Low-velocity zone

Last updated January 06, 2024

The low-velocity zone (LVZ) occurs close to the boundary between the lithosphere and the asthenosphere in the upper mantle. It is characterized by unusually low seismic shear wave velocity compared to the surrounding depth intervals. This range of depths also corresponds to anomalously high electrical conductivity. It is present between about 80 and 300 km depth. This appears to be universally present for S waves, but may be absent in certain regions for P waves.^[2] A second low-velocity zone (not generally referred to as the LVZ, but as ULVZ) has been detected in a thin ≈50 km layer at the core-mantle boundary.^[3] These LVZs may have important implications for plate tectonics and the origin of the Earth's crust.^[2]^[3]^[4]

The LVZ has been interpreted to indicate the presence of a significant degree of partial melting, and alternatively as a natural consequence of a thermal boundary layer and the effects of pressure and temperature on the elastic wave velocity of mantle components in the solid state.^[2] In any event, a very limited amount of melt (about 1%) is needed to produce these effects. Water in this layer can lower the melting point, and may play an important part in its composition.^[4]^[5]

Identification

The existence of the low-velocity zone was first proposed from the observation of slower than expected seismic wave arrivals from earthquakes in 1959 by Beno Gutenberg.^[6] He noted that between 1° and 15° from the epicenter the longitudinal arrivals showed an exponential decrease in amplitude after which they showed a sudden large increase. The presence of a low-velocity layer that defocussed the seismic energy, followed by a high-velocity gradient that concentrated it, provided an explanation for these observations.^[7]

Characteristics

The LVZ shows a reduction in velocity of about 3–6% with the effect being more pronounced with S-waves compared to P-waves.^[9] As is evident from the figure, the reduction and depth over which reduction occurs varies with the choice of tectonic province, that is, regions differ in their seismic characteristics. Following the drop, the base of the zone is marked by an increase in velocity, but it has not been possible to decide whether this transition is sharp or gradual. This lower boundary, found beneath the continental lithosphere and oceanic lithosphere away from mid-ocean ridges, is sometimes referred to as the Lehmann discontinuity and occurs at about 220±30 km depth. The interval also shows a reduction in Q, the seismic quality factor (representing a relatively high degree of seismic attenuation), and a relatively high electrical conductivity.

The LVZ is present at the base of the lithosphere except in areas of thick continental shield where no velocity anomaly is apparent.

Interpretation

The interpretation of these observations is complicated by the effects of seismic anisotropy, which may greatly reduce the actual scale of the velocity anomaly.^[7] However, because of the reductions in Q and electrical resistivity in the LVZ, it is generally interpreted as a zone in which there is a small degree of partial melting. For this to occur at the depths where the LVZ is observed, small amounts of water and/or carbon dioxide must be present to depress the melting point of the silicate minerals. Only 0.05–0.1 % water would be sufficient to cause the 1% of melting necessary to produce the observed changes in physical properties. The lack of LVZ beneath continental shields is explained by the much lower geothermal gradient, preventing any degree of partial melting.^[10]

Related Research Articles

A lithosphere is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the lithospheric mantle, the topmost portion of the upper mantle that behaves elastically on time scales of up to thousands of years or more. The crust and upper mantle are distinguished on the basis of chemistry and mineralogy.

<span class="mw-page-title-main">Asthenosphere</span> Highly viscous, mechanically weak, and ductile region of Earths mantle

The asthenosphere is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between ~80 and 200 km below the surface, and extends as deep as 700 km (430 mi). However, the lower boundary of the asthenosphere is not well defined.

<span class="mw-page-title-main">Subduction</span> A geological process at convergent tectonic plate boundaries where one plate moves under the other

Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the heavier plate dives beneath the second plate and sinks into the mantle. A region where this process occurs is known as a subduction zone, and its surface expression is known as an arc-trench complex. The process of subduction has created most of the Earth's continental crust. Rates of subduction are typically measured in centimeters per year, with rates of convergence as high as 11 cm/year.

A convergent boundary is an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other, a process known as subduction. The subduction zone can be defined by a plane where many earthquakes occur, called the Wadati–Benioff zone. These collisions happen on scales of millions to tens of millions of years and can lead to volcanism, earthquakes, orogenesis, destruction of lithosphere, and deformation. Convergent boundaries occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, and continental-continental lithosphere. The geologic features related to convergent boundaries vary depending on crust types.

