THE IMPORTANCE OF MAGMATIC PERIODICITY FOR FAULT EVOLUTION AT AN OBLIQUE SPREADING CENTER

As the Mid-Atlantic Ridge comes onshore at the Reykjanes Peninsula (RP) in SW Iceland, it bends into an 075° trend to become an oblique spreading center in which deformation is manifested as 4 NE-trending fissure swarms. The structural layout of the south-central portion of the RP is complex, composed of a combination of normal, oblique, and strike-slip faults, gaping vertical fractures, and eruptive fissures. Most of the left-lateral component of the oblique spreading is taken up by NNE-trending right-lateral strike-slip faults (bookshelf faulting). Normal faults and eruptive fissures trend ~045° (perpendicular to the NUVEL-1A plate motion direction); however, at the 100s of m scale, fracture and fault traces have a zigzag nature with two distinct orientations (~030° and 060°) where they broke through the most recent lava flows. The 030° fracture orientation is perpendicular to a 120° principal extensional strain direction that would be predicted for pure transform motion along the plate boundary. GPS data suggest that transform motion currently predominates. The 060° fracture orientation requires a different strain orientation at 150° which may occur during magmatic and eruptive periods. At the 10s of m scale, zigzag segments of normal faults and vertical fractures are made up of left-stepping, en echelon segments rotated 10-15° CW from the general trend of the array, but this is not seen in the eruptive fissures. The left-stepping geometry is indicative of right-lateral oblique slip, suggesting that there was a rotation of the regional stress field prior to initiation of slip on faults covered by young lava flows. There has been minimal slip on normal faults in recent time, yet up to 10s of m of offset exist along normal faults in 2-10 ka lava flows; therefore, there must have been periods of time in the past 10,000 yrs when normal faulting dominated. Dikes are consistently found in close proximity, subparallel to the fault traces. Numerical models indicate that these dikes would have increased normal fault slip tendency and decreased the likelihood of right-lateral slip strike-slip activity. The implications are: normal faulting predominates during magmatic periods; strike-slip faulting during amagmatic periods; the strain direction changes accordingly, causing fault slip behavior to be temporally variable.