones of North Iceland and the Kolbeinsey ridge are modelled as single offset mode I cracks, finiteelement results show that the Tjörnes fracture zone developed between these mode I cracks in a region of maximum shear stress. The underlap between these axial rift zones has, apparently, decreased during the past several million years. The model predicts that as this underlap decreased, the angle between the Husavik-Flatey fault and that of the associated axial rift zones increased and the activity of the Husavik-Flatey fault decreased. Both these predictions are borne out by the available data.