Melt infiltration of the lower lithosphere beneath the Tanzania craton
and the Albertine rift inferred from S receiver functions
Geochemistry Geophysics Geosystems, (Aug 4, 2012)DOI: 10.1029/2012GC004167
Abstract
The transition between the lithosphere and the asthenosphere is subject
to numerous contemporary studies as its nature is still poorly understood.
The thickest lithosphere is associated with old cratons and platforms
and it has been shown that seismic investigations may fail to image
the lithosphere-asthenosphere boundary in these areas. Instead, several
recent studies have proposed a mid-lithospheric discontinuity of
unknown origin existing under several cratons. In this study we investigate
the Tanzania craton in East Africa which is enclosed by the eastern
and western branches of the East African Rift System. We present
evidence from S receiver functions for two consecutive discontinuities
at depths of 50-100 km and 140-200 km, which correspond to significant
S wave velocity reductions under the Tanzania craton and the Albert
and Edward rift segments. By comparison with synthetic waveforms
we show that the lower discontinuity coincides with the LAB exhibiting
velocity reductions of 6-9\%. The shallower interface reveals a velocity
drop that varies from 12\% beneath the craton to 24\% below the Albert-Edward
rift. It is interpreted as an infiltration front marking the upper
boundary of altered lithosphere due to ascending asthenospheric melts.
This is corroborated by computing S velocity variations based on
xenolith samples which exhibit a dense system of crystallized veins
acting as pathways of the infiltrating melt. Mineral assemblages
in these veins are rich in phlogopite and pyroxenite which can explain
the reduced shear wave velocities. Melt infiltration represents a
suitable mechanism to form a mid-lithospheric discontinuity within
cratonic lithosphere that is underlain by anomalously hot mantle.