Patterns in the Mantle Discontinuity Structure of the Cratonic Lithosphere

Abstract:

Layering of seismic velocities within the cratonic lithosphere, sometimes known as mid-lithospheric discontinuities, provides information about the processes that formed the cratonic mantle or contributed to its subsequent evolution. Previous studies have identified negative shear velocity gradients in the shallow lithosphere of cratons, but the existence, characteristics, and origins of these discontinuities are disputed. To conservatively measure such discontinuities, we selected long-running stations with abundant data, deconvolved S-to-P converted phase receiver functions in the time domain, employed broad (2-100 s) bandpass filters, and selected high-quality traces using a k-means clustering algorithm. Sp phases in the resulting stacks depend on proximity to fast, thick lithospheric mantle. For stations where shear velocities (Vs) are < 4.5 km/s at 150 km depth, we observe phases indicative of negative velocity gradients in the shallow mantle. Some of these velocity gradients are potentially the lithosphere-asthenosphere boundary at the margins or cratonic regions, and are excluded from further analysis. We observe distributed, moderate amplitude negative velocity gradients in the upper mantle of regions with Vs between 4.5 and 4.675 km/s at 150 km depth, and we classify these zones as the outer belts of thick cratonic lithosphere. In the interiors of cratons (Vs > 4.675 km/s at 150 km depth) we observe fewer discontinuities in the mantle lithosphere. Negative velocity gradients in both outer belts and interiors of cratons can be explained by Vs drops of 2-4%. These velocity reductions are modest enough to be consistent with the upper boundaries of layers of low velocity minerals created by metasomatism, but other mechanisms cannot be ruled out. We also observe positive velocity gradients below 150 km in ~30% of receiver function stacks, indicating the sporadic presence of a seismic velocity increase at depths associated with the Lehmann discontinuity. We also find that negative velocity gradients near the base of the lithosphere (150-300 km) are less common in the highest velocity craton interiors, consistent with a lower or more gradual velocity contrast at the base of the lithosphere in these regions. Joint inversions of this receiver function data with Rayleigh wave phase velocities are underway.