HR: 11:20h
AN: V52B-05    [Abstracts]
TI: Tectonics of the 1783 Laki Crater Row and the Associated Graben, South Iceland
AU: * Gudmundsson, A
EM: Agust.Gudmundsson@gwdg.de
AF: Department of Structural Geology and Geodynamics, Geoscience Center, University of Gottingen, Goldschmidtstrasse 3, Gottingen, 37077, Germany
AU: Andrew, R E
EM: randrew@gwdg.de
AF: Department of Structural Geology and Geodynamics, Geoscience Center, University of Gottingen, Goldschmidtstrasse 3, Gottingen, 37077, Germany
AU: Letourneur, L
EM: lletour@gwdg.de
AF: Department of Structural Geology and Geodynamics, Geoscience Center, University of Gottingen, Goldschmidtstrasse 3, Gottingen, 37077, Germany
AB: One of the longest Holocene crater rows in Iceland is the one associated with the Laki (Skaftareldar) 1783 eruption in South Iceland. It trends northeast, is at least 27 km long, and has roughly 140 crater cones. It gave rise to the second largest historical (the past 1100 years) eruption in Iceland, the area of the issued lava flows during the eruption being about 565 km2, the lava volume 14.7 km3, and the tephra volume 0.4 km3. A part of the Laki Crater Row is associated with a graben. The main graben dissects the hyaloclastite Laki Mountain and extends to the northeast and southwest of the mountain. The graben is very narrow, mostly 200-300 m wide, but reaches 500 m in the Laki Mountain itself. The total visible length of the graben is 5-6 km, but parts of it may be buried by the lava flows. The vertical displacement on the boundary faults is variable, but reaches at least 6-7 m. Where the graben dissects the top of Laki Mountain, the normal faults are gaping by as much as 2-3 m and thus partly tension fractures and partly shear fractures. There is also a set of NW-trending faults dissecting the mountain. Our field data show that the crater row itself (and the associated feeder-dike) did not reach the top of the Laki Mountain, but rather ends vertically on its slopes. Volcanotectonic grabens of this kind are quite common in the rift zone of Iceland. Thus, for example, the Sveinagja Graben and the Sveinar-Randarholar Graben, in the North Volcanic Zone, and the graben cutting through the top of the hyaloclastite mountain Thorbjorn on the Reykjanes Peninsula, in the West Volcanic Zone, are well known and have been studied. The fundamental question as to the formation of such grabens is if they existed, partly or wholly, before the eruption and thus captured the associated feeder-dike, or if the feeder-dike itself was entirely responsible for the graben formation. To interpret the field data we provide new models of the interaction between volcanotectonic grabens and feeder- dikes. We show that in many cases an existing graben would be likely to capture an upward-propagating feeder- dike, as has been observed in the Sveinar-Randarholar Graben and during the 1975-1984 Krafla Fires in the North Volcanic Zone. For the Laki Graben, our data indicate that a segment of one of the graben faults was used by the feeder-dike as a channel to the surface, showing that this segment existed at the time of the eruption.
DE: 8010 Fractures and faults
DE: 8020 Mechanics, theory, and modeling
DE: 8178 Tectonics and magmatism
DE: 8425 Effusive volcanism
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting