Understanding the relationship between different scales of convection that drive plate motions and hotspot volcanism still eludes geophysicists. Using full-waveform seismic tomography, we imaged a ...pattern of horizontally elongated bands of low shear velocity, most prominent between 200 and 350 kilometers depth, which extends below the well-developed low-velocity zone. These quasi-periodic fingerlike structures of wavelength ~2000 kilometers align parallel to the direction of absolute plate motion for thousands of kilometers. Below 400 kilometers depth, velocity structure is organized into fewer, undulating but vertically coherent, low-velocity plumelike features, which appear rooted in the lower mantle. This suggests the presence of a dynamic interplay between plate-driven flow in the low-velocity zone and active influx of low-rigidity material from deep mantle sources deflected horizontally beneath the moving top boundary layer.
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Earth's lower mantle is dominated by a pair of antipodal large low shear velocity provinces (LLSVPs) that reach >1000km up from the core–mantle boundary (CMB). These are separated by a ring of ...faster-than-average velocities thought to be related to subduction of oceanic lithosphere. How robustly does global tomography constrain velocity structure in the lower mantle, and are there other robust large scale features that have not been identified? We use cluster analysis to identify structures and seismic characteristics common to a set of recent global tomographic models which have been derived using different data sets, parameterizations, and theory behind approximations used in inversion. We detect a pronounced asymmetry in the velocity gradient with depth between seismically fast and slow regions in the lowermost 500km of the mantle, suggesting the presence of compositional heterogeneity. We find that, in all models, there is a clear separation of lower mantle structure into one fast and two slow regions, and that the boundary of the regions is remarkably similar across models even on length scales as small as <1000km. This inter-model similarity indicates that long wavelength features are not a consequence of lack of fine-scale resolution, but that they truly dominate the structure in the lowermost mantle. There is a single exception to this separation: an isolated slow anomaly ∼900km across (at the CMB) and extending ∼500km upward from the core–mantle boundary, which we call the “Perm Anomaly”. Though it is far smaller than an LLSVP, waveform analysis confirms that this anomaly is robustly constrained and bounded by rapid lateral velocity gradients like those found around LLSVPs, suggesting that the nature and process of formation of both types of structures may be related.
► Cluster analysis detects lower mantle structures common to global tomographic models. ► Lower mantle structure divides cleanly into one fast and two slow regions (LLSVPs). ► We find a pronounced asymmetry in velocity profiles between slow and fast regions. ► Boundaries of slow regions are co-located across models even at <1000km scales. ► We identify a new class of structure, 900km across and bounded by sharp Vs gradients.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
New tungsten isotope data for modern ocean island basalts (OIB) from Hawaii, Samoa, and Iceland reveal variable 182W/184W, ranging from that of the ambient upper mantle to ratios as much as 18 parts ...per million lower. The tungsten isotopic data negatively correlate with ³He/⁴He. These data indicate that each OIB system accesses domains within Earth that formed within the first 60 million years of solar system history. Combined isotopic and chemical characteristics projected for these ancient domains indicate that they contain metal and are repositories of noble gases. We suggest that the most likely source candidates are mega–ultralow-velocity zones, which lie beneath Hawaii, Samoa, and Iceland but not beneath hot spots whose OIB yield normal 182W and homogeneously low ³He/⁴He.
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Global mantle tomography can be improved through better use of data and application of more accurate wave propagation methods. However, few techniques have been developed for objective validation and ...exploration of the resulting tomographic models. We show that cluster analysis can be used to validate and explore the salient features across such models. We present a cluster analysis of a global upper mantle radially anisotropic model SEMum developed using full waveform tomography and the Spectral Element Method. Applied to SEMum down to 350 km depth, the cluster analysis reveals that absolute shear wave velocity (Vs) depth profiles naturally group into families that correspond with known surface tectonics. This allows us to construct a global tectonic regionalization based solely on tomography, without the help of any a priori information. We find that the profiles of stable platforms and shields consistently exhibit a mid-lithospheric low velocity zone (LVZ) between 80 and 130
km depth, while the asthenosphere is found at depths greater than 250
km in both regions. This global intra-continental-lithosphere low velocity zone agrees with recent receiver function studies and regional tomographic studies. Furthermore, we identify an anomalous oceanic region characterized by slow shear wave speeds at depths below 150
km. Hotspots are found preferentially in the vicinity of this anomalous region. In the Pacific Ocean, where plate velocities are largest, these regions have elongated shapes that align with absolute plate motion, suggesting a relationship between the location of hotspots and small-scale convection in the oceanic upper mantle.
