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The nature of the Lithosphere‐Asthenosphere Boundary

The nature of the Lithosphere‐Asthenosphere Boundary
The nature of the Lithosphere‐Asthenosphere Boundary
Plate tectonic theory was developed 50 years ago and underpins most of our understanding of Earth’s evolution. The theory explains observations of seafloor spreading, linear volcanic island chains, large transform fault systems and deep earthquakes near deep sea trenches. These processes 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 imaging (MT) are consistent with general lithospheric thickening with age which suggest 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: small or large melt triangles beneath spreading ridges, channels or multiple 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 style convection, allow multiple scales of convection, and dictate the driving forces of the system. This has implications for mitigating many natural disasters caused by plate motions including volcanoes, earthquakes, and tsunamis. In addition, uplift and subsidence of the tectonic plates affects the sea level, impacting the level of the paleo‐oceans, and potentially affecting climate change estimates through geologic time. Finally, plate tectonics shapes the surface morphology of the planet, making continents that enable our existence on land and the ocean basins that hold our free‐surface water. Remarkably, despite large amounts of material transfer into and out of the mantle, and multiple scales of convection, plate tectonics has maintained a hydrosphere over billions of years that is favourable for life.
elastic thickness, heat flow, lithosphere-asthenosphere boundary, magnetotellurics, mantle rheology, seismology
2169-9356
Rychert, Catherine
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Harmon, Nicholas
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Constable, Steven
f2ffd9c4-3738-435b-8a88-38dee97de7cc
Wang, Shunguo
f935a6b8-a8c1-46f0-975a-1d4aa56b5f11
Rychert, Catherine
70cf1e3a-58ea-455a-918a-1d570c5e53c5
Harmon, Nicholas
10d11a16-b8b0-4132-9354-652e72d8e830
Constable, Steven
f2ffd9c4-3738-435b-8a88-38dee97de7cc
Wang, Shunguo
f935a6b8-a8c1-46f0-975a-1d4aa56b5f11

Rychert, Catherine, Harmon, Nicholas, Constable, Steven and Wang, Shunguo (2020) The nature of the Lithosphere‐Asthenosphere Boundary. Journal of Geophysical Research: Solid Earth, 125 (10), [e2018JB016463]. (doi:10.1029/2018JB016463).

Record type: Article

Abstract

Plate tectonic theory was developed 50 years ago and underpins most of our understanding of Earth’s evolution. The theory explains observations of seafloor spreading, linear volcanic island chains, large transform fault systems and deep earthquakes near deep sea trenches. These processes 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 imaging (MT) are consistent with general lithospheric thickening with age which suggest 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: small or large melt triangles beneath spreading ridges, channels or multiple 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 style convection, allow multiple scales of convection, and dictate the driving forces of the system. This has implications for mitigating many natural disasters caused by plate motions including volcanoes, earthquakes, and tsunamis. In addition, uplift and subsidence of the tectonic plates affects the sea level, impacting the level of the paleo‐oceans, and potentially affecting climate change estimates through geologic time. Finally, plate tectonics shapes the surface morphology of the planet, making continents that enable our existence on land and the ocean basins that hold our free‐surface water. Remarkably, despite large amounts of material transfer into and out of the mantle, and multiple scales of convection, plate tectonics has maintained a hydrosphere over billions of years that is favourable for life.

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The nature of the Lithosphere-Asthenosphere Boundary - Accepted Manuscript
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Published date: 15 September 2020
Keywords: elastic thickness, heat flow, lithosphere-asthenosphere boundary, magnetotellurics, mantle rheology, seismology

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Local EPrints ID: 444378
URI: http://eprints.soton.ac.uk/id/eprint/444378
ISSN: 2169-9356
PURE UUID: efa1bd4c-2297-429c-bb98-f05dbe609119
ORCID for Nicholas Harmon: ORCID iD orcid.org/0000-0002-0731-768X

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Date deposited: 15 Oct 2020 16:30
Last modified: 26 Nov 2021 02:54

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Author: Nicholas Harmon ORCID iD
Author: Steven Constable
Author: Shunguo Wang

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