The Nature of the Lithosphere‐Asthenosphere Boundary

Abstract

Plate tectonic theory was developed 50 years ago and underpins most of our understanding of Earth's evolution. The theory explains observations of magnetic lineations on the seafloor, linear volcanic island chains, large transform fault systems, and deep earthquakes near deep sea trenches. These features occur through a system of moving plates at the surface of the Earth, which are the surface expression of mantle convection. The plate consists of the chemically distinct crust and some amount of rigid mantle, which move over a weaker mantle beneath. However, exactly where the transition between stronger and weaker mantle occurs and what determines and defines the plate are still debated. In the classic definition the plate is defined thermally, by the geotherm-adiabat intersection, where the plate is the conductively cooling part of the mantle convection system. Many observations such as heat flow, seafloor bathymetry, seismic imaging, and magnetotelluric (MT) imaging are consistent with general lithospheric thickening with age, which suggests that temperature is an important factor in determining lithospheric thickness. However, while age averages give a good indication of overall properties, the range of lithospheric thicknesses reported is large for any given tectonic age interval, suggesting greater complexity. A number of observations including sharp discontinuities from teleseismic scattered waves and active source reflections and also strong anomalies from surface and body wave tomography and MT imaging cannot be explained by a purely thermal model. Another property or process is required to explain the anomalies and sharpen the boundary. Many subsolidus models have been proposed, although none can universally explain the variety of independent global observations. Alternatively, a small amount of partial melt can easily satisfy a range of observations. The presence of melt could also weaken the mantle over geologic timescales, and it would therefore define the lithosphere-asthenosphere boundary (LAB). The location of melt is important to mantle dynamics and the LAB, although exactly where and exactly how much melt exists in the mantle are debated. Asthenospheric melt interpretations include a variety of forms: in small or large melt triangles beneath spreading ridges, in channels, in layers, along a permeability boundary leading to the ridge, at a depth of neutral buoyancy, punctuated, or pervasively over broad areas and either sharply or gradually falling off with depth. This variability in melt character or geometry may explain the previously described variability in LAB depths. The LAB is likely highly variable laterally as are the locations, forms, and amounts of melt, and the LAB is likely dynamic, dictated by small-scale convection and the dynamics of melt generation and migration. A melt-defined, dynamic LAB and a weak asthenosphere have broad implications for our understanding of Earth systems and planetary habitability. A weak asthenosphere caused by volatiles or melt could enable plate tectonic ...

Imaging the seismic velocity structure of the crust and upper mantle in the northern East African Rift using Rayleigh wave tomography

“…The first option is sugggestive of ponding beneath a permeability barrier, and therefore likely also includes migration and/or ephemeral character, and therefore we proceed discussing the latter two options. Similar observations of disconnected slow velocity anomalies have been observed in previous studies at more developed rifts or ridges, which hypothesise lateral melt migration is occuring along a permeability boundary (Braun & Sohn, 2003;Ghods & Arkani-Hamed, 2000;Harmon et al, 2020;Holtzman & Kendall, 2010;Rychert et al, 2020;Varga et al, 2008;Wang et al, 2020) and/or melt is ephemeral (Harmon et al, 2020;Rychert et al, 2020;Wang et al, 2020). Within the MER we also observe the slowest velocity anomalies at asthenospheric depths in our model, are not located directly beneath the slowest crustal velocity anomalies (Figure 9).…”

Upper Mantle Anisotropic Shear Velocity Structure at the Equatorial Mid‐Atlantic Ridge Constrained by Rayleigh Wave Group Velocity Analysis From the PI‐LAB Experiment

“…A weak decrease in the magnitude of anisotropy beneath seafloor >80 Myr from these studies is not observed in the few points in our compilation. However, lithospheric properties from regional studies typically exhibit more variability than trends gleaned from age averages of global models (Rychert et al., 2020). Comparisons between P wave anisotropy from refraction work in the western Atlantic suggested weak lithospheric anisotropy, interpreted as the result of slower spreading, for instance, in comparison to the Pacific (e.g., Gaherty et al., 2004).…”

The Signature and Elimination of Sediment Reverberations on Submarine Receiver Functions

“…In the Pacific, a comparison with other body-wave techniques not affected by reverberations (e.g., SS precursors), indicates that our inferred seismic LAB depth is broadly consistent with early results (Gaherty et al, 1996;Ma et al, 2020;Tan & Helmberger, 2007) (see summary in Figure 12). Although the SS precursor technique seems to show an age dependence for normal Pacific lithosphere (C. Rychert et al, 2020;C. A. Rychert et al, 2018b), the very high resolution of our receiver function results (∼1.5 Hz) allows us to improve on the resolution of the inferred depth and sharpness of the seismic LAB.…”