A mantle plume is a proposed mechanism of convection within the Earth's mantle, hypothesized to explain anomalous volcanism. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic hotspots, such as Hawaii or Iceland, and large igneous provinces such as the Deccan and Siberian Traps. Some such volcanic regions lie far from tectonic plate boundaries, while others represent unusually large-volume volcanism near plate boundaries.

Seismic tomography or seismotomography is a technique for imaging the subsurface of the Earth with seismic waves produced by earthquakes or explosions. P-, S-, and surface waves can be used for tomographic models of different resolutions based on seismic wavelength, wave source distance, and the seismograph array coverage. The data received at seismometers are used to solve an inverse problem, wherein the locations of reflection and refraction of the wave paths are determined. This solution can be used to create 3D images of velocity anomalies which may be interpreted as structural, thermal, or compositional variations. Geoscientists use these images to better understand core, mantle, and plate tectonic processes.

Island arcs are long chains of active volcanoes with intense seismic activity found along convergent tectonic plate boundaries. Most island arcs originate on oceanic crust and have resulted from the descent of the lithosphere into the mantle along the subduction zone. They are the principal way by which continental growth is achieved.

Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic cumulates. The crust overlies the rigid uppermost layer of the mantle. The crust and the rigid upper mantle layer together constitute oceanic lithosphere.

Earth's mantle is a layer of silicate rock between the crust and the outer core. It has a mass of 4.01×10²⁴ kg (8.84×10²⁴ lb) and thus makes up 67% of the mass of Earth. It has a thickness of 2,900 kilometers (1,800 mi) making up about 46% of Earth's radius and 84% of Earth's volume. It is predominantly solid but, on geologic time scales, it behaves as a viscous fluid, sometimes described as having the consistency of caramel. Partial melting of the mantle at mid-ocean ridges produces oceanic crust, and partial melting of the mantle at subduction zones produces continental crust.

The core–mantle boundary (CMB) of Earth lies between the planet's silicate mantle and its liquid iron–nickel outer core, at a depth of 2,891 km (1,796 mi) below Earth's surface. The boundary is observed via the discontinuity in seismic wave velocities at that depth due to the differences between the acoustic impedances of the solid mantle and the molten outer core. P-wave velocities are much slower in the outer core than in the deep mantle while S-waves do not exist at all in the liquid portion of the core. Recent evidence suggests a distinct boundary layer directly above the CMB possibly made of a novel phase of the basic perovskite mineralogy of the deep mantle named post-perovskite. Seismic tomography studies have shown significant irregularities within the boundary zone and appear to be dominated by the African and Pacific Large Low-Shear-Velocity Provinces (LLSVP).

Earth's crust is its thick outer shell of rock, referring to less than 1% of the planets radius and volume. It is the top component of the lithosphere, a division of Earth's layers that includes the crust and the upper part of the mantle. The lithosphere is broken into tectonic plates whose motion allows heat to escape the interior of the Earth into space.

The Lehmann discontinuity is an abrupt increase of P-wave and S-wave velocities at the depth of 220 km (140 mi), discovered by seismologist Inge Lehmann. The thickness is 220 km. It appears beneath continents, but not usually beneath oceans, and does not readily appear in globally averaged studies. Several explanations have been proposed: a lower limit to the pliable asthenosphere, a phase transition, and most plausibly, depth variation in the shear wave anisotropy. Further discussion of the Lehmann discontinuity can be found in the book Deformation of Earth Materials by Shun-ichirō Karato.

The term "mesoplates" has been applied in two different contexts within geology and geophysics. The first is applicable to much of the Earth's mantle, and the second to distinct layering within the Earth's crust.

<span class="mw-page-title-main">Marquesas hotspot</span> Volcanic hotspot in the Pacific Ocean

The Marquesas hotspot is a volcanic hotspot in the southern Pacific Ocean. It is responsible for the creation of the Marquesas Islands – a group of eight main islands and several smaller ones – and a few seamounts. The islands and seamounts formed between 5.5 and 0.4 million years ago and constitute the northernmost volcanic chain in French Polynesia.

