[S22-1] Lithosphere structure and dynamics

[S22-1-04] Bookshelf faulting in Iceland: Characteristic of oblique rifts and unstable transforms

^○Pall Einarsson (Institute of Earth Sciences, University of Iceland, Reykjavik, Iceland)

Oblique and immature plate boundaries are frequently characterized by complicated fault patterns, which makes their seismogenic potential and seismic hazard difficult to assess. The structure of the plate boundaries in Iceland is relatively complex. Two of the plate boundary segments are highly oblique to the over-all plate velocity vector between the North America and Eurasia Plates, i.e. the Reykjanes Peninsula and the Grimsey oblique rifts. They contain both volcanic systems and seismogenic strike-slip faults. Oblique spreading leads to extensive volcanism and large earthquakes, a combination that is otherwise uncommon in Iceland. The fissure swarms of individual volcanic systems contain normal faults and fissures, arranged en echelon along the plate boundary. The fissure swarms fade out as they extend into the plates on either side. Overprinting this pattern of volcano-tectonic structures are sets of parallel, transversly striking transcurrent faults that generate the largest earthquakes in the zones, up to M 6.5. Their surface expressions are en echelon fracture arrays and push-up structures. The distance between them varies from 0.5 to 5 km, and together they form a bookshelf-type fault system taking up the shear component of plate movements across the oblique rift zones.
The interaction of the Iceland hotspot with the mid-Atlantic plate boundary leads to frequent ridge jumps by rift propagation and the formation of temporary microplates between the propagating and receding rifts. At present, the Hreppar Microplate is bounded by a transform zone, the South Iceland Seismic Zone, that is the source of many of the most damaging earthquakes in the history of Iceland, up to M 7. The earthquakes occur on many parallel strike-slip faults that are perpendicular to the zone, i.e. by bookshelf faulting. It is postulated that bookshelf faulting is one of the characteristics of unstable or immature plate boundaries.