The X-discontinuity: Seismic signatures of a heterogeneous upper mantle

Change log

The structure and composition of the Earth’s interior provokes great debate within the Earth Science community. While the bulk composition and density of the Earth are largely known, many discrepancies exist between different probes of the Earth, and little is understood of the distribution of compositional heterogeneity within the Earth. Seismic discontinuities arise from jumps in velocity and density, with global discontinuities manifesting from mineral phase transitions and large-scale changes in composition like the core-mantle boundary. Widely distributed previous seismic observations of localised reflectors in the upper mantle have been termed the X-discontinuity (X; 230-350 km depth) and allude to the widespread presence of chemical heterogeneity, but the exact cause of the X is yet to be determined.

Receiver function (RF) techniques using P-to-S converted waves are capitalised upon to study the X at higher spatial resolution than previously attainable. Stacking RFs in the depth and time-slowness domains provides identification of the X beneath 15 hotspots. The X appears more prevalent at hotspots compared to six reference locations. Amplitude information from recorded and synthetically modelled RFs constrains the causal mechanism of the X in mantle plume settings to the coesite-stishovite phase transition and/or the presence of carbonated silicate melt, shedding insight into the nature of mantle plumes and their role in mantle convection.

I study the presence of the X beneath Africa, using continuous observations and a vote map style procedure to understand the length scales of the X, its short wavelength variation, and the presence of topography across the discontinuity. The X continues to be co-located with regions of mantle upwelling and observations appear to show a strong dependence on back azimuthal distribution, suggesting that the X may possess short wavelength topography.

Finally, with the potential to add further constraints on the X and increase data coverage, I determine the feasibility of using S-to-P converted RFs for studying the X. While it is possible to record signals from the upper mantle using these RFs, the presence of interfering phases greatly limits the epicentral distance range available for study and requires careful windowing of the source for iterative deconvolution. I do not recommend this potential complimentary probe.

Cottaar, Sanne
Seismology, Receiver Functions, X-discontinuity, Upper Mantle, Hotspot
Doctor of Philosophy (PhD)
Awarding Institution
University of Cambridge
Natural Environment Research Council (2073237)