Geophysical evidence from the MELT area for compositional controls on oceanic plates

Evans, Robert; Hirth, Greg; Baba, Kiyoshi; Forsyth, Donald W.; Chave, Alan D.; Mackie, Randall L.

doi:10.1038/nature04014

Cited by 217 publications

(218 citation statements)

References 27 publications

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“…Alternatively, water, in the form of dissolved hydrogen, has been invoked to explain elevated conductivities in the oceanic mantle (Lizarralde et al 1995;Evans et al 2005) and in the asthenosphere beneath continents (Hirth et al, 2000). Although there is still some disagreement between different laboratory measurements, the balance of evidence suggests that hydrogen enhances conductivity (Yoshino et al, 2006;Wang et al, 2006).…”

Section: Discussionmentioning

confidence: 99%

The elusive lithosphere–asthenosphere boundary (LAB) beneath cratons

Eaton

Darbyshire

Evans

et al. 2009

Lithos

379

284

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The lithosphere-asthenosphere boundary (LAB) is a first-order structural discontinuity that accommodates differential motion between tectonic plates and the underlying mantle. Although it is the most extensive type of plate boundary on the planet, its definitive detection, especially beneath cratons, is proving elusive. Different proxies are used to demarcate the LAB, depending on the nature of the measurement. Here we compare interpretations of the LAB beneath three The seismic LAB beneath cratons is typically regarded as the base of a high-velocity mantle lid, although some workers infer its location based on a distinct change in seismic anisotropy.Surface-wave inversion studies provide depth-constrained velocity models, but are relatively insensitive to the sharpness of the LAB. The S-receiver function method is a promising new seismic technique with complementary characteristics to surface-wave studies, since it is 2 sensitive to sharpness of the LAB but requires independent velocity information for accurate depth estimation. Magnetotelluric (MT) observations have, for many decades, imaged an "electrical asthenosphere" layer at depths beneath the continents consistent with seismic lowvelocity zones. This feature is most easily explained by the presence of a small amount of water in the asthenosphere, possibly inducing partial melt. Depth estimates based on various proxies considered here are similar, lending confidence that existing geophysical tools are effective for mapping the LAB beneath cratons.

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Section: Discussionmentioning

confidence: 99%

The elusive lithosphere–asthenosphere boundary (LAB) beneath cratons

Eaton

Darbyshire

Evans

et al. 2009

Lithos

379

284

View full text Add to dashboard Cite

show abstract

“…No discontinuity is expected from temperature sensitivity alone and there is no indication that the depth of the discontinuity is dependent on the age of the seafloor. Second, a magnetotelluric study of the East Pacific Rise (EPR) found a resistive layer that is about 60 km thick, independent of age (Evans et al, 2005), extending roughly to the depth to the seismic discontinuity reported elsewhere. Third, seismic velocity anomalies extend to greater depth and change more rapidly with distance from the East Pacific Rise axis than expected if simple conductive cooling from above is the primary control on structure (Dunn and Forsyth, 2003;Forsyth, 1977;Hammond and Toomey, 2003).…”

Section: Introductionmentioning

confidence: 86%

Thickening of young Pacific lithosphere from high-resolution Rayleigh wave tomography: A test of the conductive cooling model

Harmon

Forsyth

Weeraratne

2009

Earth and Planetary Science Letters

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a b s t r a c t a r t i c l e i n f oDirect seismic measurements of the thickening of oceanic lithosphere away from the spreading axis are rare due to the remoteness of mid ocean ridges. On the East Pacific Rise, at 17°S, there were two long-term broadband ocean bottom seismometer deployments, the MELT and GLIMPSE experiments. These arrays spanned young Pacific plate seafloor ranging in age from 0 to 8 Ma. Combining these two data sets, we describe the increase in Rayleigh wave phase velocities for 16-100 s period as function of distance from the ridge with two parameterizations: an arbitrary function of seafloor age and a simple polynomial function of age on the Pacific and Nazca plates. Although resolution analysis shows that 10 to 15 independent pieces of information about the age variation of phase velocity on the Pacific plate can be resolved at periods of ≤50 s, the three parameter polynomial model fits the data almost as well, indicating a simple pattern of the evolution of structure with age. To compare our observations to the predictions for the conductive cooling of the lithosphere, we convert a thermal half-space cooling model to shear velocity using anharmonic and anelastic contributions. The patterns in the velocities are not consistent with simple conductive cooling. The conductive cooling model under predicts the changes in phase velocity with age observed in the 20 to 78 s period range. We observe significant changes in shear velocity in the asthenosphere at depths greater than conductive cooling should extend. The asthenospheric low velocity zone is asymmetric, dipping to the west with the lowest velocities beneath the Pacific plate west of the spreading center. The conductive cooling model also over predicts the shear velocities in the lithosphere by~0.2 km/s. These anomalies indicate that more complicated mantle flow and significant mantle heterogeneities such as melt are required.

