CRUSTAL SPREADING DUE TO DIKES AND FAULTS IN SOUTHWEST ICELAND

A 30 km long profile, extending from 3 million year-old Pleistocene rocks to theHolocene rocks of the rift zone in southwest Iceland, is dissected by 443 dikes and 156 normal faults. Nearly all the dikes are basaltic, with an average strike of N26-degrees-E, which is subparallel to the trend of fractures in the nearby rift zone. Their average dip is 69-degrees and average thickness 1.37 m, both values much lower than averages for regional Tertiary dike swarms in Iceland. Many dikes are seen to change into sills. The great majority of the faults are pure dip-slip, with an average throw of 10.2 m. Their average strike, N37-degrees-E, is different from the average strike of the dikes in this area and the fractures in the nearby rift zone. Their average dip is 75.2-degrees, which is somewhat steeper than in the Tertiary areas, but the dips are less in the interbedded sedimentary layers than in the lava flows. Growth faults are very rare in the area and most of the faults appear to have completed their growth in less than 10,000 years. In subsections, crustal dilation due to dikes is 0.4-3.7%, that due to faults is 0.6-5.3%, and that due to both dikes and faults is 1.04-6.44%. In these subsections, average fault spacing is one fault per 170-563 m. Spacing decreases toward the active rift zone. It is concluded that in typical fissure swarms of the rift zone, the dilation at the surface is almost entirely due to extensional fractures and gaping vertical normal faults. In the depth range of a couple of hundred metres to perhaps 1 km, the non-vertical faults contribute most to the dilation. At deeper crustal levels, however, dilation due to dikes becomes dominant.