HR: 0800h
AN: V21D-0638    [Abstracts]
TI: Tectonics of the Hengill Volcano, Southwest Iceland
AU: Friese, N
EM: nfriese@gwdg.de
AF: Department of Structural Geology and Geodynamics, Geoscience Center, University of Gottingen, Goldschmidstrasse 3, Gottingen, 37077 Germany
AU: Krumbholz, M
EM: krumbholz@freenet.de
AF: Department of Structural Geology and Geodynamics, Geoscience Center, University of Gottingen, Goldschmidstrasse 3, Gottingen, 37077 Germany
AU: Burchardt, S
EM: sburcha@gwdg.de
AF: Department of Structural Geology and Geodynamics, Geoscience Center, University of Gottingen, Goldschmidstrasse 3, Gottingen, 37077 Germany
AU: * Gudmundsson, A
EM: Agust.Gudmundsson@gwdg.de
AF: Department of Structural Geology and Geodynamics, Geoscience Center, University of Gottingen, Goldschmidstrasse 3, Gottingen, 37077 Germany
AB: The Hengill Volcano is the center of a 5-10 km wide and 60-70 km long volcanic system in the West Volcanic Zone of Iceland. The volcano is mostly composed of hyaloclastites (basaltic breccias), but contains also pillow lavas, pillow breccias, as well as andesitic and acid rocks, mainly dacite and rhyolite. The volcano formed in the past 0.8 Ma, is the site of powerful geothermal fields, many of which are related to large normal faults that dissect the volcano and generate a major graben through its top region (at 800 m.a.s.l.). This graben continues north along the volcanic system and is referred to as the Thingvellir Graben in the Holocene lava flows at Lake Thingvallavatn. Hengill has no collapse caldera. Drill-hole data indicate that inclined, basaltic sheets make up 50% of the rock at 1.5 km depth and nearly 100% below 2 km depth. Hengill was subject to intense seismic activity in 1994-2000 when more than 80,000 earthquakes (reaching up to M5) were recorded in the volcano and its surrounding. Most earthquakes occurred on NNE-trending dextral and ENE-trending sinistral faults, but some on NE-trending normal faults. Coinciding with the seismic activity, there was crustal doming, with a maximum uplift rate of about 2 cm/a. Most of the seismicity and deformation came to an end following two M6.6 earthquakes in South Iceland in June 2000. To understand better the deformation, seismicity, and geothermal activity in Hengill, we made a detailed tectonic study of the volcano in 2005. The data obtained include measurements of 2044 joints and minor faults (many conjugate), 970 mineral veins, 60 large faults, and 7 dykes. The veins have a general circular strike distribution, but a noticeable peak in the NE direction. Some 35 large normal faults dissect an E-W profile across Hengill and its graben: they trend mostly NNE and range in apparent displacement from about 10 m to 160 m. The minor faults have a general circular strike distribution, but peak at two main trends: NNE and ENE. Displacements are normally difficult to determine, because of lack of marker horizons. Nevertheless, these minor faults clearly correspond to main trends of the dextral and sinistral faults generating the intense seismic activity in 1994-2000.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8010 Fractures and faults
DE: 8178 Tectonics and magmatism
DE: 8419 Volcano monitoring (7280)
DE: 8424 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8135)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005