Non-volcanic passive margins (NVPM) constitute one end member of the transitional crustal types that lie beneath passive continental margins; the other end member being volcanic passive margins (VPM). Transitional crust welds continental crust to oceanic crust along the lines of continental break-up. Both VPM and NVPM form during rifting, when a continent rifts to form a new ocean basin. NVPM are different from VPM because of a lack of volcanism. Instead of intrusive magmatic structures, the transitional crust is composed of stretched continental crust and exhumed upper mantle. NVPM are typically submerged and buried beneath thick sediments, so they must be studied using geophysical techniques or drilling. NVPM have diagnostic seismic, gravity, and magnetic characteristics that can be used to distinguish them from VPM and for demarcating the transition between continental and oceanic crust.

<span class="mw-page-title-main">Society hotspot</span> Pacific volcanic hotspot

The Society hotspot is a volcanic hotspot in the south Pacific Ocean which is responsible for the formation of the Society Islands, an archipelago of fourteen volcanic islands and atolls spanning around 720 kilometres (450 mi) of the ocean which formed between 4.5 and <1 Ma.

<span class="mw-page-title-main">Large low-shear-velocity provinces</span> Structures of the Earths mantle

Large low-shear-velocity provinces, LLSVPs, also called LLVPs or superplumes, are characteristic structures of parts of the lowermost mantle of Earth. These provinces are characterized by slow shear wave velocities and were discovered by seismic tomography of deep Earth. There are two main provinces: the African LLSVP and the Pacific LLSVP. Both extend laterally for thousands of kilometers and possibly up to 1,000 kilometres vertically from the core–mantle boundary. The Pacific LLSVP is 3,000 kilometers across, and underlies four hotspots that suggest multiple mantle plumes underneath. These zones represent around 8% of the volume of the mantle. Other names for LLSVPs include "superswells", "thermo-chemical piles", or "hidden reservoirs". Most of these names, however, are more interpretive of their proposed geodynamical or geochemical effects. For example, the name "thermo-chemical pile" interprets LLSVPs as lower-mantle piles of thermally hot and/or chemically distinct material. LLSVPs are still relatively mysterious, and many questions remain about their nature, origin, and geodynamic effects.

The lithosphere–asthenosphere boundary represents a mechanical difference between layers in Earth's inner structure. Earth's inner structure can be described both chemically and mechanically. The lithosphere–asthenosphere boundary lies between Earth's cooler, rigid lithosphere and the warmer, ductile asthenosphere. The actual depth of the boundary is still a topic of debate and study, although it is known to vary according to the environment.

The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust and ends at the top of the lower mantle at 670 km (420 mi). Temperatures range from approximately 500 K at the upper boundary with the crust to approximately 1,200 K at the boundary with the lower mantle. Upper mantle material that has come up onto the surface comprises about 55% olivine, 35% pyroxene, and 5 to 10% of calcium oxide and aluminum oxide minerals such as plagioclase, spinel, or garnet, depending upon depth.

Intraplate volcanism is volcanism that takes place away from the margins of tectonic plates. Most volcanic activity takes place on plate margins, and there is broad consensus among geologists that this activity is explained well by the theory of plate tectonics. However, the origins of volcanic activity within plates remains controversial.

