Deep Earth Structure: Lower Mantle and D″

Lay, Thorne

doi:10.1016/b978-0-444-53802-4.00019-1

Cited by 58 publications

(30 citation statements)

References 777 publications

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“…Analyses of S waveforms offered some support for flat regions of high speed at the transition zone (e.g., Brudzinski et al, 1997;Tseng & Chen, 2004). Subsequent tomographic studies have shown evidence for such flattening of high-speed zones in the western Pacific and elsewhere (e.g., Ferreira et al, 2019;French & Romanowicz, 2014, 2015Simmons et al, 2012). Although Goes et al (2017) stated "Our compilation of transition-zone slab morphologies in Figure 2 shows, however, that slabs very rarely stagnate below 660, if at all," it seems to me, following Ballmer et al (2015), that images like those in Figure 8, and others of Fukao and Obayashi (2013), make it hard not to ignore the possibility that downgoing slabs hit a resistant layer at~1,000-km depth.…”

Section: 1029/2019gc008416mentioning

confidence: 98%

“…Proclaiming lateral and vertical variations in the chemical composition of the lower mantle has proven to be easier than resolving the differences in chemistry. On the one hand, the lower mantle might seem chemically simple, for it is dominated by one mineral, bridgmanite (or perovskite), (Mg,Fe)SiO 3 , which is commonly thought to constitute 70% of the lower mantle, plus 20% ferropericlase (or magnesiowüstite), (Mg, Fe)O, and calcium perovskite, CaSiO 3 , comprising the remaining~10% (e.g., Lay, 2015). As the bulk modulus of ferropericlase is lower than that of bridgmanite, it is compressed more at higher pressures at greater depths than at the top of the lower mantle.…”

Section: 1029/2019gc008416mentioning

confidence: 99%

“…The Central America region is the only one for which Fukao and Obayashi (2013) show such deep penetration of high-speed material. In fact, the profiles across the southern United States (Figure 7) (e.g., French & Romanowicz, 2014, 2015Grand et al, 1997;Ren et al, 2007;Simmons et al, 2012) and those crossing Central America (e.g., Li et al, 2008;van der Hilst et al, 1997) share a rather small, common, high-speed region deep below the western Atlantic ocean just east of the southeastern United States. Moreover, regarding deep penetration of high-speed material beneath this region, Goes et al (2017) noted "this is clear only along a few cross sections."…”

Section: 1029/2019gc008416mentioning

confidence: 99%

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Lower Mantle Dynamics Perceived With 50 Years of Hindsight From Plate Tectonics

Molnár

2019

Geochem Geophys Geosyst

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Continually improving resolution of lower mantle structure and understanding of mantle dynamics suggest that, as in the plate-tectonics era, the upper and lower mantle may undergo little exchange. Although the phase transition at~660 km forms the sharpest seismologically mapped boundary within the mantle, another at 1,000 (±100) km might be more important for geodynamics. Contrasts in density and viscosity may limit penetration of upper and lower mantle through this boundary. Strong evidence, if still not definitive, calls for chemical heterogeneity of the lower mantle that manifests itself in a few energetic plumes rising from near the core mantle boundary and slowly evolving, larger piles or blobs of chemically distinct material. Those blobs or piles may deform little on billion-year timescales. Convection in the lower mantle might dictate some aspects of flow in the upper mantle, such as the positions of plumes and hot spots and the breakup of supercontinents, but its contribution to the speeds and directions that plates move might be negligible. The improved understanding of mantle convection has not overturned the simple, 50-year-old image that plates "drive themselves," with forces per unit length pushing them apart at spreading centers, excess mass in downgoing slabs pulling them down, and viscous drag on their bases (and tops at subduction zones) retarding movement. Remaining challenges include determining how convection in the lower mantle affects geologic history and using that history to constrain lower mantle dynamics.Plain Language Summary Fifty years ago, plate tectonics united many aspects of the surface geology of the Earth, but little connection to the lower mantle was seen. Today, most view plate tectonics as the relative movements of cold, top, stiff boundary layers of a convecting system that reaches to the core-mantle boundary and with aspects of the deep structure not foreseen decades ago. Large provinces in the deepest~1,000 km, in which P and S wave speeds are relatively low, not only seem to be chemically different from the neighboring mantle and from that at shallower depths, but their distribution also correlates with some aspects of the overlying surface geology, including the positions of major plumes rising from deep in the mantle and the positions of continents 100 to 200 Ma. These correlations imply a geodynamic connection between the lower mantle and the crust. Scaling laws derived from experiments in geophysical fluid mechanics suggest that the chemically distinct provinces may be relics from the earliest formation of the earth, but if not, they nevertheless have evolved slowly on the timescales of geologic eras. A concurrent emerging view of the lower mantle, however, also places increased emphasis on a boundary at~1,000 (±100) km depth, and this boundary might define a barrier to cold sinking slabs of lithosphere. A few isolated plumes of hot material that are also chemically different from most of the mantle penetrate this interface at 1,000 km, but it seems possible that this...

