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AfricaArray Science Projects

A number of AfricaArray science projects are under way or are in various stages of planning.

Seismic Structure of the Crust and Upper Mantle Beneath Ethiopia/Afar Hotspot and Kenya
Cameroon Broadband Seismic Experiment
Imaging the African Superplume
Projects Under Development

Seismic Structure of the Crust and Upper Mantle Beneath Ethiopia/Afar Hotspot and Kenya

In these projects, broadband seismic data recorded by deployments of 25 seismic stations in Ethiopia and 10 stations in Kenya are being used to interrogate crust and upper mantle structure under the Ethiopian Plateau, the Main Ethiopian Rift, the Afar depression, and the Kenya Rift for seismic evidence of a mantle plume(s). The field deployment in Ethiopia began in March 2000 and was completed in March 2002.  The field deployment in Kenya began in July 2001 and was completed in July 2002.  Data recovery exceeded ~80%, and the data was released to the community July 2004 through the IRIS data management center. 

Several major finding have come from this project so far.

1. Crustal structure has been modeled using receiver functions and the results show normal Precambrian crust under the volcanic cover on the Ethiopian Plateau and Kenya Dome.  In the Main Ethiopian Rift and Afar Depression, the crust has been thinned and the high Poisson’s ratios indicate the presence of partial melt within the crust (Dugda et al., 2005). 
2. Body wave tomography reveals that upper mantle structure under the Afar Hotspot is characterized by a low wave speed region that is deep (> 400 km) and trends to the southwest from the Afar Depression across western Ethiopia.
3. Group velocity maps have been obtained for Rayleigh and Love waves and are being inverted for anisotropic structure.
4. Local/near regional events are being located using P and S arrival times, and focal mechanisms are being determined for the larger events (M> 3.5) using grid searches and waveform modeling. 
5. Waveforms from regional events have been modeled for average upper mantle structure across both the East African and Ethiopian Plateaus.
6. Shear wave splitting analysis has been completed for Tanzania, Kenya and Ethiopia (Walker et al, 2004; Gashawbeza et al., 2004) showing a rift-parallel trend of fast directions. 

We are also in the process of inverting receiver functions and Rayleigh wave group velocities for crust and uppermost mantle structure, and stacking receiver functions for transition zone structure (410 and 660 km discontinuities).

Papers

Nyblade, A.A., and C. A. Langston, Broadband seismic experiments probe the East Africa Rift, EOS Trans. AGU, 83, 405-409, 2002.

Benoit, M., A.A. Nyblade, J. Vandecar, and H. Gurrola, Upper mantle P velocity structure and mantle transition zone thickness beneath the Arabian Shield, in press, Geophysical Research Letters, 30, 1153, doi:10.1029/2002GL016436, 2003.

Walker, K., A. Nyblade, S. Klemperer , G. Bokelmann, and T. Owens, On the relationship between extension and anisotropy: Constraints from shear wave splitting across the East African Plateau, J. Geophys. Res., 109, B08302, doi:10.1029/2003JB002866 , 2004

Gashawbeza, E. M, S. L. Klemperer, A. A. Nyblade, K. T. Walker, K. M.Keranen, Shear-wave splitting in Ethiopia: Precambrian mantle anisotropy locally modified by Neogene rifting, Geophy. Res. Lett. 31, L18602, doi:10.1029/2004GL020471, 2004.

Dugda, M. A. Nyblade, and J. Julia, Crustal structure in Ethiopia and Kenya from receiver function analysis; Implications for rift development in eastern Africa, J. Geophys. Res., in press, 2005.

Cameroon Broadband Seismic Experiment

 In this project we will critically evaluate models for the origin of volcanic lines without age progression by imaging the seismic structure of the upper mantle beneath the Cameroon Volcanic Line (CVL).  Volcanic lines without age progression cannot be explained by the conventional plume model, and there is no consensus about their origin.  One family of models proposes that hot lines can be explained by added complexities within the plume framework.  These models include lateral transport of plume material, recurrent volcanism due to relict plume material in the mantle along an old plume track, and multiple plume models.  Several investigators have proposed lithospheric tensional cracks to explain simultaneous volcanism along linear features.  Another interesting proposal suggests that hot lines may mark the upwelling limb of small scale convection cells. 

The CVL, a 1500 km long line of Cenozoic volcanic constructs, provides an excellent opportunity to investigate the mantle structure of a hot line.  The CVL is one of the most prominent volcanic lines lacking a discernable age progression, and it is relatively active, with holocene volcanism along nearly its entire length.  About one-half of the length of the CVL occurs on land, allowing extensive study of upper mantle structure without the expense of seafloor instrumentation. 

To image the seismic structure of the upper mantle beneath the CVL, we will carry out a 21-month passive-source broadband seismic experiment in Cameroon beginning in early 2005.  For the first 9 months of the experiment, the seismic network will consist of 8 stations.  For the final 12 months, the network will be densified to 30 stations.  The broadband seismic data will be analyzed using a number of proven modeling techniques, including body and surface wave tomography, receiver functions, and shear wave splitting.  The results will enable us to test candidate models for the origin of the CVL and will provide important constraints on the distribution of hot mantle material and the dynamics of the mantle beneath hot lines in general.

Imaging the African Superplume

The African Superplume is a large region of low seismic wave speeds in the lower mantle under southern Africa that has long been recognized as one of the most prominent features of the mantle. Above the African Superplume lies the African Superswell, suggesting a geodynamic link between lower mantle dynamics and geologic processes shaping the African plate. The origin and nature of the African Superplume is controversial. The initial interpretation of the low wave speed region under southern Africa attributed it to a long-lived, hot mantle upwelling. A number of seismic studies since then have suggested the presence of chemical heterogeneity within the superplume. 

What parts of the superplume anomaly are thermal vs. chemical (or both) remains uncertain, as does its origin. In this project, the structure, composition and origin of the African Superplume will be investigated using the first 3 years of broadband seismic data from AfricaArray, together with existing data, and concentrating on four types of analyses; 1) tomographic imaging of the upper and lower mantle using body wave travel times, 2) modeling waveforms of teleseismic body wave phases that sample the Superplume, 3) jointly inverting receiver functions and surface wave dispersion measurements for crust and uppermost mantle structure, and 4) stacking and migrating receiver functions to image topography on the 410 and 660 km discontinuities. 

Projects in various stages of development:

• Congo Craton
• Bushveld Complex
• Northern East African Plateau
• East African Rift Basins ( Karoo through Cenozoic)
• Rungwe Volcanics and Natural Hazards
  

For more information please contact:
Dr. Andrew Nyblade, The Pennsylvania State University, email: andy@geosc.psu.edu, 814.863.8341
Dr. Paul Dirks, University of the Witwatersrand, email: dirksp@geosciences.wits.ac.za, phone +27 11 717-6547