Dynamics of dyke intrusion in the mid-crust of Iceland

We have captured a remarkable sequence of microearthquakes showing progressive melt intrusion of a dyke into the mid-crust of the northern volcanic rift zone in Iceland. Two-thirds of the earth’s crust is created at mid-ocean rifts. Two-thirds of that crust is formed by intrusion and freezing in the mid-crust of molten rock generated within the underlying mantle. So this must be a widespread phenomenon. I discuss seismicity recorded on a 27-seismometer array caused by a dyke intruding at 45–50º dip from 18–13 km depth on the mid-ocean Icelandic rift. Moment tensor solutions show dominantly double-couple failure, with fault mechanisms sometimes flipping between normal and reverse faulting within minutes in the same location. The inferred fault planes from microearthquakes align precisely with the overall plane of the dyke delineated by hypocentres. We attribute the reverse faults to melt injecting along the dyke, sometimes breaking plugs of previously frozen melt. Changes in fault polarity are caused either by breaking the plugs of frozen melt or by dyke propagation above a deflating sill. Although the crust at these depths is normally aseismic, high strain rates as melt intrudes generate microearthquakes up to magnitude 2.2. Melt injection occurs in bursts propagating at 2–3 m/min along channels 0.15–0.20 m wide, producing swarms of microearthquakes lasting several hours. Intervening quiescent periods last tens to hundreds of hours. We have captured the igneous crust in the process of being generated by melt moving upward along an inclined plane from a sill at 18 km depth to another shallower crustal sill at 14 km depth.