Ian J. Hamling

Evidence for focused magmatic accretion at segment centers from lateral dike injections captured beneath the Red Sea rift in Afar

Continental breakup occurs through repeated episodes of mechanical stretching and dike injection within discrete, narrow rift segments. However, the time and length scales of the dike intrusions, along with the source regions of melt within continental and oceanic rifts, are poorly constrained. We present measurements of spatial and temporal variability in deformation from the currently active 60-km-long Dabbahu segment of the Red Sea rift in Afar, using satellite radar, global positioning system, and seismicity data sets, that capture emplacement of two ~10-km-long, ~1–2-m-wide dike intrusions in June and July 2006. Our observations show that the majority of strain is accommodated by dikes that propagate laterally over ~4–5 h time scales along the rift axis and are sourced from a reservoir in the middle to lower crust, or upper mantle, beneath the center of the rift segment. New intrusions during the ongoing rifting episode in Afar show that the injection of lateral dikes fed from magma reservoirs beneath rift segment centers is a key component in creating and maintaining regular along-axis rift segmentation during the final stages of continental breakup. Our observations also provide evidence that the focused magmatic accretion at segment centers observed in slow-spreading mid-ocean ridges occurs prior to the onset of seafloor spreading.

Post‐rifting relaxation in the Afar region, Ethiopia

[1]xa0Crustal accretion at divergent plate boundaries typically occurs via the periodic intrusion of dikes, but their emplacement and the associated deformation are rarely observed. The few existing observations at subaerial rifts show that these diking events are followed by a decadal-scale period with extension rates faster than the secular divergent plate motion. This transient accelerated deformation has been explained by continued subsurface magma injection or by relaxation, in the viscoelastic mantle, of the stress changes imparted by dike opening. For the first time, GPS measurements were collected within a few months of a rifting event at a major plate boundary, the September 2005, 60 km-long dike intrusion in the Dabbahu segment, Afar, Ethiopia. Extension rates for the first 3 years greatly exceed the plate motion (Nubia-Arabia) secular divergence rate, even at sites located more than 60 km from the rift axis. Here we show that these observations are consistent with stress relaxation in a viscoelastic upper mantle with a viscosity of about 5 × 1018 Pa·s overlain by a 12–14 km-thick elastic crust. The alternative model of continued diking requires continuous opening well below the Moho and is therefore unlikely. Instead, magma injection in Afar since June 2006 has taken the form of smaller discrete diking events, tapping into a mid-crustal melt reservoir under the segment center.