Shift of a rift by a transform zone: a case from the Northern Rift Zone and Tjörnes Fracture Zone of Iceland
Transform zones present fascinating geometry and tectonic deformation, some of which can be confusing, especially, when rift and transform zones interact, and rift structures are displaced by transform fractures. A case study from north Iceland includes the transform zone of Tjörnes (the Tjörnes Fracture Zone) and the Theistareykir Fissure Swarm of the Northern Rift Zone. This fissure swarm contains two north-trending central graben that are apparently shifted in a sinistral motion across a WNW-trending fault of the transform zone (the Stórihver Fault). The present analysis takes into account the number of faults and their relative displacement as criteria, and demonstrates that the configuration of the north-trending rift structures is compatible with the dextral slip on thisWNWblind leaky fault of the transform zone. The dextral motion along the WNW fault explains more coherently the greater number and greater throw of the north-trending normal faults in two of the blocks on the shoulder of the graben, as well as with the formation of the central graben themselves. A dextral motion is also compatible with the regional tectonics and the mechanism of transform faulting. The apparent displacement of tectonic structures at diverging plate boundaries can be misleading. Therefore, identifying the correct sense of motion is essential for exploration and structural drilling targets where rift and transform of any age interact.