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“…Thermal [Stixrude and Lithgow-Bertelloni, 2005], hydration [Evans et al, 2005;Karato, 1990;Karato and Jung, 1998], and melt mechanisms Kawakatsu et al, 2009;Lambert and Wyllie, 1970;Schmerr, 2012] have been used to explain the observed behavior at the LAB and the presence of a seismic low velocity zone (LVZ). The thermal mechanism requires that the temperature gradient of the upper asthenosphere results in a region of low viscosity at the LAB allowing the rigid plate to slide above the underlying mantle.…”

Section: Oceanic Plate Motionmentioning

confidence: 99%

“…My magnetotelluric models showed that six tectonic elements are present in the defined region of study -two of which (the Mwembeshi Suture Zone and the Niassa Craton) previous studies could only speculate and not explicitly map. The Evans, R. L., G. Hirth, K. Baba, D. Forsyth, A. Chave, and R. Mackie (2005) Hirth, G., and D. L. Kohlstedt (1996), Water in the oceanic upper mantle: implications for rheology, melt extraction and the evolution of the lithosphere, Earth and Planetary Science Letters,144(1), 93-108, doi:10.1016/0012-821X(96) …”

Section: Thesis Objectivesmentioning

confidence: 99%

Geophysical and petrological constraints on ocean plate dynamics

Sarafian¹

2017

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This thesis investigates the formation and subsequent motion of oceanic lithospheric plates through geophysical and petrological methods. Ocean crust and lithosphere forms at mid-ocean ridges as the underlying asthenosphere rises, melts, and flows away from the ridge axis. In Chapters 2 and 3, I present the results from partial melting experiments of mantle peridotite that were conducted in order to examine the mantle melting point, or solidus, beneath a mid-ocean ridge. Chapter 2 determines the peridotite solidus at a single pressure of 1.5 GPa and concludes that the oceanic mantle potential temperature must be ~60 ºC hotter than current estimates. Chapter 3 goes further to provide a more accurate parameterization of the anhydrous mantle solidus from experiments over a range of pressures. This chapter concludes that the range of potential temperatures of the mantle beneath mid-ocean ridges and plumes is smaller than currently estimated. Once formed, the oceanic plate moves atop the underlying asthenosphere away from the ridge axis. Chapter 4 uses seafloor magnetotelluric data to investigate the mechanism responsible for plate motion at the lithosphere-asthenosphere boundary. The resulting two dimensional conductivity model shows a simple layered structure. By applying petrological constraints, I conclude that the upper asthenosphere does not contain substantial melt, which suggests that either a thermal or hydration mechanism supports plate motion. Oceanic plate motion has dramatically changed the surface of the Earth over time, and evidence for ancient plate motion is obvious from detailed studies of the longer lived continental lithosphere. In Chapter 5, I investigate past plate motion by inverting magnetotelluric data collected over eastern Zambia. The conductivity model probes the Zambian lithosphere and reveals an ancient subduction zone previously suspected from surface studies. This chapter elucidates the complex lithospheric structure of eastern Zambia and the geometry of the tectonic elements in the region, which collided as a result of past oceanic plate motion. Combined, the chapters of this thesis provide critical constraints on ocean plate dynamics.

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Geophysical evidence from the MELT area for compositional controls on oceanic plates

Cited by 217 publications

References 27 publications

The elusive lithosphere–asthenosphere boundary (LAB) beneath cratons

The elusive lithosphere–asthenosphere boundary (LAB) beneath cratons

Thickening of young Pacific lithosphere from high-resolution Rayleigh wave tomography: A test of the conductive cooling model

Geophysical and petrological constraints on ocean plate dynamics

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