P AND S BODY WAVE TOMOGRAPHY OF THE NORTHERN LAKE MALAWI RIFT BASIN AND RUNGWE VOLCANIC PROVINCE

Open Access
Author:
Grijalva, Ashley Nichole
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
March 27, 2017
Committee Members:
  • Andrew Nyblade, Thesis Advisor
  • Charles Ammon, Committee Member
  • Christelle Wauthier, Committee Member
Keywords:
  • seismology
  • crust
  • mantle
  • east african rift zone
  • velocity
  • tomography
  • rift
Abstract:
The Lake Malawi Rift, situated at the southern end of the East African Rift System, is in an early stage of rifting. The estimated available tectonic forces are insufficient to initiate rupture, which suggests a weakening mechanism such as magmatism and/or dike intrusions is assisting rift development. Magmatism can thermally weaken the lithosphere and thus enable continental rupture to occur at a lower tectonic stress. To determine if there is seismic evidence for thermal perturbations in the upper mantle, I investigated the P- and S-wave velocity structure of the upper mantle beneath the northern end of the Lake Malawi Rift basin and Rungwe Volcanic Province (RVP). The upper mantle velocity structure has been tomographically imaged using P- and S-wave relative arrival time residuals from earthquakes recorded on the SEGMeNT seismic network and several previous networks in eastern Africa. Tomographic images of P- and S-wave velocity models reveal a prominent low wave speed anomaly (LWA) with reductions in Vp of 1.5 - 2.2% and Vs of 2 - 3% beneath the RVP between depths of ~50 to 300 km. This LWA can be attributed to a 100 - 200 K thermal anomaly. The tomographic images indicate that there is no basin wide ( > 75 km) thermal anomaly extending beneath the northern part of the Lake Malawi rift from the RVP. However, the presence of localized heating and/or magmatic modification of the lithosphere beneath the Lake Malawi rift from magmatic activity such as dike swarms and small pockets of melt on a sub-basin scale cannot be ruled out. Three geodynamic models for the origin of the LWA beneath the RVP, superplume, lithospheric drip, and small-scale convection, are examined. The lithospheric drip and small-scale convection models cannot readily explain magmatism in the RVP and thus are not favored. In examining the superplume model, a direct connection is not seen in the tomographic models between the African superplume and the RVP LWA. However, the LWA could be linked to the superplume structure by a small upwelling that is too narrow to be imaged or alternatively, via a small thermal plumelet that rose from the superplume structure in the mantle transition zone.