Superior Layer-Cake: Gradual Lower Boundary and Textured Interior of the Lithosphere

English Abstract:

The lithosphere of Archean cratons reaches deeper than 200 km, and may be delineated by the distribution of elevated seismic wave speed in the mantle. The precise location of the decrease in speed that marks the lower boundary of the lithosphere has been difficult to define, leading to the inclusion of the Lithosphere-Asthenosphere Boundary (LAB) among the main challenges in modern seismological studies.

We investigate the properties of the mantle lithosphere beneath the Superior province of Quebec using teleseismic receiver function analysis. We rely on a data set spanning over 5 years from a number of long-running observatories in Quebec. Long duration of observations ensures excellent directional coverage at all sites we examine.

The appearance of a converted phase in a receiver function documents the presence of a vertical gradient in properties that takes place over a distance that is "short" relative to the wavelength of the phase. Our examination of the receiver functions from the Superior province fails to identify consistent P-to-SV converted phases with delay times ~20 s that would be consistent with the likely depth to the LAB. This is true for all frequencies down to 0.125 Hz, and implies that changes in isotropic seismic properties associated with the LAB take place over vertical distances similar to, or larger than the corresponding shear-wave wavelength (~36 km).

Interestingly, numerous converted phases are detected between 10 and 20 s at most sites we have examined. This range of delays corresponds to depths in excess of 100 km. On the transverse component of receiver functions these phases display directional polarity changes characteristic for conversions from boundaries that have significant dip, or else have anisotropic seismic velocity adjacent to them. On the Radial component the interval between 10 and 20 s delays is overprinted by crustal multiples that are excited well by the sharp Moho of the Superior province crust. Directional patterns of SH-polarized (transverse) phases are distinct from those of the crustal multiples. As we do not find corresponding SV-polarized phases, we favor an interpretation in terms of the abrupt changes in mantle rock texture, as might result from episodes of shearing, delamination or underplating, all likely to have affected the long-lived lithosphere of the craton.