The KRISP and EAGLE projects: crustal structure in the East African Rift

Posted by pkm at Jan 15, 2015 07:24 AM |
G. Randy Keller - School of Geology and Geophysics, University of Oklahoma

The Kenya Rift International Seismic Project (KRISP) started in 1968 as a largely British effort and blossomed into a large multi-national cooperative effort that involved experiments in 1985, 1989-90, and 1994. The seismic refraction/wide-angle reflection profiles and teleseismic data produced several results that were transformative within the larger perspective of the East African rift system and for the understanding of continental rifting in general. One result was the discovery that the amount of magmatic modification of the pre-rift crust is modest rather than involving a large axial “dike” that affected the entire crustal column. Mafic underplating of the crust was observed in the Kenya topographic dome area and axial variation of crustal thickness by 50% was observed. Additionally, the crust and upper mantle anomaly associated with the Kenya dome area is deep-seated, only slightly wider than the rift valley, and surprisingly steep-sided. The seismic data agree very well with the observed north to south decrease of Bouguer gravity anomaly values. The area of thickest crust correlates with the apex of the Kenya dome where the elevation of the rift valley floor is highest. As one proceeds northward along the rift valley from the Lake Naivasha area, the physiographic expression of the rift valley widens from its minimum of about 60 km to about 180 km in the area of thinnest crust, and the elevation of the rift valley floor decreases from ~2 km to ~440 m that is the level of Lake Turkana. In addition to these observations, published seismic reflection results near Lake Turkana indicate about 35-40 km of extension across the rift while it is only 5-10 km in the region of Naivasha-Nakuru. These observations provide an internally consistent relationship between crustal thickness, amount of extension, width of the rift zone, and topographic relief.

Following KRISP, The Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE) was conducted mostly in 2002 and included a significant number of broadband instruments. A major goal was achieving a better understanding of the processes involved in continental break-up. Axial and cross-rift crustal structures in Ethiopia were similar to those in Kenya, but across the Ethiopian Plateau the thick crust is probably due to lower crustal underplating associated with extensive flood basalts. In addition, the mantle results showed that the upper mantle across the entire Ethiopian topographic dome has been modified by the presence of the Afar plume head, which is in contrast to the narrow zone of modification across the Kenya dome.

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