OFF-RIFT AND RIFT-ZONE PALAEOSTRESSES IN NORTHWEST ICELAND

The Vestfirdir peninsula of Northwest Iceland consists mainly of gentl y dipping basaltic lava flows 8-14 Ma old. Detailed studies of its pri ncipal tectonic elements (basaltic dykes, normal faults and minor faul ts) and observations of the trends of the major fjords and valleys mak e it possible to reconstruct the evolution of the associated stress fi eld. 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 propos ed that the tectonic elements of the Vestfirdir peninsula were generat ed 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 dire ction inferred from the minor faults. Conversely, the off-rift stress field was responsible for the formation of several Holocene normal fau lts in the southern part of the peninsula, the direction of the second ary extension (inferred from the minor faults) as well as for most maj or fjords and valleys in the area. Boundary-element studies suggest th at the off-rift stress field generated on the concave side of a curved rift zone is different from that generated on its convex side. Absolu te tensile stresses that encourage the formation of valleys and fjords concentrate very near to, or at, the convex margin, but at a consider able distance from the concave margin. Applying these model results to Iceland as a whole, it is concluded that the valley formation in Nort hwest Iceland occurred at a large distance form the associated rift zo ne, whereas in East Iceland the valleys formed near to the margin of t he rift zone. Furthermore, the model predicts that zones of relative t ension may propagate radially from the convex side of the rift zone an d act as magma channels for volcanoes. The location and orientation of major fjords and valleys in East and Northwest Iceland, as well as th e existence of several major off-rift Holocene volcanoes, support the model predictions.