Fracture pattern of Thjórsárdalur central volcano with respect to rift-jump and a migrating transform zone in South Iceland

An analysis of dykes, mineral veins, and tectonic joints in the Matuyama-age Thjórsárdalur volcano provides insights into the relative development of rift and transform zones at slow-spreading centres. Rifting resulted in the formation of NNE fractures. Transform faulting gave rise to shear fractures striking predominantly N-S and ENE, parallel to the main earthquake fractures of the South Iceland Seismic Zone (SISZ), but also WNW, E-W, and NNW. All fractures were filled with the same geothermal fluid and magma. The strikes, motions, and fracture cross-cuttings show that the transform fault was active early in the history of the volcano, coeval with the rift(s). The shear fractures are consistent with a Riedel shear model of an E-W transform zone, and with a minimum principal stress, σ3, striking N135°E to N150°E. These values are close to the direction of extension across the Eastern Rift Zone (ERZ), supporting an interpretation that the E-W shear zone has migrated from the latitude of Thjórsárdalur to the SISZ with the southward propagation of the ERZ. Migration of synchronous transform and rift systems at a slow-spreading centre yields a distinctive fracture pattern, but lacks some of the elements found at similar migration sites for fast-spreading centres.