References

↑ GR Helffrich & BJ Wood (2002). "The Earth's Mantle" (PDF). Nature. Macmillan Magazines. 412 (2 August): 501–507. doi:10.1038/35087500. PMID 11484043. S2CID 4304379.
1 2 3 L Stixrude & C Lithgow-Bertolloni (2005). "Mineralogy and elasticity of the oceanic upper mantle: Origin of the low-velocity zone". Journal of Geophysical Research. 110: B03204. Bibcode:2005JGRB..11003204S. doi: 10.1029/2004JB002965 . hdl: 2027.42/94924 .
1 2 EJ Garnero, MS Thorne, A McNamara & S Rost (2007). "Chapter 6: Fine-scale ultra-low-velocity zone layering at the core-mantle boundary and superplumes". In David A Yuen; Shigenori Maruyama (eds.). Superplumes: beyond plate tectonics. Springer. p. 139. ISBN 978-1-4020-5749-6.{{cite book}}: CS1 maint: multiple names: authors list (link)
1 2 Philip Kearey; Keith A. Klepeis; Frederick J. Vine (2009). Global tectonics (3rd ed.). Wiley-Blackwell. p. 32. ISBN 978-1-4051-0777-8.
↑ It is hypothesized that the absence of plate tectonics on the planet Venus is due to the absence of water in its crust and upper mantle. Cooling occurs largely through mantle plumes. See Gillian R. Foulger (2005). Plates, plumes, and paradigms; Volume 388 of Special papers. Geological Society of America. p. 857. ISBN 0-8137-2388-4.
↑ Gutenberg, B. (1959). Physics of the Earth's Interior . New York: Academic Press. pp. 240. ISBN 0-12-310650-8.
1 2 Anderson, D.L. (1989). "3. The Crust and Upper Mantle". Theory of the Earth (PDF). Boston: Blackwell Scientific Publications. ISBN 0-521-84959-4. Archived from the original (PDF) on 2010-06-23. Retrieved 2010-02-20.
↑ Figure patterned after Don L Anderson (2007). New theory of the earth (2nd ed.). Cambridge University Press. p. 102, Figure 8.6. ISBN 978-0-521-84959-3.; Original figure attributed to Grand & Helmberger (1984)
↑ Brown, G.C.; Mussett A.E. (1981). The inaccessible earth. Taylor & Francis. p. 235. ISBN 978-0-04-550028-4 . Retrieved 2010-02-20.
↑ Condie, K.C. (1997). Plate tectonics and crustal evolution. Butterworth-Heinemann. p. 282. ISBN 978-0-7506-3386-4 . Retrieved 2010-02-20.

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.

[Helffrich-1] GR Helffrich & BJ Wood (2002). "The Earth's Mantle" (PDF). Nature. Macmillan Magazines. 412 (2 August): 501–507. doi:10.1038/35087500. PMID 11484043. S2CID 4304379.

[Stixrude-2] 1 2 3 L Stixrude & C Lithgow-Bertolloni (2005). "Mineralogy and elasticity of the oceanic upper mantle: Origin of the low-velocity zone". Journal of Geophysical Research. 110: B03204. Bibcode:2005JGRB..11003204S. doi: 10.1029/2004JB002965 . hdl: 2027.42/94924 .

[Rost-3] 1 2 EJ Garnero, MS Thorne, A McNamara & S Rost (2007). "Chapter 6: Fine-scale ultra-low-velocity zone layering at the core-mantle boundary and superplumes". In David A Yuen; Shigenori Maruyama (eds.). Superplumes: beyond plate tectonics. Springer. p. 139. ISBN 978-1-4020-5749-6.{{cite book}}: CS1 maint: multiple names: authors list (link)

[Vine-4] 1 2 Philip Kearey; Keith A. Klepeis; Frederick J. Vine (2009). Global tectonics (3rd ed.). Wiley-Blackwell. p. 32. ISBN 978-1-4051-0777-8.

[Venus-5] It is hypothesized that the absence of plate tectonics on the planet Venus is due to the absence of water in its crust and upper mantle. Cooling occurs largely through mantle plumes. See Gillian R. Foulger (2005). Plates, plumes, and paradigms; Volume 388 of Special papers. Geological Society of America. p. 857. ISBN 0-8137-2388-4.

[Bruno-6] Gutenberg, B. (1959). Physics of the Earth's Interior . New York: Academic Press. pp. 240. ISBN 0-12-310650-8.

[Don-7] 1 2 Anderson, D.L. (1989). "3. The Crust and Upper Mantle". Theory of the Earth (PDF). Boston: Blackwell Scientific Publications. ISBN 0-521-84959-4. Archived from the original (PDF) on 2010-06-23. Retrieved 2010-02-20.

[Anderson1-8] Figure patterned after Don L Anderson (2007). New theory of the earth (2nd ed.). Cambridge University Press. p. 102, Figure 8.6. ISBN 978-0-521-84959-3.; Original figure attributed to Grand & Helmberger (1984)

[Brown-9] Brown, G.C.; Mussett A.E. (1981). The inaccessible earth. Taylor & Francis. p. 235. ISBN 978-0-04-550028-4 . Retrieved 2010-02-20.

[Condie-10] Condie, K.C. (1997). Plate tectonics and crustal evolution. Butterworth-Heinemann. p. 282. ISBN 978-0-7506-3386-4 . Retrieved 2010-02-20.

[2]

[3]

[4]

[5]

[6]

[7]

[9]

[10]