『Abstract
S and P receiver functions from the Southern African Seismic
Experiment are analyzed for lithospheric discontinuities beneath
the Kalahari craton. Besides the Moho, the most prominent feature
is a discontinuous reduction in seismic velocity of about 4.5％
at approximately 150 km depth. The discontinuity appears to have
a width of about 10 km. Termed the K-discontinuity, this feature
is restricted to the northern half of the array, extending from
the Zimbabwe craton south to the TML (Thabazimbi-Murchison lineament),
and in the west from Botswana to the edge of the Kalahari craton
in the east. It spans several Archean sutures and is thus unlikely
related to Archean tectonics. It does, however, appear to be related
to subsequent magmatic episodes. The strongest anomaly id coincident
with the most intense Karoo volcanism, and it extends to the northern
edge of the Bushveld intrusion. From mantle xenoliths and xenocrysts,
the entire lithosphere in this region appears to have experienced
a long-term infiltration of basaltic melt and metasomatic fluids.
We propose that the K-discontinuity reflects the influence of
this melt/metasomatic infiltration, which has, over time, intruded
and refertilized the lithosphere. Based on kimberlites pipes that
show obvious signs of melt metasomatism and likely Karoo influence,
the observed reduction in seismic velocity is plausibly consistent
with the observed major-element and volatile enrichment at 150
km depth in such kimberlites. If this interpretation is correct,
then the high-temperature kimberlite nodules that most clearly
reflect this perturbation likely represent the general state of
the lower lithosphere, rather than only reflecting local mantle
properties in the immediate vicinity of the kimberlite eruption.
Keywords: lithosphere; Karoo; Craton; flood-basalt; seismology;
metasomatism』
1. Introduction
2. Methods
2.1. P to S conversions
2.2. S to P conversions
3. Results
4. Discussion
4.1. Comparison with previous studies
4.2. Possible origin of the K-discontinuity
5. Conclusions
Acknowledgments
References