S-Velocity Structure of Cratons, from Broad-Band Surface-Wave Dispersion

S-Velocity Structure of Cratons, from Broad-Band Surface-Wave Dispersion Top left: phase-velocity measurements sampling Precambrian continental lithosphere. Top right: summary profile of the isotropic-average shear speed beneath Archean cratons. The ranges of Vs values include the best-fitting profiles from all the cratonic locations sampled. Bottom: radial anisotropy within the upper Precambrian lithosphere. Anisotropy with horizontally polarised shear waves propagating faster than vertically polarised ones (Vsh > Vsv) is observed in the lower crust and mantle lithosphere and indicates horizontally oriented fabric. Anisotropy with Vsh < Vsv is observed in the upper crust beneath a number of locations and suggests the occurrence of vertically oriented fabric.
Despite recent progress in mapping the lateral extent of cratonic roots worldwide [e.g. Lebedev & van der Hilst, 2008], profiles of seismic velocities within them remain uncertain. Here, a novel combination of waveform-analysis techniques was used to measure inter-station, Rayleigh- and Love-wave phase velocities in broad period ranges. The new measurements yield resolution from the upper crust to deep upper mantle beneath a selection of cratons from around the globe and provide new constraints on the thermal and compositional structure and evolution of Precambrian lithosphere.
</p><p>Shear-wave speed Vs is consistently higher in the lithosphere of cratons than in the lithosphere of Proterozoic foldbelts. Because known effects of compositional variations in the lithosphere on Vs are too small to account for the difference, this confirms that temperature in the cratonic lithosphere is consistently lower. This is in spite of sub-lithospheric mantle beneath continents being thermally heterogeneous, with some cratons currently underlain by a substantially hotter asthenosphere compared to others.
</p><p>An increase in Vs between the Moho and a 100-150 km depth is consistently preferred by the data and is likely to be due to phase transformations, in particular the transition from spinel peridotite to garnet peridotite, proposed previously to give rise to the “Hales discontinuity” within this depth interval. The depth and the width of the phase transformation depend on mantle composition; it is likely to occur deeper and over a broader depth interval beneath cratons than elsewhere because of the high Cr content in the depleted cratonic lithosphere, as evidenced by a number of xenolith studies. Seismic data available at present would be consistent with both a sharp and a gradual increase in Vs in the upper lithosphere (a Hales discontinuity or a “Hales gradient”).
</p><p>Radial anisotropy in the upper crust indicates vertically oriented anisotropic fabric (Vsh < Vsv); this may yield a clue on how cratons grew, lending support to the view that distributed crustal shortening with sub-vertical flow patterns occurred over large scales in hot ancient orogens. In the lower crust and upper lithospheric mantle, radial anisotropy reveals horizontal fabric (Vsh > Vsv), likely to be a record of horizontal ductile flow in the lower crust and lithospheric mantle at the time of the formation and stabilisation of the cratons.
</p><p>Lebedev, S., J. Boonen, J. Trampert, Seismic structure of Precambrian lithosphere: New constraints from broadband surface-wave dispersion, Lithos, Special Issue "Continental Lithospheric Mantle: the Petro-Geophysical Approach", 109, 96-111, 2009.
</p><p>Lebedev, S., R. D. van der Hilst, Global upper-mantle tomography with the automated multimode inversion of surface and S-wave forms, Geophys. J. Int., 173, 505- 518, 2008.</p>


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