Off-rift and rift-zone palaeostresses in Northwest Iceland

Abstract

The Vestfirdir peninsula of Northwest Iceland consists mainly of gently dipping basaltic lava flows 8–14 Ma old. Detailed studies of its principal tectonic elements (basaltic dykes, normal faults and minor faults) and observations of the trends of the major fjords and valleys make it possible to reconstruct the evolution of the associated stress field. The results show that a kink-shaped (curved) rift zone existed in this area until about 5 Ma ago. In the northern part of the peninsula the rift zone trended NNE, but ENE in the southern part. It is proposed that the tectonic elements of the Vestfirdir peninsula were generated in two stress fields, one operating inside the rift zone, the other operating outside it. The rift-zone stress field was responsible for the formation of most normal faults, dykes and the main extension direction inferred from the minor faults. Conversely, the off-rift stress field was responsible for the formation of several Holocene normal faults in the southern part of the peninsula, the direction of the secondary extension (inferred from the minor faults) as well as for most major fjords and valleys in the area. Boundary-element studies suggest that the off-rift stress field generated on the concave side of a curved rift zone is different from that generated on its convex side. Absolute tensile stresses that encourage the formation of valleys and fjords concentrate very near to, or at, the convex margin, but at a considerable distance from the concave margin. Applying these model results to Iceland as a whole, it is concluded that the valley formation in Northwest Iceland occurred at a large distance form the associated rift zone, whereas in East Iceland the valleys formed near to the margin of the rift zone. Furthermore, the model predicts that zones of relative tension may propagate radially from the convex side of the rift zone and act as magma channels for volcanoes. The location and orientation of major fjords and valleys in East and Northwest Iceland, as well as the existence of several major off-rift Holocene volcanoes, support the model predictions.