Magma Capturing in Lower Ductile Crust: Reawakening of Northern Iceland's Divergent Plate Boundary

Beginning in Februaury 2007, anomalous swarms of deep earthquakes were detected in the region of Upptyppingar/Álftadalsdyngja, which forms part of the Kverkfjöll volcanic system in north-east Iceland. The focal depths concentrate in the lower ductile region of the crust at 14-22 km. Previous episodes of deep- seated earthquake activity in Iceland have usually been linked with magma unrest, some of which have resulted in eruptions, and these latest swarms are the most intensive and persistent deep-seated seismicity yet observed. Associated deformation has also been observed by GPS and InSAR. The high strain rates required to explain the brittle fracturing under ductile conditions and the observed deformation lead us to conclude that magma has being emplaced into the lower crust. Moreover, the depth of intrusion has decreased with time. Although the activity continued into the first half of 2008, it has ceased for the time being with a total intruded volume of ~0.04 km3. Both the pattern of the seismicity and our modeling of the deformation data strongly indicate that the geometry of the intrusion approximates a tilted sheet. The strike and dip imply that either the stress field in the lower crust is different to the regional stress field, or that the sheet orientation is dictated by some other factor, such as varying rheological properties. Emplacement of a large magma chamber below the intrusion could indeed be expected to perturb the stress field in such a way as to favor the intrusion of tilted sheet, and exposed examples of such relict relationships have been observed in Iceland. However, there is no other evidence to suggest the current existence of a large magma chamber, and the stress perturbation due to any relict chamber would have been relaxed by viscous flow at these depths. Alternatively, relict intrusions leading from a relict magma chamber could have a similar orientation to the tilted sheet. These relict intrusions might then guide the recent intrusion due to their contrasting rheological properties. We do not know if the intrusion is the prelude to a new eruption or represents the natural processes of lower crustal generation, but in any case, this event differs from processes that have been observed in upper crustal generation episodes.