Evolution of vertical faults at an extensional plate boundary, southwest Iceland

Vertical faults having both opening and vertical displacements are common in southwest Iceland, and hypotheses vary regarding whether they propagated to the surface from below or from the surface downward. We address this issue through a study of vertical faults and associated surface fracture zones in regions of both oblique and normal spreading in southwest Iceland. Individual fracture segments are commonly rotated out of the general trend of the fracture zone, suggesting oblique motion along subsurface normal faults. These faults commonly breach the upper hinge lines of narrow monoclinal folds that flank many fault traces on the hanging wall side. Based on these field observations and the results of numerical models, we propose that 60–75° dipping normal faults in the subsurface propagated to the surface from below. Vertical fractures formed at the upper tips of the faults at depths of between 250 and 500 m (25–50% of the fault length) in response to stress concentrations along the tip line. Model results indicate that narrow monoclinal folds develop at the surface above these vertical fractures, which subsequently breach the monoclines along the upper hinge line, forming vertical fault scarps and open fissures at the surface. If vertical fractures utilize pre-existing cooling joints in basalt to connect directly to the surface, the hanging wall is able to pull apart from the footwall without the development of a surface monocline along the fault trace.