Geophysical Evidence for Magmatic Transport in the Lower Crust in Iceland
Abstract
An event characterized by deep seated earthquake activity, started near Upptyppingar, N-Iceland, in early 2007 and has been ongoing episodically since. The area is part of the Kverkfjöll Volcanic System, lying in the fissure swarm extending northeast. Iceland GeoSurvey has carried out a resistivity survey, measuring two parallel, approximately 20 km long E-W profiles with 5 km apart, centred immediately north of Upptyppingar area. Using both TEM and MT method, the resistivity structure of the area was modelled to 30 km depth below the profile. Two pronounced horizontal low resistivity layers are observed; the upper at 1-2 km depth and the lower at 7-13 km depth. Beneath the deeper layer a low resistivity column is observed right under the Upptyppingar area, extending as deep as can be detected. The upper low resistivity layer reflects clay rich rock caused by thermal alteration. Its lower boundary corresponds to the change in dominant alteration minerals at temperatures above 230°C. The deeper low resistivity layer appears to be the same as present beneath most of Iceland according to the results of previous MT-surveys. We interpret this layer to mark the boundary between the brittle and the ductile crust for normal strain rates, supposedly close to the 700°C isotherm. Contrary to most earthquakes in Iceland, most of the earthquake hypocenters beneath Upptyppingar are below the low resistivity layer, i.e. at 10 to 20 km depth beneath the surface, moving upwards with time. They occur within the deep low resistivity column and line up at the lower boundary of the deeper conductor. Very few earthquakes occur within the low resistivity layer itself, but some activity is above it. The low resistivity column probably represents hotter rocks than the surroundings, due to recent magma intrusions. The composition of the crust below the low resistivity layer must then be different and mechanically stronger than above. Resistivity data along with the earthquake distribution suggest magma upwelling from the mantle to the lower part of the crust, causing the deep earthquakes due to high strain rates. Near the lower boundary of the deep conductor the rock becomes more ductile and the high density magma accumulates as intrusions.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFMMR43A1803V
- Keywords:
-
- 0930 Oceanic structures;
- 3619 Magma genesis and partial melting (1037);
- 5109 Magnetic and electrical properties (0925);
- 8416 Mid-oceanic ridge processes (1032;
- 3614)