Mechanisms and geologic significance of the mid-lithosphere discontinuity in the continents

Nature of the seismic lithosphere‐asthenosphere boundary within normal oceanic mantle from high‐resolution receiver functions

“…However, recent high-resolution body-wave studies show fine structure at the inferred LAB depth that may not be explained by standard models. A similar seismic feature at similar depths has been observed in stable continental regions and ascribed to other causal factors [Wirth and Long, 2014;Yuan and Levin, 2014;Selway et al, 2015;Karato et al, 2015;Rader et al, 2015]. Seismic observations respond to mechanical properties at seismic frequencies.…”

Lithospheric low-velocity zones associated with a magmatic segment of the Tanzanian Rift, East Africa

“…16c) provides an alternative explanation for a velocity drop at mid lithospheric depth (Karato 2012;Karato et al 2015). At moderate temperatures, grain boundaries weaken and lead to the reduction of the elastic modulus for polycrystalline aggregates (Zener 1941;Kê 1947).…”

Synthesizing Seemingly Contradictory Seismic and Magnetotelluric Observations in the Southeastern United States to Image Physical Properties of the Lithosphere

“…Theories of anelasticity are parameterized in terms of a critical frequency at which a continuum of viscoelastic/viscous grain-boundary processes begins to exert significant control on seismic observables (e.g., Jackson & Faul, 2010;Olugboji et al, 2013;Karato et al, 2015). Anelasticity has long been known to be an important control on seismic velocities (e.g., Karato, 1993), but only recently have rigorous attempts been made to quantify the magnitude of this effect (e.g., Jackson & Faul, 2010;Karato et al, 2015) and to integrate its consequences into geophysical interpretation (e.g., Goes et al, 2012). At very low frequencies (very long periods, hundreds of seconds for the specified conditions), steady state creep further significantly increases attenuation and decreases the Figure 4, from the five surface wave models (Pollitz & Mooney, 2016;Porter et al, 2016;Schmandt et al, 2015;Shen & Ritzwoller, 2016;Wagner et al, 2018) and the electrical resistivity inverse solutions.…”

“…Around this critical frequency, the value of which depends on temperature, pressure, and grain size, elastically accommodated grain-boundary sliding (EAGBS) causes a peak in attenuation and a major reduction in shear modulus, which consequently decreases both Vp and Vs. (For example, at 1000°C and 4 GPa, for a grain size of 5 mm, the critical frequency is~5 Hz, or~0.2 s.) At lower frequencies (longer periods, seconds to hundreds of seconds for the specified conditions), diffusionally accommodated grain-boundary sliding (also termed "absorption band behavior") leads to a slow but continual increase in attenuation and decrease in shear modulus. Though much of the current research on anelasticity has been specifically targeted at understanding the cause of midlithospheric discontinuities (MLDs; e.g., Karato et al, 2015;Selway et al, 2015) and at characterizing the nature of the LAB (e.g., Olugboji et al, 2013;Olugboji et al, 2016), the proposed models of anelastic controls on seismic observables can, and indeed should, be applied more broadly when formulating geodynamic interpretations of observed seismic anomalies. The solid line is the median value of all profiles, and the shaded regions indicate the range of extracted profiles (darker region is the 25th-75th percentile; lighter region is the full range).…”