Grant, J.V. & Kattenhorn, S.A (2000)
Origin of Tensile Stresses at the Earth's Surface at an Extensional Plate Boundary
An unresolved problem with regards to the state of stress at an extensional plate margin is whether absolute tension can be produced at the Earthıs surface purely as a result of the relative plate motions (i.e., regional tectonic stresses), or if an alternative explanation is necessary. Vertical tensile surface fractures (joints) in basalt at several locations on the Reykjanes Peninsula, Iceland, attest to the occurrence of absolute tensile stresses at or near the surface, in excess of the rock strength. We investigate whether these fractures developed in response to regional tension at the surface, as suggested by previous researchers, or if they have an alternative genetic origin, specifically due to localized tensile stresses associated with stress perturbations near to faults. Surface fractures are typically arranged into discrete fracture clusters, several hundreds of meters to several kilometers in length, separated by regions of little or no fracturing. We interpret this pattern of fracturing as being the result of subsurface normal faults that locally perturbed the stress field near the surface. The normal faults accommodate regional extensional strain at depth due to crustal spreading at the plate boundary.
Linear elastic fracture mechanics theory predicts that tensile stresses, of sufficient magnitude to break intact rock, can be induced above the upper tip of a fault during slip events. We hypothesize that joints forming at the fault tip propagated vertically towards the surface producing linear fracture clusters along the surface projection of the fault tipline. At the surface, individual fractures in the fracture clusters utilized the pre-existing columnar joint network in the basalt, producing zigzag fracture geometries. The depth to the normal faults, and hence the height of the joints, is unknown, however, previous researchers have used fracture spacing data to infer fracture heights on the order of several hundreds of meters. At both the Thingvellir and Vogar fracture swarms, several fracture clusters are comprised of echelon fractures that are slightly rotated from the general trend of the fracture cluster, perhaps indicative of a component of strike slip motion on the underlying fault. At Thingvellir, right-stepping echelon fractures range from N10-20E where the general trend of the fracture zone is N33E. At Vogar, left-stepping echelon fractures range from N65-72E where the general trend of the fracture zone is N55E. As slip continued on faults at depth, surface fractures interacted and linked together and accumulated heave and throw components of displacement, forming wide fissures at the surface. We conclude that the tensile stresses that induced fractures at the Earthıs surface on the Reykjanes Peninsula were not necessarily induced by regional tectonic stresses, but may be a localized phenomenon associated with subsurface normal faults that induced tensile stresses above their upper tips.