New Seismic Observables Constrain Structure within the Continental Lithosphere

Abstract:

The origin and stability of the continental lithosphere play a fundamental role in plate tectonics and enable the survival of Archean crust over billions of years. Recent advances in seismic data and imaging have revealed a velocity drop with depth within continental cratons too shallow to be interpreted as the lithosphere asthenosphere boundary (Rychert and Shearer 2009). The significance of this “mid lithospheric discontinuity” (MLD) – or multiple MLDs as suggested recently (Lekic & Fischer, 2013) – is not fully understood, and its implications for continental formation and stability are only beginning to be explored. Discrepancies call for both improving the constraints on the nature of the MLD, and relating these observations to tectonic setting and deformation history. The extensive coverage of the EarthScope USArray presents an unprecedented opportunity to systematically map the structure of the continental lithosphere. We use receiver functions (RFs) to isolate converted phases (Ps or Sp) produced across velocity discontinuities beneath a seismometer, and thereby constrain vertical density and seismic velocity variations. We show that at some stations, the apparent velocity contrast across the MLD demonstrates a dependence on seismic wave frequency, being greater at low frequencies than at high frequencies. This suggests that the MLD – at least in certain locations – is distributed across tens of kilometers in depth. The gradient of the MLD fingerprints physical process at play; a weak gradient indicates thermal origin, while an abrupt discontinuity implicates change in composition or partial melting. Furthermore, we map the strength, depth, and ratio of amplitudes of waves converted across the MLD and the Moho throughout the US. Because these receiver function based measurements only reveal relative velocity variations with depth, we combine them with frequency-dependent measurements of apparent incidence angles of P and S waves. Doing so allows us to present new constraints on absolute velocity variations in the crust, and relate them to variations in lithospheric structure.

References

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V.Lekic,K.Fischer Earth Planet. Sci. Lett. (2013)