Global Patterns in Cratonic Mid‐Lithospheric Discontinuities From Sp Receiver Functions

Abstract

The goal of this study is to constrain the origins of layering in the seismic velocity structure within the cratonic mantle lithosphere (i.e. mid‐lithospheric discontinuities [MLDs]). For long‐lived stations in cratons worldwide, we calculated S‐to‐P converted phase receiver function stacks using time domain deconvolution and a k‐means algorithm to select robust, consistent receiver functions. Negative MLDs appear in only 50% of the receiver function stacks, indicating that negative MLDs are common but intermittent. The negative MLDs correspond to shear velocity drops of 1%–4%, which could be caused by layers of minerals created by metasomatism, although vertical layering in seismic anisotropy cannot be ruled out. In craton interiors, negative MLDs have a lower amplitude (<3% velocity drops) and can be explained by metasomatism of the original Archean mantle. Negative MLD amplitudes increase with decreasing upper mantle shear velocity (toward the outer margins of the cratons), but do not depend on the age of the craton. Thus, negative MLD amplitudes are not dominated by age‐related variations in the cratonic mantle composition, and, instead, are more strongly correlated with proximity to tectonic and metasomatic activity that occurred long after craton formation. Negative MLDs are less numerous among stations that have Paleoproterozoic and Archean thermotectonic ages, consistent with the view that shallow release of slab‐derived fluids during early “warm” subduction was less favorable for negative MLD formation. We also observe velocity gradients below 150 km at stations in craton boundaries and interiors, indicating the presence of seismic velocity changes at the cratonic lithosphere‐asthenosphere boundary and/or Lehmann discontinuity.

New Insights Into Lithospheric Structure and Melting Beneath the Colorado Plateau

“…Below the Colorado Plateau, this discontinuity becomes deeper and more diffuse, indicating a deeper LAB that is not defined by partial melt. In the Colorado Plateau interior and further east multiple NVGs are occasionally found, which is consistent with global observations of an intermittent MLD (e.g., Abt et al, 2010;Ford et al, 2010;Hansen et al, 2015;Krueger et al, 2021;Rader et al, 2015;Selway et al, 2015).…”

“…Using time-domain deconvolution, and to a secondary degree the other methodological improvements in this study, has removed spurious phases surrounding the Moho and at depth; we also image a more continuous Moho. We echo the conclusions of Krueger et al (2021) with regards to the importance of time-domain deconvolution: in the new stack the NVGs located in the shallow mantle are lower in amplitude and less complex than in Hopper and Fischer (2018). Moreover, our NVGs, deep and shallow alike, do not evidently correlate with the Moho PVGs.…”

“…The abrupt beginning of a melt‐rich layer at the top of the asthenosphere, just below the solidus, has been proposed as an explanation for strong LABs, particularly in the western US (Hansen et al., 2015), and would be a plausible source for volcanism. Similarly, although many theories have been proposed to explain MLDs (Karato et al., 2015), an origin related to mantle melting is a popular candidate: a volatile‐rich layer of present or frozen melt would provide a sufficient velocity decrease to be consistent with Sp RFs (Hansen et al., 2015; Krueger et al., 2021; Rader et al., 2015). We therefore propose that melt pooling below or within the lithosphere at 80 km acts as a source for volcanism and explains the seismic observations.…”

“…Despite these differences, NVGs are still observed at 80 km depth, which could be explained by patches of melt that pool within the lithosphere (Hansen et al., 2013). Such patches could represent a juvenile form of the MLDs that are observed in less recently active, and perhaps cratonic, lithosphere (Hansen et al., 2015; Hopper & Fischer, 2018; Krueger et al., 2021; Rader et al., 2015; Selway et al., 2015). However, this model does not preclude the existence of other mechanisms that have potential to cause MLDs, such as metasomatic processes (Selway et al., 2015), changes in anisotropic fabric (Wirth & Long, 2014), and grain‐scale deformation processes (Karato et al., 2015).…”

“…Krueger et al. (2021) demonstrated that time‐domain deconvolution mitigates these concerns and that with this approach MLDs are still observed, though with less frequency than in previous studies. Furthermore, recent work characterizing the sensitivity of Sp phases to structure (Hansen & Schmandt, 2017; Hua, Fischer, Mancinelli, & Bao, 2020; Hua, Fischer, Wu, & Blom, 2020; Lekić & Fischer, 2017; Mancinelli et al., 2017) allow for improved stacking and open new avenues for quantification of error.…”