Multigenetic Origin of the X-discontinuity Below Continents: Insights
from African Receiver Functions
Abstract
Constraints on chemical heterogeneities in the upper mantle may be
derived from studying the seismically observable impedance contrasts
that they produce. Away from subduction zones, several causal mechanisms
are possible to explain the intermittently observed X-discontinuity (X)
at 230-350km depth: the coesite-stishovite phase transition, the
enstatite to clinoenstatite phase transition and/or carbonated silicate
melting, all requiring a local enrichment of basalt. Africa hosts a
broad range of terranes, from Precambrian cores to Cenozoic hotspots
with or without lowermost mantle origins. With the absence of subduction
below the margins of the African plate for >0.5Ga, Africa
presents an ideal study locale to explore the origins of the
X.
Traditional receiver function (RF) approaches used
to map seismic discontinuities, like common conversion-point stacking,
ignore slowness information crucial for discriminating converted upper
mantle phases from surface multiples. By manually assessing depth and
slowness stacks for 1° radius overlapping bins, normalized vote mapping
of RF stacks is used to robustly assess the spatial distribution of
converted upper mantle phases. The X is mapped beneath Africa at
233-340km depth, revealing patches of heterogeneity proximal to mantle
upwellings in Afar, Canaries, Cape Verde, East Africa, Hoggar, and
Réunion with further observations beneath Cameroon, Madagascar, and
Morocco. There is a lack of an X beneath southern Africa, and
strikingly, the magmatic eastern rift branch of the southern East
African Rift. With no relationships existing between depth and
amplitudes of observed X and estimated mantle temperatures, multiple
causal mechanisms are required across a range of continental geodynamic
settings.