TIME-VARIABLE FAULTING BEHAVIOR AT AN OBLIQUE SPREADING CENTER, SOUTHWEST ICELAND

The Reykjanes Peninsula (RP), SW Iceland, is an on-land extension of the mid-Atlantic Ridge where spreading is 30° oblique to the plate boundary in a left-lateral sense. Clay analog models of fault development in transtension for 30° left-lateral obliquity predict normal faults and coeval right-lateral oblique-slip faults oriented at ~30° and ~60° to the plate boundary, respectively. On the RP, normal, oblique-slip and strike-slip faults are present; however, orientations differ somewhat from 30° transtension models and are spatially variable. Only the strike-slip faults have been seismically active in recent history; behavior more reminiscent of a purely transform environment. Inversion of GPS data is in agreement with the NUVEL-1A plate motion model which predicts a velocity component of 16.8 mm/yr of slip parallel to the plate boundary. The rifting component of 8.5 mm/yr has not been measured by GPS but is evidenced by the existence of dikes and normal faults which form during magmatic periods, the most recent of which occurred 779-765 yrs ago. Normal faults are currently inactive, yet scarp heights in <8000 yr lava flows can be >10m. The dominant active fault set and the overall strain field is thus temporally variable. The regional stress field must oscillate from magmatic to amagmatic periods, causing nonuniform slip behavior on individual fault sets. The sense of motion on any particular fault depends on the stage within the oscillatory magmatic cycle. Normal faults have undergone periods of right-lateral oblique motion manifested by left-stepping en echelon geometries in postglacial lava flows. Multiple fault orientations in close proximity (e.g., at least 3 in the region of Vogar) also suggest a variable stress history. Activity on any one fault set tends to reactivate portions of older, differently oriented faults, resulting in zigzag fault patterns and abrupt doglegs along strike. At Burfell, a cluster of normal faults with scarps up to 10s of m high formed at the north tip of a right-lateral oblique fault, indicating that part of the structural fabric of the RP is a secondary phenomenon produced by local stress fields generated by motion along pre-existing faults. Fault patterns and activity on the RP thus appear to be both temporally and spatially variable.