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

At the junction between an oceanic transform fault and the adjacent ri dge segments, there is normally a curved fabric. The Trollaskagi penin sula is located just south of the junction between the Kolbeinsey ridg e and the Tjornes fracture zone and offers a rare opportunity for stud ying the three-dimensional tectonics of the associated curved fabric. For the purpose of elucidating its infrastructure, we measured the att itudes of tens of lava flows and several hundred dykes, faults, and mi neral veins in 28 profiles in the northern half of the Trollaskagi pen insula. 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 d egrees-36 degrees, with an average of 22 degrees. Most dykes are basal tic and strike north or NNE. They are exceptional as regards thickness , frequency of multiple dykes and crustal dilation. The average thickn ess, 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. Mo st faults are normal faults. The curved fabric is limited to the north ernmost part of the Trollaskagi peninsula and consists of two main set s of fractures. One set includes north and NNE striking faults, minera l veins, and dykes. The other set consists of NW striking (oblique) fa ults, mineral veins, and tilted lava flows. A boundary element model i ndicates that the abnormal lava dip and the NW striking (oblique) exte nsional structures in the northernmost part of the peninsula are attri butable to the stress field associated with the junction between the K olbeinsey ridge and the Tjornes fracture zone.