CRUSTAL STRUCTURE OF THE RUNGWE VOLCANIC PROVINCE AND REGION SURROUNDING THE NORTHERN LAKE MALAWI RIFT BASIN

Open Access
Author:
Borrego, David James
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
August 07, 2016
Committee Members:
  • Andy Nyblade, Thesis Advisor
  • Charles J. Ammon, Committee Member
  • Christelle Wauthier, Committee Member
Keywords:
  • Africa
  • East African Rift System
  • Receiver Functions
  • Joint Inversion
  • Rungwe Volcanic Province
  • Magmatism
  • Crustal Structure
  • Malawi Rift
Abstract:
The crustal structure around the northern end of the Lake Malawi rift and beneath the Rungwe Volcanic Province (RVP) in southern Tanzania has been investigated using teleseismic receiver functions from SEGMeNT broadband seismic stations to determine the extent to which the Precambrian crust has been modified by the Cenozoic rifting and magmatism. The SEGMeNT network included 57 broadband seismic stations deployed in northern Malawi and southern Tanzania between August 2013 and October 2015. Estimates of crustal structure have been obtained by modeling P-wave receiver functions using the H-k stacking method and jointly inverting the receiver functions with Rayleigh wave phase velocities. The average crustal thickness for the four stations in the RVP is 39 km, the average Poisson’s ratio is 0.28, and the average crustal shear wave velocity is 3.6 km/s. There is little evidence that the composition of the lower crust or the crustal thickness beneath the RVP has been modified by Cenozoic magmatism, but Poisson’s ratios 0.29 to 0.31 at three of the stations in the RVP may indicate the presence of partial melting associated with the RVP magmatism. The average crustal thickness of Proterozoic terrains surrounding the Lake Malawi rift is between 38 and 42 km. In most of the terrains the average Poisson’s ratios are between 0.25 and 0.26, with the exception of the Irumide belt, which has an average Poisson’s ratio of 0.23. The average crustal shear wave velocities for all the Proterozoic terrains are between 3.6 and 3.7 km/s. These results indicate a bulk felsic to intermediate composition for the Proterozoic crust and show that there is little, if any, crustal thinning beneath the rift flanks. Because crustal thinning does not extend far beneath either rift flank, as would be expected for the simple shear model of rifting, rift models invoking pure shear extension or extension aided by magmatic processes are favored for the formation of the Lake Malawi rift. However, it is also possible that crustal extension may not have progressed to a stage where the differences in crustal structure between simple and pure shear models of extension would be evident.