ABSTRACT

The formation of a new ocean basin begins with the rupture of a >100 km-thick continental plate, but only after millions of years of heating from Earth's deep mantle, and stretching in response to forces at the plate boundaries. Deep, fault-bounded valleys with active volcanoes above these zones of plate stretching and heating are termed continental rift zones. The East African rift system in the Afar Depression is one of few places worldwide where this process of continental rupture is occurring on land, affording an opportunity to directly observe and quantify the plate separation process.

The time-averaged rate of the continental rupture process is extremely slow; about the rate of fingernail growth. Earthquakes and volcanic activity demonstrate that the plate also sustains stresses for periods of 10's to 100's of years. When the plate can no longer support the stresses from the plate stretching, much more rapid and sometimes catastrophic deformation occurs in a swarm of earthquakes. Likewise, molten rock (magma) from the hot zone beneath the plate is less dense than solid rock, and it may pond within the plate, leading to volcanic eruptions. If molten rock has accumulated beneath a weak or highly stressed plate zone, it will rise as a thin sheet called a dike that fills the fissures and cracks in the brittle crust. Volcanic and earthquake activity in these episodes affects only a sector of the long narrow rift zones, producing a regular along-axis rift segmentation that is maintained in subsequent episodes. One of these 60 km-long segments of the East African rift system in Ethiopia experienced an intense period of localized deformation in September-October, 2005. Over 163 moderate (5.3 > mb > 3.9) earthquakes and an explosive volcanic eruption occurred over a 3 week-period. Field, earthquake, remote sensing, and modelling studies show that molten rock (magma) was intruded into the plate beneath this 60 km-long rift segment, with cracks and faults forming in brittle rocks above the narrow zones of magma injection. The rifting episode was probably triggered by the injection of a 60 km-long, ~ 8m-wide sheet of basaltic magma (dike), with faults slipping above the dikes. The basalts did not reach the surface in this episode, or a second, less intense episode in June, 2006.

The ongoing monitoring of earthquake activity, vertical and horizontal motion of the ground surface relative to stable areas ~100 km away from the active zone, and monthly satellite radar images allow us to directly measure one of these rare episodes of plate rupture with magma injection. The observations are then compared with computer simulations of the rise of magma through a plate that is being stretched apart. Among the significant broader impacts of this study are that continued monitoring of the activity provides vital information for seismic and volcanic hazard mitigation in East Africa.

This project is co-funded by two NSF programs: Geophysics and the Office of International Science and Engineering.

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