Enigmatic rift-parallel, strike-slip faults around Eyjafjorthur, Northern Iceland

Strike-slip faults along mid-ocean ridge and subaerial spreading centers are generally thought to be restricted to transform boundaries connecting rift segments. Faults that are parallel to spreading centers are generally assumed to be normal faults associated with tectonic extension. However, clear evidence of north-south (rift-parallel), strike-slip displacements occur widely around the southern portion of Eyjafjorthur, northern Iceland about 50 km west of the actively spreading Northern Rift Zone. The area is south of the southernmost strand (Dalvik Lineament) of the NW-SE-trending, dextral-slip, Tjornes Fracture Zone (where N-S, sinistral, strike-slip "bookshelf" faulting occurs). Faults in the Eyjafjorthur area cut 8.5-10 Ma basaltic crust, parallel to spreading-related dikes, and commonly cut preexisting dike margins. Fault rocks range from fault breccia to gouge. Riedel shears and other kinematic indicators provide unambiguous evidence of shear sense. Most faults show evidence of sinistral, strike-slip movement; less common normal and oblique-slip faults also are present. Cross-cutting relations among the different types of faults are inconsistent thus they are interpreted as being related to a single deformation event. Fault slip-line kinematic analysis yields solutions indicating an overall oblique-slip system with sinistral sense of shear. These results may be interpreted as previously unrecognized transform-fault bookshelf faulting or slip accommodating block rotation associated with northward propagation of the Northern Rift Zone.