Brittle deformation and fracture patterns on oceanic rift shoulders: the esja peninsula, SW Iceland

Abstract

This paper provides detailed structural data on the crustal deformation of the Esja peninsula, at the southwestern margin of the rift zone in SW Iceland. Forty percent of the faults are strike-slip with steep dips, 35% are normal dip-slip with dips ranging from 50° to 90°. For normal faults, E-W, NE-SW and NNE-SSW are the three major trends. The paleostress tensors determined using the fault data, indicate three main states of stress. Two extensional regimes ( σ 3 N180°E ±10° and N110°E ±10°) are related to normal slip faults. The third is a compressive stress regime with σ 1 trending N10°E-N30°E. Associations between strike-slip and dip-slip faults are often observed, resulting from both the geometrical requirements of interactions between faulted blocks and from local-regional permutations between σ 1 and σ 2. We note a constant obliquity of 10–20° between the trend perpendicular to the regional structures and the computed direction of extension. This conclusion is also supported by the major trends of dykes and tensional fractures. There are two mechanisms of volcanic injection (local sheets and regional dykes) resulting in power-law cumulative thickness distributions with exponents of 1.5 and 3.0, respectively.