Crustal Structure at a Young Continental Rift: A Receiver Function Study From the Tanganyika Rift

Isabel Hodgson,Finnigan Illsley‐Kemp,Cynthia J Ebinger,Ryan J Gallacher,Derek Keir,Khalfan Mtelela

doi:10.1002/2017tc004477

Abstract

AbstractThe southern Tanganyika Rift, within the Western rift, Africa, has earthquakes to depths of 37 km, yet few constraints exist on crustal thickness, or of early stage rifting processes in apparently amagmatic rift sectors. The aim of the TANGA14 experiment was to constrain bulk crustal properties to test whether magmatic processes modify the lithosphere in areas of deep seismicity, and the degree of lithospheric thinning. We use 11 broadband seismometers to implement receiver function analysis using H‐κ stacking, a method sensitive to crustal thickness and VP/VS ratio, to determine bulk crustal properties. Analyses include extensive error analysis through bootstrap, variance, and phase‐weighted stacking. Results show the Archean Tanzanian Craton and Bangweulu Block are characterized by VP/VS ratios of 1.75–1.77, implying a felsic bulk composition. Crust beneath the fault‐bounded basins has high VP/VS (>1.9). Anorthosite bodies and surface sediments within the region may contribute to localized high VP/VS. However, elevated VP/VS values within fault‐bounded extensional basins where elevated heat flow, hydrothermal vent sites, and deep earthquakes are observed suggest that magma may be intruding the lower crust beneath the southern Tanganyika Rift. Crustal thicknesses on/near the relatively unextended Tanzanian craton and Bangweulu Block are 41.6–42.0 km. This contrasts with the Tanganyika Rift where crustal thicknesses are 31.6 km to 39 km from north to south. Our results provide evidence for ~20% crustal thinning localized to fault‐bounded basins. Taken together, they suggest a previously unrecognized role of magma intrusion in early stage continental rifting in the Western rift, Africa.

Highlights

The southern Tanganyika Rift, within the Western rift, Africa, has earthquakes to depths of 37 km, yet few constraints exist on crustal thickness, or of early stage rifting processes in apparently amagmatic rift sectors
Numerical models predict that tectonic forces are insufficient to initiate rifting of thick, strong continental lithosphere and favor magmatic processes for helping to initiate extension of the East African rift system (EARS) (Bialas et al, 2010; Buck, 2004; Koptev et al, 2015)
The Tanzanian Craton and Bangweulu Block were linked by several Proterozoic belts in the Precambrian (e.g., Lenoir et al, 1994; McConnell, 1950) (Figure 2)

Summary

Introduction

Numerical models predict that tectonic forces are insufficient to initiate rifting of thick, strong continental lithosphere and favor magmatic processes for helping to initiate extension of the East African rift system (EARS) (Bialas et al, 2010; Buck, 2004; Koptev et al, 2015). Regional surface wave studies of the Rukwa-Malawi region show mantle low-velocity zones localized to the Rungwe volcanic province (Accardo et al, 2017). Faults bounding the Tanganyika Rift transect three major geological terrains: the Tanzanian Craton, the Bangweulu Block, and a series of Proterozoic belts bounding each craton (Figure 2). The Tanzanian Craton comprises Archean terrains that coalesced ~2.6 Ga (Bellucci et al, 2011 and references therein) which can be characterized by very low surface heat flow (~34 mW/m2) (Nyblade et al, 1990). The Tanzanian Craton and Bangweulu Block were linked by several Proterozoic belts in the Precambrian (e.g., Lenoir et al, 1994; McConnell, 1950) (Figure 2). The most prevalent lithology within the belt is amphibolebearing orthogneiss (diorite to granite) with rare quartzites and metacalcareous rocks hosting remnants of high P-T rock associations (Lenoir et al, 1994)

Methods

Discussion

Conclusion