Faulting and Magmatism on the Reykjanes Peninsula, SW Iceland

As the Mid-Atlantic Ridge comes onshore at the Reykjanes Peninsula (RP), it bends into an 075° trend marked by a 2-5 km wide seismic belt. The RP is an oblique spreading environment (30° obliquity) manifested as 4 or 5 NE-trending fissure swarms. We used 0.5 m/pixel resolution aerial photographs and field mapping to analyze fracture, fault, and dike orientations, geometries, and interactions along the Krisuvik fissure swarm (KFS) in the central RP. This region is within the active rift zone (plate boundary). The plate spreading is marked at the surface by vertical (and hence dilational) normal faults, strike-slip faults, spatter cone chains, hyaloclastite ridges, eruptive fissures, and tension fractures. KFS normal faults are oriented 035-055° . In basalts, the faults have vertical fault scarps with offsets of a few cm to 10s of m. These offsets can be partially taken up by fault related surface monoclines. In hyaloclastites, the faults occur as steeply dipping shear planes and deformation bands, indicating a behavioral difference between the two rock types. At the km scale, normal faults are made up of numerous segments 100s of meters long with both right- and left-stepping corrugated geometries. These large segments, in turn, are comprised of smaller (10-40 m) segments, most of which have a left-stepping geometry. Similar fault orientations and en echelon geometries occur in the Reykjanes fissure swarm to the west of the KFS. Left-lateral motion along the 075° rift axis is partially accommodated by 010-030° right-lateral strike-slip faults (i.e., bookshelf style faulting) in the central RP. These faults are characterized by a series of elliptical mounds or push-up structures connected by NE-trending en echelon fractures. Strike-slip faults are sometimes connected to the tips of 055° trending normal faults suggesting a genetic relationship between the two fault types, although the relative timing of activity of each is unclear. Spatter cones in the KFS are surface expressions of underlying dikes that form long, segmented chains trending ∼ 040° (i.e., sub-parallel to the normal fault traces). Segment lengths range from 40-200 m and have both right- and left-stepping geometries. Postglacial lava flows covered pre-existing faults south of the ends of the hyaloclastite ridges. This region is riddled with vertical fractures that are the surface expression of buried faults. These 025-035° fractures (i.e., different to older faults) are highly segmented and have a consistent left-stepping geometry, suggesting right-lateral oblique motion. There has been no apparent change in the rift spreading direction in the last 10,000 years so the stress rotation evidenced by changed fracture orientations is likely the result of a local effect. During the emplacement of a dike the magma pressure pushes against the dike walls and creates a locally perturbed stress field. Where dikes are not parallel to normal faults in the subsurface, magmatism may resolve shear stress onto the faults, inducing oblique slip. In response, faults break through the youngest lava flows at the surface as en echelon vertical tension fractures oriented obliquely to the trend of the underlying fault. The fault geometries suggest a likely control on the mechanics of fault evolution that varies as a function of magmatic activity. During amagmatic periods, oblique rifting tectonic stresses dominate, driving the linkage of the en echelon segments that developed during a prior magmatic episode.