BRITTLE DEFORMATION AND FRACTURE PATTERNS ON OCEANIC RIFT SHOULDERS -THE ESJA PENINSULA, SW ICELAND

This paper provides detailed structural data on the crustal deformatio n of the Esja peninsula, at the southwestern margin of the rift zone i n SW Iceland. Forty percent of the faults are strike-slip with steep d ips, 35% are normal dip-slip with dips ranging from 50 degrees to 90 d egrees. For normal faults, E-W, NE-SW and NNE-SSW are the three major trends. The paleostress tensors determined using the fault data, indic ate three main states of stress. Two extensional regimes (sigma(3) N18 0 degrees E +/- 10 degrees and N110 degrees E +/- 10 degrees) are rela ted to normal slip faults. The third is a compressive stress regime wi th sigma(1) trending N10 degrees E-N30 degrees E. Associations between strike-slip arid dip-slip faults are often observed, resulting from b oth the geometrical requirements of interactions between faulted block s and from local-regional permutations between sigma(1) and sigma(2). We note a constant obliquity of 10-20 degrees between the trend perpen dicular to the regional structures and the computed direction of exten sion. This conclusion is also supported by the major trends of dykes a nd tensional fractures. There are two mechanisms of Volcanic injection (local sheets and regional dykes) resulting in power-law cumulative t hickness distributions with exponents of 1.5 and 3.0, respectively.