THE PRECAMBRIAN CRUSTAL STRUCTURE OF EAST AFRICA

Open Access
Author:
Tugume, Fred Alex
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 03, 2011
Committee Members:
  • Andrew A. Nyblade, Committee Chair
  • Charles J. Ammon, Committee Member
  • S. Anandakrishnan, Committee Member
  • Derek Elsworth, Committee Member
Keywords:
  • Precambrian crustal structure
  • Receiver functions
Abstract:
ABSTRACT In this thesis, the Precambrian crustal structure of East African is investigated along with the crustal structures of three Cenozoic rift basins located in the western branch of the East African Rift System (EARS). In the first part of the thesis, P-wave receiver functions are modeled using the H-k method to obtain new insights about the bulk composition and thickness of the crust for Precambrian terrains throughout East Africa. The average crustal thickness for all but one of the terrains is between 37 and 39 km. An exception is the Ubendian terrain, which has an average crustal thickness 42 km. In all terrains, the average Poisson’s ratio is similar, ranging from 0.25 to 0.26, indicating a bulk crustal composition that is felsic to intermediate. The main finding of this study is that crustal structure is similar across all terrains, which span more than 4.0 Ga of earth history. There is no discernable difference in the crustal thicknesses and Poisson’s ratios between the Archean and Proterozoic terrains, or between the Proterozoic terrains, unlike the variability in Precambrian crustal structure found in many other continents. In the second part of the thesis, a joint inversion of Rayleigh wave phase and group velocities and receiver functions was used to investigate the shear wave velocity structure of the crust and uppermost mantle beneath the Precambrian terrains of East Africa. In comparison with other areas of similar age in southern and western Africa where the same joint inversion method has been applied, I find that while there is little difference in the mean shear wave velocities for the entire crust across all of the Precambrian terrains, and also few differences in the thickness of the crust, there exists substantial variability in lower crustal structure. This variability is reflected primarily in the thickness of the lower crustal layers with shear wave velocities ≥ 4.0 km/s. This variability is found both within terrains of the same age (i.e., Archean) as well as terrains of different ages. In global studies of continental crustal structure, it is shown that high velocity (Vp ~ 7km/s and Vs ~4.0 km/s) lower crustal layers indicate the presence of mafic rocks, and that such layers are common in most Precambrian terrains. In contrast, my results show large variability in lower crustal structure between terrains of similar age within Africa, suggesting that making generalizations about the structure and evolution of continental crust based on global averages of the crustal velocity structure is difficult to do. The results of this study indicate that the local geological history of each terrain can lead to significant variability in crustal structure, making the use of global averages less representative of global processes than previously thought. In the third part of this thesis, forward modeling of receiver functions is used to investigate the crustal structure beneath three of the rift basins in the western branch of the East African Rift System (Lake Albert, Lake Edward and the northern part of the Lake Malawi rifts). Modeling results reveal sediment thickness of about 1 km beneath stations BUTI and KATE, located at the margins of the Lake Albert and Lake Edward rifts, respectively, and about 1.5 km beneath station KYLA in the northern part of the Lake Malawi rift. The Poisson’s ratio obtained (0.4) indicate that the sediments are poorly consolidated. A thin sedimentary layer (1 – 1.5 km) and high Poisson’s ratio is consistent with the young age (Miocene) of the rift basins. The Ps from the Moho cannot be easily identified because the first 5-6 sec of data are dominated by the Ps conversion from the sediment-bedrock interface and its reverberations. Therefore crustal thickness beneath these stations remains unknown.