Ethiopia is an ideal natural laboratory for the study of rifting and hotspot tectonism. In contrast to the passive margins, where rifting processes must be inferred from the geological record or theoretical models for rifting, the Main Ethiopian Rift (MER) and Red Sea rift in Afar expose subaerially, several stages of seismically- and volcanically-active rift development from embryonic continental rifting in the south, to incident ocean spreading in the north. Flanking Ethiopia’s rift zones, uplifted plateau areas are home to Earth’s youngest continental flood basalt province: the Ethiopian Traps. Erupted ~30Ma during the development of the Red Sea rift, these thick (2-3km) basalt flows were likely the result of a mantle plume that continues to provide the gravitational potential energy required to rift a continent that is flanked to the East and west by divergent plate boundaries.
Beginning in 2001 with the Ethiopia, Afar, Geoscientific Lithospheric Experiment, broadband seismograph networks throughout Ethiopia have provided unprecedented opportunity to study crust and upper mantle structure beneath Ethiopia. In this talk, I will report on some of the results emerging from these experiments and their implications or rifting and hotspot tectonism. Magma intrusion, not plate stretching and faulting, dominate extension throughout much of Ethiopia’s rift zones, with the implication that traditional models for extension and melt production are inappropriate for the region. In the Danakil Depression, however, a late-stage of basin development, documented by thick (~1km) sequences of evaporites that post-date the last marine incursion, only ~100ka ago. A simple explanation for the observations in Afar is that a late-stage of plate stretching has occurred, following heating and weakening of the lithosphere during protracted extension by dyke intrusion. Such a process may provide an elegant explanation for the deposition of thick evaporite sequences at some rifted margins such as those observed along the South Atlantic.
Broadband seismograph deployments across Ethiopia (and beyond) have also provided an excellent opportunity to image the African super plume. Most body-wave tomographic models are constructed using relative, not absolute, arrival-times because of the greater accuracy with which the former can be calculated from the often-noisy seismograms recorded by temporary networks. To address this issue, and to glean more useful absolute wave-speed information about the African mantle, we have developed a new strategy for picking first-breaking energy from regional network data. The African superplume is a striking feature of our new P-wave model, with remarkably slow (~4%) structure below Ethiopia clearly requiring more than just temperature to explain the observations.
Looking to the future, I will describe the early stages of project TRAILS: the Turkana Rift Arrays Investigating Lithospheric Seismology experiment, which will record earthquakes in the low-lying Turkana Depression between the uplifted Ethiopian and East African plateaus. In the absence of excess gravitational potential energy, what drives rifting in Turkana? Can the lack of elevation be explained by Mesozoic- or EAR-associated rifting, or does the African Superplume provide no dynamic uplift in this region? Watch this space in 2019 and beyond!