Multiple melt injection along a spreading segment at Askja, Iceland

Lower crustal earthquakes (12-25 km depth) have been detected since August 2005 in the Askja volcanic system along the north Iceland rift, in the normally ductile part of the crust. The earthquakes occur in three clusters, which have stable dimensions and locations through time and are interpreted as positions of repeated melt supply from the mantle to the lower crust. Seismic velocity V_p/V_s ratios are consistent with the presence of partial melt in the lower crust at Askja. The spatial separation of the clusters shows that there are multiple positions of melt injection within this one magmatic segment and all three positions are currently active. This pattern of melt supply is more like that observed on fast spreading ridges than slow spreading ridges and is probably a consequence of the increased melt production beneath Iceland compared to the rest of the Mid-Atlantic Ridge. However, the relative number of earthquakes in each cluster shows that two thirds of the melt is supplied to the central volcano Askja (i.e., segment center). During the last major rifting episode shallow lateral melt migration occurred from the magma chamber beneath the volcano. Therefore on long time scales melt supply is probably greater at the segment center, with melt redistribution in the upper crust, even though there are multiple points of lower crustal injection along the segment.