Structure and development of the Sveinagja graben, Northeast Iceland

Abstract

The Sveinagja graben strikes N8° E, is 34 km long and as much as 2.5 km wide, and is located in the Holocene basaltic (mostly pahoehoe) lava flows of the rift zone in Northeast Iceland. During a rifting episode in 1872–1875, the slip on the boundary faults of the northern part of the graben was as much as 3–6 m, but this part was mostly buried under a lava formed in the 1875 Sveinagja eruption. The southern part of the graben, referred to as Sveinagja, is itself an exceptionally well-defined graben, 9 km long and 1–1.7 km wide. Over one hundred accurate field measurements of the width and throw were made at intervals of 100 m along the length of the boundary faults of this part. These faults are normal faults, continuous for 5.2–5.8 km, as much as 10–13 m wide and with throws of up to 16–17 m. The arithmetic mean width at all points of measurement is 4.2 m. It is proposed that the boundary faults of Sveinagja developed from (mostly columnar) joints in the lava pile. The faults were generated either directly from sets of inclined joints, when the lava pile had become tilted, or from large-scale tension fractures which in turn developed from joints. In the rift zone of Iceland, where the in situ tensile strength ranges from 1 MPa to 6 MPa, large-scale tension fractures rarely reach crustal depths in excess of 0.5–0.8 km; at greater depths they usually change into normal faults. Theoretical considerations suggest that, whether they developed from sets of inclined joints or from large-scale tension fractures, the boundary faults of Sveinagja initiated at crustal depths of 0.5–1.5 km. Using continuum crack models in combination with the aspect ratios of the boundary faults and the appropriate elastic moduli, the driving tensile and shear stresses associated with the fault development are estimated as 4–5 MPa and 2–5 MPa, respectively. From field data, historical accounts and theoretical considerations, it is concluded that the Sveinagja graben was mostly, or completely, generated during the Holocene.