Grant, J.V. & Kattenhorn, S.A (2001)
The Evolution of Fracture Swarms Induced by Subsurface Normal Faults in Basalt Lava Flows, Southwest Iceland
Segmented, vertical normal faults characterize much of the surface deformation along fracture swarms in recent basalt lava flows on the Reykjanes Peninsula, southwest Iceland. We propose that these vertical normal faults developed at the surface by the linkage of, and subsequent throw along, tension fractures that clustered above the upper tip lines of upward-propagating normal faults having typical normal fault dip values (~60o) in the subsurface. This model contradicts existing hypotheses that propose tension fracturing at the surface in response to regional absolute tension. In those models, normal faults initiate at the surface as tension fractures that propagate downward to some critical depth where they become normal faults. Our model is supported by field observations of fracture characteristics and linear elastic fracture mechanics based numerical models that predict a region of tensile stresses above the upper tip of normal faults during slip events.
At Vogar fracture swarm, two independent processes control fracture orientations, both related to deformation above the upper tips of segmented, subsurface normal faults. In areas where the fractures lack heave and throw components of displacement, fracture swarms are usually coincident with a surface monocline above the buried tipline, with tension fractures along the hinge line induced by tensile stresses in the monoclinal flexure. The second style of fault-related fractures at Vogar are arranged into linear fracture clusters that exhibit heave and throw displacements at the surface. Many of these fracture clusters contain left-stepping echelon fractures rotated out of the general trend of the cluster. This fracture pattern is indicative of localized stress perturbations associated with right-lateral oblique slip along the upper tip line of subsurface normal faults. The oblique slip implies a clockwise rotation of the stress field between the time of initial normal fault growth at depth, and the development of the surface rupture.
At Thingvellir fracture swarm, clusters of fault-related echelon surface fractures are also rotated out of the general trend of the fracture cluster. However, unlike Vogar, these echelon fractures only occur at bends in the fault trace, above subsurface fault segment linkage sites. The fracture geometry is consistent with dip-slip subsurface faults, with oblique slip above fault linkages. There is no evidence of a rotation of the stress field at Thingvellir between the time of formation of the subsurface normal faults and the development of the surface fracture clusters.
In all areas studied, vertical faults at the surface clearly developed from the linkage of tension fractures. As the subsurface faults evolved and propagated towards the surface, surface fractures that formed above their tips mechanically interacted and linked together, at which time they developed vertical displacements. In summary we propose the following steps in the evolution of the fracture swarms: (1) Normal faults initiate in the subsurface in response to regional extension, with orientations possibly related to the obliquity of the spreading direction with respect to the rift axis. (2) As the normal faults evolve, fault segments mechanically interact and link. Stress field rotations may initiate a change from dip-slip to oblique slip motion. (3) The normal faults propagate upwards and induce localized tensile stresses at the surface causing surface fracturing. (4) Further slip on the subsurface faults results in the linkage of the surface fractures and the development of vertical components of displacement.