EXTENSIONAL TECTONICS IN THE VICINITY OF A TRANSFORM-FAULT IN NORTH ICELAND

At the junction between an oceanic transform fault and the adjacent ridge segments, there is normally a curved fabric. The Trollaskagi peninsula is located just south of the junction between the Kolbeinsey ridge and the Tjornes fracture zone and offers a rare opportunity for studying the three-dimensional tectonics of the associated curved fabric. For the purpose of elucidating its infrastructure, we measured the attitudes of tens of lava flows and several hundred dykes, faults, and mineral veins in 28 profiles in the northern half of the Trollaskagi peninsula. At sea level the general dip of the lava pile is 6 degrees-14 degrees, but in the northernmost part of the peninsula the dip is 10 degrees-36 degrees, with an average of 22 degrees. Most dykes are basaltic and strike north or NNE. They are exceptional as regards thickness, frequency of multiple dykes and crustal dilation. The average thickness, 5.9 m, and a crustal dilation of 28% in a 4.5-km-long profile are the highest figures reported from regional dyke swarms in Iceland. Most faults are normal faults. The curved fabric is limited to the northernmost part of the Trollaskagi peninsula and consists of two main sets of fractures. One set includes north and NNE striking faults, mineral veins, and dykes. The other set consists of NW striking (oblique) faults, mineral veins, and tilted lava flows. A boundary element model indicates that the abnormal lava dip and the NW striking (oblique) extensional structures in the northernmost part of the peninsula are attributable to the stress field associated with the junction between the Kolbeinsey ridge and the Tjornes fracture zone.