Oblique rift opening and graben reactivation revealed by reoccurring magma intrusions in central Iceland

Abstract:

More than 98% of volcanic activity at divergent magmatic plate boundaries occurs on the seafloor while the remaining 2% take place on land, such as in Afar (Ethiopia) and in Iceland. Radar satellite data, dense seismic networks and field structural measurements allow analyzing in detail the spatial and temporal propagation of magma during on land rifting events and the related volcano-tectonic processes. We present results of one of the best-monitored rifting events so far, which occurred in Iceland in 2014-2015, and discuss how such results may benefit submarine volcanism research. During the 2014-15 Bárðarbunga rifting event in central Iceland, magma propagated over 40 km to the northeast from a central volcano where the intrusion eventually made it to the surface to start an eruption. It produced a 10 km long by 800 meters wide graben with multi-meter deformation and a lava field of 1.5 km³ leading to the largest eruption in Iceland for the last 200 years. Using radar image offsets, seismicity and structural field measurement we analyzed the spatio-temporal evolution of the event. We found new evidence of oblique rift opening influenced by pre-existing fractures and two centuries of extension deficit accumulation. Our results show that the opening was initially accompanied by left-lateral shear that ceased with increasing opening. They also show that the magma opened pre-existing fractures and propagated aseismically at shallower depths and that those rapidly opening fractures possibly extended into the brittle-ductile boundary causing new fracturing and seismicity at the brittle – ductile boundary. Our findings directly challenge current dyke propagation models that classically do not consider pre-fractured medium. All together, our results strongly suggest that fault reactivation plays a key role in magma propagation at divergent plate boundaries.