Interaction of Tectonics and Magmatism in the Askja spreading segment of Iceland

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There is a long-standing 'chicken and egg' debate over rifting and the injection of molten rock into the crust at rifted spreading centres: does the molten rock injection drive rifting or vice-versa? A further question is whether melt flows from the deep mantle where it is generated into the overlying crust of a segment of the rift that is spreading at a central point, and then flows within the crust along the segment, or whether it moves into the crust in sub-vertical feeders at multiple points along the entire segment, ponding at different depths in the crust prior to eventual eruption from a high-level magma chamber a few kilometres below the surface. Seventy percent of the earth's crust is generated at oceanic spreading segments: in Iceland the presence of an underlying mantle plume pushes the spreading segments above sea level, allowing us better access to instrument them with seismometers than below the oceans. The 80-km long Askja spreading segment in northern Iceland is a remarkable natural laboratory: it is an extremely quiet environment seismically because there is no vegetation, and no noise created by human activity (it is only accessible in the summer, and then only by 4-wheel drive vehicles). We routinely record down to extremely small local magnitude 0.2 events. There has been an extraordinary increase of activity in the Askja segment since we started deploying seismometers (earthquake detectors) in it seasonally around Askja volcano in the summer months of 2006. So far we have recorded over 10,000 earthquakes, including episodes of melt injection at 15-25 km depth near Upptyppingar, followed some months later by hundreds of tectonic earthquakes in the overlying brittle crust at 3-7 km depth. We model these earthquakes as caused by melt injection in a dyke at depth triggering tectonic failure in the overlying crust. The time-scale of the intrusions and succeeding brittle failure is 18-24 months, so it requires long-term monitoring to capture an entire cycle. So far we have only captured portions of the cycle. Building on our identification of lower-crustal earthquakes thought to be caused by melt moving in the central part of the Askja rift segment, we propose to deploy an array of 15 seismometers along the entire Askja segment to record seismicity for 3 years from September 2010. This will be supplemented by a further 10 seismometers which will be moved once per year along the segment to provide extremely densely sampled coverage overall.We will map the development in space and time of earthquake seismicity along the Askja segment and will investigate the relationship between lower crustal earthquakes attributed to melt movement and those caused by failure in the overlying brittle crust. If we are fortunate we shall capture an entire cycle including a volcanic eruption, although since 80% of the igneous crust in the spreading segment consists of intrusive rocks that freeze at depth before they reach the surface, it is more likely that we shall record seismicity resulting from dyke or sill intrusion. The crustal structure of the northern Icelandic rifts is only poorly known, especially in the lower crust. We shall use well constrained earthquakes as seismic sources to map the crustal structure and anisotropy along the entire Askja segment. This will give not only a picture of the variation in crustal structure along the segment away from its central volcano, but by using earthquakes, which are rich in shear waves, we shall be able to constrain the presence and distribution of melt in the lower crust.This is possible because shear waves are very sensitive to the presence of molten rock, because they do not travel through liquids. In summer 2011 we shall install additional temporary seismometers to increase their density, and then fire large explosive sources into the array to provide a conventional wide angle controlled source profile to calibrate and to supplement the earthquake tomography.