Preferential detection of the Lehmann discontinuity beneath continents

Gu, Yu Jeffrey; Dziewoński, Adam M.; Ekström, Göran

doi:10.1029/2001gl013679

Cited by 75 publications

(66 citation statements)

References 22 publications

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“…The 220 is attributed to several different mechanisms; the tectosphere hypothesis of Jordan (1975) suggested the 220 is a chemically distinct boundary layer beneath the cratons, while Karato (1992) attributes the 220 to a switch from anisotropy to isotropy in the mantle, resulting from a change in the deformation mechanism from dislocation creep to diffusion creep. Seismically, the 220 is observed regionally, with the most robust observations occurring beneath the continents (e.g., Gu et al 2001;Deuss and Woodhouse 2002) and a few detections beneath the oceans (Rost and Weber 2001). For the 520 km discontinuity, Ringwood (1975) predicted a discontinuity near this depth arising from the mineralogic phase transformation of wadsleyite to ringwoodite, though a global seismic study by Deuss and Woodhouse (2001) has suggested multiple 520 km discontinuities that may also be attributed to phase changes in the garnet system.…”

Section: Introductionmentioning

confidence: 95%

Imaging Mantle Heterogeneity with Upper Mantle Seismic Discontinuities

Schmerr

2015

The Earth's Heterogeneous Mantle

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We use underside reflections of S wave seismic energy arriving as precursors to the seismic phase SS to image the depth and impedance contrast present across mantle discontinuities in the depth range of 230-380 km beneath the Pacific basin. A number of past studies have identified seismic discontinuities at these depths, known as the X-discontinuities, ascribing the interfaces to a variety of mineral physical mechanisms, including the coesite to stishovite phase transition, the formation of hydrous Phase A, and/or the reorganization of orthopyroxene into a C2/c monoclinic structure. Thus, the presence of the X-discontinuity (abbreviated here as the X) may be indicative of the nature of mantle heterogeneity. This study finds discontinuities associated with the X Pacific-wide, with SS precursory reflections present beneath the subduction, hot spots, and ridges. Where detected, the X is at an average depth of 293 ± 65 km and the precursor amplitudes indicate a mean shear impedance contrast of 2.3 ± 1.6 %. We model mantle heterogeneity by comparing the depth and the impedance contrasts at the X with predictions for seismic structure from a mineral physics model in which the mantle is considered to be a mechanically mixed bulk assemblage of subducted basalt and harzburgite. In this model, the average mantle composition of the Pacific is fit by a mixture of ~20 % basalt and 80 % harzburgite, roughly consistent with the bulk chemistry of mantle peridotite (18 % basalt, 82 % harzburgite). In some regions beneath the hot spots and subduction, there is evidence for a bulk chemistry enriched in basalt (basalt fraction ~30-35 %), lending evidence to the hypothesis that the mantle is laterally heterogeneous and that dynamics are stirring an enriched component, perhaps from the deep or shallow Earth, into the upper mantle.

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Section: Introductionmentioning

confidence: 95%

Imaging Mantle Heterogeneity with Upper Mantle Seismic Discontinuities

Schmerr

2015

The Earth's Heterogeneous Mantle

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“…It has been suggested [22] that a rheological boundary separates fossil lithospheric anisotropy, preserved since the last major orogeny, from current asthenospheric anisotropy due to contemporaneous mantle deformation, which may be consistent with the strati¢cation of the lithosphere suggested by some seismological studies [23,24]. However, the interpretation of seismic layering is diverse [25^27], and the boundary^or transition^be-tween regimes characterized by 'frozen' anisotropy, and 'present-day' lithospheric anisotropy has remained imprecisely mapped.…”

Section: Introductionmentioning

confidence: 99%

Seismic and mechanical anisotropy and the past and present deformation of the Australian lithosphere

Simons

Hilst

2003

Earth and Planetary Science Letters

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We interpret the three-dimensional seismic wave-speed structure of the Australian upper mantle by comparing its azimuthal anisotropy to estimates of past and present lithospheric deformation. We infer the fossil strain field from the orientation of gravity anomalies relative to topography, bypassing the need to extrapolate crustal measures, and derive the current direction of mantle deformation from present-day plate motion. Our observations provide the depth resolution necessary to distinguish fossil from contemporaneous deformation. The distribution of azimuthal seismic anisotropy is determined from multi-mode surface-wave propagation. Mechanical anisotropy, or the directional variation of isostatic compensation, is a proxy for the fossil strain field and is derived from a spectral coherence analysis of digital gravity and topography data in two wavelength bands. The joint interpretation of seismic and tectonic data resolves a rheological transition in the Australian upper mantle. At depths shallower than V150^200 km strong seismic anisotropy forms complex patterns. In this regime the seismic fast axes are at large angles to the directions of principal shortening, defining a mechanically coupled crust^mantle lid deformed by orogenic processes dominated by transpression. Here, seismic anisotropy may be considered 'frozen', which suggests that past deformation has left a coherent imprint on much of the lithospheric depth profile. The azimuthal seismic anisotropy below V200 km is weaker and preferentially aligned with the direction of the rapid motion of the Indo-Australian plate. The alignment of the fast axes with the direction of present-day absolute plate motion is indicative of deformation by simple shear of a dry olivine mantle. Motion expressed in the hot-spot reference frame matches the seismic observations better than the no-net-rotation reference frame. Thus, seismic anisotropy supports the notion that the hot-spot reference frame is the most physically reasonable. Independently from plate motion models, seismic anisotropy can be used to derive a best-fitting direction of overall mantle shear. ß

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“…The Lehmann discontinuity is widely considered as the bottom of the asthenosphere. A global study of the Lehmann discontinuity is given by Gu et al (2001). Theoretical travel-time curves of S precursors at the above mentioned discontinuities are marked by dashed black lines in Fig.…”

Section: Datamentioning

confidence: 99%

Structure of the upper mantle in the north-western and central United States from USArray S-receiver functions

et al. 2015

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Abstract. We used more than 40 000 S-receiver functions recorded by the USArray project to study the structure of the upper mantle between the Moho and the 410 km discontinuity from the Phanerozoic western United States to the cratonic central US. In the western United States we observed the lithosphere-asthenosphere boundary (LAB), and in the cratonic United States we observed both the midlithospheric discontinuity (MLD) and the LAB of the craton. In the northern and southern United States the western LAB almost reaches the mid-continental rift system. In between these two regions the cratonic MLD is surprisingly plunging towards the west from the Rocky Mountain Front to about 200 km depth near the Sevier thrust belt. We interpret these complex structures of the seismic discontinuities in the mantle lithosphere as an indication of interfingering of the colliding Farallon and Laurentia plates. Unfiltered S-receiver function data reveal that the LAB and MLD are not single discontinuities but consist of many small-scale laminated discontinuities, which only appear as single discontinuities after longer period filtering. We also observe the Lehmann discontinuity below the LAB and a velocity reduction about 30 km above the 410 km discontinuity.

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Preferential detection of the Lehmann discontinuity beneath continents

Cited by 75 publications

References 22 publications

Imaging Mantle Heterogeneity with Upper Mantle Seismic Discontinuities

Imaging Mantle Heterogeneity with Upper Mantle Seismic Discontinuities

Seismic and mechanical anisotropy and the past and present deformation of the Australian lithosphere

Structure of the upper mantle in the north-western and central United States from USArray S-receiver functions

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