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Section: 1029/2019gc008416mentioning

confidence: 98%

Section: 1029/2019gc008416mentioning

confidence: 99%

Section: 1029/2019gc008416mentioning

confidence: 99%

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Lower Mantle Dynamics Perceived With 50 Years of Hindsight From Plate Tectonics

Molnár

2019

Geochem Geophys Geosyst

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“…The D" region is the lowest 200-300 km of the mantle, atop the core-mantle boundary (CMB). D" is characterized by ultra-low-velocity zones (ULVZs), seismic discontinuities, anisotropy, CMB topography, and, most prominently, by large-low-shear-velocity provinces (LLSVPs) below Africa and the Pacific [e.g., Lay, 2007;Garnero and McNamara, 2008]. The LLSVPs extend hundreds of kilometers both laterally and vertically into the lower mantle [Ritsema et al, 1999].…”

Section: Introductionmentioning

confidence: 99%

Observations of core‐mantle boundary Stoneley modes

Koelemeijer

Deuss

Ritsema

2013

Geophysical Research Letters

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[1] Core-mantle boundary (CMB) Stoneley modes represent a unique class of normal modes with extremely strong sensitivity to wave speed and density variations in the D" region. We measure splitting functions of eight CMB Stoneley modes using modal spectra from 93 events with M w > 7.4 between 1976 and 2011. The obtained splitting function maps correlate well with the predicted splitting calculated for S20RTS+Crust5.1 structure and the distribution of S diff and P diff travel time anomalies, suggesting that they are robust. We illustrate how our new CMB Stoneley mode splitting functions can be used to estimate density variations in the Earth's lowermost mantle. Citation: Koelemeijer, P., A. Deuss, and J. Ritsema (2013), Observations of core-mantle boundary Stoneley modes, Geophys. Res. Lett., 40,

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“…The D region acts as a thermomechanical boundary layer [9] between the Earth's liquid outer core and the solid mantle. While the bulk of the lower mantle is mostly isotropic [10,11], seismic observations have revealed strong seismic anisotropy in D where upwelling and subduction occur [10,[12][13][14][15]. With the discovery of the Mg-perovskite (Pv, also known as bridgmanite [16]) Mg-post-perovskite (PPv) phase transition [17,18] at deep mantle conditions, highly anisotropic PPv emerged as a likely candidate to explain the observed anisotropy in D .…”

Section: Introductionmentioning

confidence: 99%

A Refined Approach to Model Anisotropy in the Lowermost Mantle

Chandler

Yuan

et al. 2018

IOP Conf. Ser.: Mater. Sci. Eng.

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Abstract.Seismic anisotropy in the lowermost mantle has been attributed to texture development during mantle convection. This study models texture evolution in a subducting slab impinging on the core-mantle boundary. Using a 3-dimensional geodynamic model with tracers recording the deformation history, a visco-plastic self-consistent (VPSC) model for polycrystal deformation, and relying on experimentally determined slip systems, aggregate grains with volume fractions of 60% orthorhombic silicate perovskite (MgSiO 3), 20% cubic calcium perovskite (CaSiO3), and 20% cubic ferropericlase ((Mg,Fe)O) were deformed plastically and developed crystal preferred orientation. Forward and reverse perovskite (Pv)-postperovskite (PPv) phase transitions were included by allowing for likely orientation variant selections. Grain orientations, P (compression) and S (shear) wave velocity pole gures were calculated for each phase as well as the aggregate. The results show that dominant (001) slip on PPv can produce strong texture and shear wave anisotropy of 0.01-3.07% with V S H >VSV which agrees with seismological observations in selected areas of the D layer.

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Deep Earth Structure: Lower Mantle and D″

Cited by 58 publications

References 777 publications

Lower Mantle Dynamics Perceived With 50 Years of Hindsight From Plate Tectonics

Lower Mantle Dynamics Perceived With 50 Years of Hindsight From Plate Tectonics

Observations of core‐mantle boundary Stoneley modes

A Refined Approach to Model Anisotropy in the Lowermost Mantle

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