CONTINENTAL RIFTING IN CENTRAL ETHIOPIA: GEOCHEMICAL AND ISOTOPIC CONSTRAINTS FROM LAVAS AND XENOLITHS

Open Access
Author:
Rooney, Tyrone O.
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 22, 2006
Committee Members:
  • Richard Rudolph Parizek, Committee Member
  • Tanya Furman, Committee Chair
  • Andrew Arnold Nyblade, Committee Member
  • David H Eggler, Committee Member
  • Derek Elsworth, Committee Member
  • Barry B Hanan, Committee Member
Keywords:
  • Ethiopia
  • continental rift
  • Debre Zeyit
  • Butajira
  • Wonjii
  • geochemistry
  • fault belt
  • cinder cone
  • basalt
  • xenolith
  • crustal structure
Abstract:
This dissertation will integrate geochemical and geophysical constraints from central-northern Ethiopia, moving closer to an integrated tectonic, structural and magmatic model of rifting process, from its initiation through the final transition to seafloor spreading. I focus on Quaternary magmatism within the Main Ethiopian Rift (MER), which is concentrated in extensional faults belts on the rift floor, specifically - the Wonjii Fault Belt and the Silti-Debre Zeyit Fault Zone. The location of these Quaternary eruptives within presently active extensional fault belts of the MER, presents the opportunity to deduce the primitive source(s) of rift magmatism, probe crustal structure and assess the role of magmatism in continental rifting. Mass balance of bulk rock & phenocryst composition and thermodynamic modeling of mafic lavas from both zones of extension indicates heterogeneity between the two extensional fault belts in the MER. Fractionation occurs at a shallower level (~1 kb) beneath the Wonjii Fault Belt in comparison to the Silti-Debre Zeyit Fault Zone, where fractionation occurs at various levels in the crust (<1–35 km). Shallow fractionation of the Wonjii Fault Belt lavas is consistent with calderas observed along the belt (e.g. Kone, Gedemsa) that are largely absent from the Silti-Debre Zeyit Fault Zone, where explosive maars associated with basaltic eruptions are more common. Given the strong association between these extensional fault belts and magma facilitated crustal modification (e.g. dyking, calderas), the sub-aerial distribution of Quaternary volcanism outlines the spatial distribution of lithospheric modification attendant to rifting and extension in central Ethiopia. We present lithospheric xenolith data that clearly show magmatic intrusions in the lithosphere beneath these active zones of extension, supporting geodynamic models of magma-assisted rifting. The broadly basaltic composition of these lithospheric xenoliths confirms the important role of mafic volcanism in modifying crustal structure. The Debre Zeyit and Butajira volcanic zones exhibit non-overlapping 87Sr/86Sr and 207Pb/204Pb ratios (0.7038-0.7045, 15.57-15.60 in Debre Zeyit; and 0.7036-.7038, 15.54-15.56 in Butajira). While other isotopic systems do not exhibit such variation (143Nd/144Nd 0.51261-0.51283, Debre Zeyit and 0.51278 to 0.51282, Butajira; 206Pb/204Pb 18.15-18.58, Debre Zeyit and 18.17-18.52, Butajira; 208Pb/204Pb 38.38-38.60, Debre Zeyit and 38.17 to 38.45, Butajira). The ubiquitous signature of the Afar Plume in Quaternary volcanism throughout the region supports magma assisted rifting models. However, a depleted mantle signature is observed in southern samples (Butajira), indicative of intrusion into the SCLM prior to the current episode of volcanism. This intrusion of depleted mantle derived melt into the lithosphere may reflect decompression melting due to the northward propagation of the MER, suggesting that the widespread influence of the Afar plume may be a more recent phenomenon. Combining these observations with existing geophysical data, I present a new geodynamic model for rift evolution in Ethiopia. This model associates processes active to the northeast of the Boru-Toru Transfer Zone with the Afar Depression and incipient seafloor spreading. To the southwest, continental rifting dominates and extension occurs in tectono-magmatic belts close to the rift margins with an overall orientation similar to the rift border faults. Within the framework the Wonjii Fault Belt is considered the southward propagation of the Red Sea Rift while the Silti-Debre Zeyit Fault Zone represents the northward propagation of the Main Ethiopian Rift.