Magma-Tectonic Processes in an Active Transitional Rift
from Seismic, GPS, and Modelling Studies in Afar

Collaboration:
C. Ebinger, University of Rochester, NY
R. Buck, Lamont-Doherty Observatory, NY
T. Wright (Univ. Leeds, UK) E. Lewi, L. Asfaw (Geophysical Institute, Addis Ababa)
Ethiopian Mapping Agency, Addis Ababa

NSF Grant EAR-


Project Summary

Magma intrusion via diking is the quantum event of oceanic crustal accretion. Observations in subaerial rift zones in Iceland and Afar suggest that diking events are associated with instantaneous localized extension and with shallow, migrating earthquake swarms. These episodes have one or a sequence of discrete dike intrusions over periods of days to years, followed by a decadal-scale relaxation strain transient. The periodic repetition of this process along individual tectono-magmatic segments of a divergent plate boundary eventually leads to opening rates consistent with far-field plate motions.

From September 4 to October 4, 2005, a tectono-magmatic event of unprecedented scale and intensity occurred along the Dabbahu segment of the Afar rift, Ethiopia, with a 60 km-long dike intrusion, 162 5.6 > mb > 3.9 earthquakes, and the emplacement of about ~2.5 km3 of magma. It is the largest single rifting event on land since the Laki (Iceland) eruption in 1783, and the first to occur in the era of satellite geodesy. This event offers an unprecedented opportunity to directly observe and quantify diking and post-diking processes because this team and UK colleagues deployed seismometers and GPS receivers within the critical first few weeks of the rift episode. These short-term, as well as longer-term measurements, are needed to differentiate between models of strain accommodation and plate rheology in active continental rift zones, as well as ongoing -- but not directly accessible processes at most oceanic spreading centers.


View of the Dabbahu rift, Afar region of Ethiopia. Recent lava flows are cut by subvertical normal faults. In the background, an older felsic volcano is also dissected by normal faults.

We propose to combine a collocated seismic and geodetic field experiment n Afar with dynamic deformation models to test rifting cycle models at divergent plate boundaries. The data acquisition program will quantify strain distribution across the recently activated Dabbahu segment and the adjacent, but currently inactive Hararo segment. These studies will capture different stages in the rift deformation cycle. We will use these data sets, together with our seismicity and geodetic data from the first year after the Afar event commenced, to constrain dynamic deformation models that will investigate the diking and post-diking processes, and the long-term development of rift segments.

This work will address some of the fundamental questions of plate tectonics: How does episodic strain release at divergent plate boundaries relate to continuous plate motions? How much post-diking deformation is accommodated by continued dike intrusions versus viscoelastic relaxation in the lower crust and mantle? What is the strength/viscosity structure of the lithosphere at the transition between continental and oceanic rifting? How do spreading rate, magma supply, dike propagation, and faulting maintain the across and along-axis topography of divergent plate boundaries?