A contrast in anisotropy across mid-lithospheric discontinuities beneath the central United States—A relic of craton formation

Wirth, Erin A.; Long, Maureen D.

doi:10.1130/g35804.1

Cited by 86 publications

(127 citation statements)

References 31 publications

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“…MLDs have been associated with formation and thickening of cratonic nuclei (Abt et al, 2010;Fischer et al, 2010;Miller and Eaton, 2010;Yuan and Romanowicz, 2010;Snyder et al, 2013;Cooper and Miller, 2014;Wirth and Long, 2014). If they represent palaeo-LABs prior to root thickening, azimuthal anisotropy could result by the same mechanisms as proposed for modern LABs, i.e.…”

Section: Frozen Rock Fabrics: Mlds As Palaeo-labsmentioning

confidence: 97%

“…Discontinuities in seismic velocity and anisotropy in the crust and upper mantle are imaged using receiver functions (RF), based on different arrival times for primary and converted P-and S-waves (termed PRF and SRF, respectively) and back-azimuthal differences in amplitudes (Kind et al, 2012;Wirth and Long, 2014;Selway et al, 2015).…”

Section: Geophysical Observationsmentioning

confidence: 99%

“…While negative velocity gradients beneath intact cratons at ~170-250 km depth are generally interpreted as the LAB (section 4.1), seismic discontinuities, with a thickness of ~30-40 km and occurring at 60-160 km depth (predominantly between 80-120 km depth) beneath continental lithosphere (Thybo, 2006;Rychert and Shearer, 2009) have been interpreted as MLDs (Fischer et al, 2010;Yuan and Romanowicz, 2010;Ford et al, 2010;Foster et al, 2014;Wirth and Long, 2014;Selway et al, 2015; for a compilation of MLD locations, depths and associated velocity decreases see Rader et al, 2015) (Fig. 3D).…”

Section: Palaeo-labs or Future Labs ?mentioning

confidence: 99%

“…MLDs are also detected when adjacent layers show seismic anisotropy contrasts with regard to strength and orientation (Wirth and Long, 2014) and in some instances they correlate with anomalous conductivity (e.g. Jones et al, 2001).…”

mentioning

confidence: 99%

“…An important point that has been emphasised in recent studies is that the results of seismic studies depend critically on the energy spectrum used to filter the data. Thus, low frequencies favour the appearance of single coherent large phases, whereas higher frequencies allow the detection of multiple conversions with lower amplitudes that appear as multiple MLDs (Wirth and Long, 2014). As a consequence, single larger MLDs may be an artefact of lower frequency filtering (Wirth and Long, 2014).…”

mentioning

confidence: 99%

See 4 more Smart Citations

Origins of cratonic mantle discontinuities: A view from petrology, geochemistry and thermodynamic models

Aulbach

Massuyeau

Gaillard

2017

Lithos

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Section: Frozen Rock Fabrics: Mlds As Palaeo-labsmentioning

confidence: 97%

Section: Geophysical Observationsmentioning

confidence: 99%

Section: Palaeo-labs or Future Labs ?mentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

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Origins of cratonic mantle discontinuities: A view from petrology, geochemistry and thermodynamic models

Aulbach

Massuyeau

Gaillard

2017

Lithos

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Characterization and Petrological Constraints of the Midlithospheric Discontinuity

Rader

Emry

Schmerr

et al. 2015

Geochem Geophys Geosyst

111

143

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Within continental lithosphere, widespread seismic evidence suggests a sharp discontinuous downward decrease in seismic velocity at 60–160 km depth. This midlithospheric discontinuity (MLD) may be due to anisotropy, melt, hydration, and/or mantle metasomatism. We survey global seismologic observations of the MLD, including observed depths, velocity contrasts, gradients, and locales across multiple seismic techniques. The MLD is primarily found in regions of thick continental lithosphere and is a decrease in seismic shear velocity (2–7% over 10–20 km) at 60–160 km depth, the majority of observations clustering at 80–100 km. Of xenoliths in online databases, 25% of amphibole‐bearing xenoliths, 90% of phlogopite‐bearing xenoliths, and none of carbonate‐bearing xenoliths were formed at pressures associated with these depth (2–5 GPa). We used Perple_X modeling to evaluate the elastic moduli and densities of multiple petrologies to test if the MLD is a layer of crystallized melt. The fractional addition of 5–10% phlogopite, 10–15% carbonate, or 45–100% pyroxenite produce a 2–7% velocity decrease. We postulate this layer of crystallized melt would originate at active margins of continents and crystallize in place as the lithosphere cools. The concentration of mildly incompatible elements (Y, Ho, Er, Yb, and Lu) in xenoliths near the MLD is consistent with higher degrees of melting. Thus, we postulate that the MLD is the seismological signature of a chemical interface related to the paleointersection of a volatile‐rich solidus and progressively cooling lithosphere. Furthermore, the MLD may represent a remnant chemical tracer of the lithosphere‐asthenosphere boundary (LAB) from when the lithosphere was active and young.

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Teleseismic P wave spectra from USArray and implications for upper mantle attenuation and scattering

Cafferky

Schmandt

2015

Geochem Geophys Geosyst

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Teleseismic P wave amplitude spectra from deep earthquakes recorded by USArray are inverted for maps of upper mantle Δt* for multiple frequency bands within 0.08–2 Hz. All frequency bands show high Δt* regions in the southwestern U.S., southern Rocky Mountains, and Appalachian margin. Low Δt* is more common across the cratonic interior. Inversions with narrower frequency bands yield similar patterns, but greater Δt* magnitudes. Even the two standard deviation Δt* magnitude for the widest band is ∼2–7 times greater than predicted by global QS tomography or an anelastic olivine thermal model, suggesting that much of the Δt* signal is nonthermal in origin. Nonthermal contributions are further indicated by only a moderate correlation between Δt* and P travel times. Some geographic variations, such as high Δt* in parts of the cratonic interior with high mantle velocities and low heat flow, demonstrate that the influence of temperature is regionally overwhelmed. Transverse spectra are used to investigate the importance of scattering because they would receive no P energy in the absence of 3‐D heterogeneity or anisotropy. Transverse to vertical (T/Z) spectral ratios for stations with high Δt* are higher and exhibit steeper increases with frequency compared to T/Z spectra for low Δt* stations. The large magnitude of Δt* estimates and the T/Z spectra are consistent with major contributions to Δt* from scattering. A weak positive correlation between intrinsic attenuation and apparent attenuation due to scattering may contribute to Δt* magnitude and the moderate correlation of Δt* with travel times.

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A contrast in anisotropy across mid-lithospheric discontinuities beneath the central United States—A relic of craton formation

Cited by 86 publications

References 31 publications

Origins of cratonic mantle discontinuities: A view from petrology, geochemistry and thermodynamic models

Origins of cratonic mantle discontinuities: A view from petrology, geochemistry and thermodynamic models

Characterization and Petrological Constraints of the Midlithospheric Discontinuity

Teleseismic P wave spectra from USArray and implications for upper mantle attenuation and scattering

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