Showing papers by "Gezahegn Yirgu published in 2018"

Using Lake Sediment Cores to Improve Records of Volcanism at Aluto Volcano in the Main Ethiopian Rift

Abstract: Aluto is a silicic volcano in central Ethiopia, flanked by two large population centers and home to an expanding geothermal power plant. Here we present data from two lake sediment cores sampled 12 and 25 km from the volcano, which record at least 24 distinct eruptions in the Holocene. Tephra layers from the two cores are correlated using a variety of techniques, including major and trace element geochemistry as well as textural and morphological features from scanning electron microscopy-backscatter electron imaging. The purpose is to provide a Holocene reference section for further tephrostratigraphic studies of the volcano as well as to provide information on eruption frequency. The lake cores suggest that Aluto has had a variable eruption rate, with three eruption clusters in the Holocene at ~3, 6.5, and 11 ka, with small Vulcanian-to sub-Plinian eruptions separated by larger, Plinian eruptions. We infer that the smaller tephras are likely the product of pumice coneand dome-forming eruptions. In addition, modern wind data suggest that the likely direction of an ash cloud from Aluto is to the west and south west, which is toward population centers and is in agreement with thickness data from the cores. We conclude that current records underestimate the volcano’s eruptive history and that hazard assessments should be updated accordingly. Plain Language Summary In this study we explore the history of a populated volcano in central Ethiopia called Aluto. The volcano is close to several large towns and home to an expanding geothermal power plant. Aluto has not erupted in the last few hundred years, and so there are no eye witness accounts of volcanic activity. In lieu of this, we use volcanic material deposited in the surrounding environment during an eruption as a record of the volcano’s activity. In this study, we examine the volcanic material deposited in two lakes using lake cores drilled ~12 and 27 km from the volcano that record the eruptive history from 2,000 to 12,000 years ago. These cores record 24 volcanic eruptions; which indicates an eruption every ~250 years. We use features of the volcanic ash particles to match up eruptions in the cores and to establish when the eruptions occurred. We find that the volcano has had three periods of increased activity, each lasting around 1,000 years. As such, we can infer that the volcano has erupted frequently over the last 10,000 years and hazard assessments of the volcano should be updated accordingly.

From initiation to termination: a petrostratigraphic tour of the Ethiopian Low-Ti Flood Basalt Province

Abstract: Continental flood basalts (CFBs), thought to preserve the magmatic record of an impinging mantle plume head, offer spatial and temporal insights into melt generation processes in large igneous provinces (LIPs). Despite the utility of CFBs in probing mantle plume composition, these basalts typically erupt fractionated compositions, suggestive of significant residence time in the continental lithosphere. The location and duration of residence within the lithosphere provide additional insights into the flux of plume-related magmas. The NW Ethiopian plateau offers a well-preserved stratigraphic sequence from flood basalt initiation to termination, and is thus an important target for study of CFBs. This study examines modal observations within a stratigraphic framework and places these observations within the context of the magmatic evolution of the Ethiopian CFB province. Data demonstrate multiple pulses of magma recharge punctuated by brief shut-down events, with initial flows fed by magmas that experienced deeper fractionation (lower crust). Broad changes in modal mineralogy and flow cyclicity are consistent with fluctuating changes in magmatic flux through a complex plumbing system, indicating pulsed magma flux and an overall shallowing of the magmatic plumbing system over time. The composition of plagioclase megacrysts suggests a constant replenishing of new primitive magma recharging the shallow plumbing system during the main phase of volcanism, reaching an apex prior to flood basalt termination. The petrostratigraphic data sets presented in this paper provide new insight into the evolution of a magma plumbing system in a CFB province.

Magmatically assisted off-rift extension—The case for broadly distributed strain accommodation

Abstract: Within continental rift settings, extensional strain is initially accommodated along the nascent rift margins, subsequently localizing to zones of focused magmatic intrusion. The migration of strain from rift-border faults to diking places an emphasis on constraining the magmatic plumbing system of zones of focused intrusion to resolve how extension is accommodated in the rift lithosphere. While existing rifting models concentrate on the relationship between extension and focused magmatism within the rift, there is increasing evidence of rift-related magmatic activity outside the rift valley. We examine the Galema range, an area of focused magmatic activity along the eastern margin of the Central Main Ethiopian Rift, which is morphologically similar to areas of focused magmatism within the rift. We find that whole-rock thermodynamic modeling and thermobarometric calculations on mineral-liquid pairs suggest that fractionation (and hence magma stalling depths) within the Galema range is polybaric (~7 and ~3 kbar). These results, when compared to zones of focused intrusion within the rift, indicate an incipient magmatic plumbing system. We contend that diking associated with the Galema range, which predates magmatic belts within the rift, thermomechanically modified the lithosphere along this margin. While the cessation of magmatism within the Galema range may have been precipitated by a change in magma flux, the now thermomechanically modified lithospheric mantle along this margin facilitated the subsequent development of within-rift magmatic chains. The implications of this are that off-rift magmatic activity may play an integral role in facilitating the development of rift architecture.

Constraining the Magmatic Plumbing System in a Zoned Continental Flood Basalt Province

Abstract: The geographic heterogeneities in lava composition observed in continental flood basalt provinces could provide a probe of material upwelling from the deep mantle and their length scales, but their utility is limited by uncertainties in the locus of magmatism. We examine the magma plumbing system for the Oligocene Ethiopian flood basalts. The province, which exhibits domains defined by the eruption of low‐Ti (LT) and high‐Ti (HT) lavas, requires a magmatic plumbing system that facilitates the transit of compositionally distinct magmas through the crust without mixing. Here we present a geochemical and geochronological study of a suite of 43 dikes from western Ethiopia. We find that the dikes were dominantly emplaced contemporaneously with the Oligocene flood basalt phase of activity. The composition of the dikes is overwhelmingly LT in character, typified by an overall flat rare earth element pattern (median value of La/LuCN = 2.6), and a lack of enrichment in incompatible trace elements in comparison to the HT lavas. These observations confirm the western Ethiopian dike swarm as a source for the LT flood basalts in the Ethiopian flood basalt province. We also present tentative evidence for an eastward migration in the LT dike system over time. These observations are consistent with the terminal stages of the LT magmatism being centered on the Simien shield volcano. We conclude that the apparent separation of ~400 km between the LT and HT magma plumbing systems allowed for the development of a strongly geochemically zoned continental flood basalt province.