► Cluster analysis can be used to verify, compare, and explore tomographic models. ► Shear wavespeeds naturally cluster into regions that correspond to tectonic provinces. ► Hotspots surround a region that may be associated with small scale convection. ► Hotspots have a deep low-velocity signature extending to at least 300
km depth. ► A mid-lithospheric low velocity zone is found beneath stable continental regions.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Mapping variations in the attenuation of seismic energy is important for understanding dissipative mechanisms in the lithosphere, and for modeling ground shaking associated with earthquakes. We ...cross-correlate ambient seismic signal recorded across the EarthScope Transportable Array in the 3-15 s period range. We apply to the resulting cross correlations a new method to estimate lateral variations in Rayleigh-wave attenuation, as a function of period, beneath North America. Between 3 and 6 s, our maps are dominated by a strong eastward decrease in attenuation. This pattern vanishes at longer periods, confirming early observations based on regional earthquakes. Attenuation maps and phase-velocity maps are anti-correlated at periods between 3 and 6 s, but the anti-correlation is also largely lost at longer periods. This corresponds to the attenuation coefficient decreasing with period more rapidly in the west than in the east, while the change in phase velocity with period is more uniform across the continent. Our results point to a transition in the properties of upper-crustal materials with depth, probably related to the closure of fluid-filled cracks and pores, and imply that measures of attenuation from seismic noise carry significant information on crustal rheology.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Single seismometer structure
Because of the lack of direct seismic observations, the interior structure of Mars has been a mystery. Khan
et al.
, Knapmeyer-Endrun
et al.
, and Stähler
et al.
used ...recently detected marsquakes from the seismometer deployed during the InSight mission to map the interior of Mars (see the Perspective by Cottaar and Koelemeijer). Mars likely has a 24- to 72-kilometer-thick crust with a very deep lithosphere close to 500 kilometers. Similar to the Earth, a low-velocity layer probably exists beneath the lithosphere. The crust of Mars is likely highly enriched in radioactive elements that help to heat this layer at the expense of the interior. The core of Mars is liquid and large, ∼1830 kilometers, which means that the mantle has only one rocky layer rather than two like the Earth has. These results provide a preliminary structure of Mars that helps to constrain the different theories explaining the chemistry and internal dynamics of the planet.
Science
, abf2966, abf8966, abi7730, this issue p.
434
, p.
438
, p.
443
see also abj8914, p.
388
Data from the InSight mission on Mars help constrain the structure and properties of the martian interior.
For 2 years, the InSight lander has been recording seismic data on Mars that are vital to constrain the structure and thermochemical state of the planet. We used observations of direct (
P
and
S
) and surface-reflected (
PP
,
PPP
,
SS
, and
SSS
) body-wave phases from eight low-frequency marsquakes to constrain the interior structure to a depth of 800 kilometers. We found a structure compatible with a low-velocity zone associated with a thermal lithosphere much thicker than on Earth that is possibly related to a weak
S
-wave shadow zone at teleseismic distances. By combining the seismic constraints with geodynamic models, we predict that, relative to the primitive mantle, the crust is more enriched in heat-producing elements by a factor of 13 to 20. This enrichment is greater than suggested by gamma-ray surface mapping and has a moderate-to-elevated surface heat flow.
Close examination of the long wavelength shear velocity signal in the lowermost mantle in the wavenumber domain ties several geophysical observations together and leads to fundamental inferences. ...When mantle shear velocity model S362ANI at a depth of 2800
km is expanded in spherical harmonics up to degree 18, more than one half of the seismic model's total power is contained in a single spherical harmonic coefficient: the “recumbent”
Y
20 spherical harmonic; a
Y
20 with its axis of symmetry rotated to the equatorial plane. This degree 2 signal, which continues with decreasing amplitude for more than 1000
km above the core–mantle boundary (CMB), is characterized by two antipodal regions of low velocities, separated by a circum-polar torus of higher than average velocities. If the slow regions are associated with net excess mass, then any axis of rotation located in the plane of the circum-polar torus will be close to the maximum moment of inertia axis; this includes, of course, the current axis of rotation. We suggest that the recumbent
Y
20 is a very stable feature: once established, it is difficult to erase, and only relatively small departures from this equilibrium configuration are possible. This anomaly correlates strongly with the degree 2 terms of the residual geoid expansion, distribution of the hot spots above the slow regions, high attenuation in the transition zone, and position of subduction zones above the fast band during the last 200
Ma. Also, the preferred paths of the virtual geomagnetic pole and true polar wander locations for the last 200
Ma lie within the fast band. Since the non-hydrostatic perturbation of the moment of inertia tensor depends only on degree-2 anomalies in the density distribution and deformation of discontinuities, it is natural to infer that rotational dynamics of the Earth have influenced the distribution of heterogeneities in the Earth's deep interior. We propose that the large-scale heterogeneity at the base of the mantle, which we name Mantle Anchor Structure (MAS) may have formed early in the history of the convecting mantle, remained locked in place with respect to the Earth's rotation axis ever since, and is currently imposing the planform of flow in the mantle and of plate tectonics at the surface.
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The large low shear‐wave velocity provinces (LLSVP) are thermochemical anomalies in the deep Earth's mantle, thousands of km wide and ∼1800 km high. This study explores the hypothesis that the LLSVPs ...are compositionally subdivided into two domains: a primordial bottom domain near the core‐mantle boundary and a basaltic shallow domain that extends from 1100 to 2300 km depth. This hypothesis reconciles published observations in that it predicts that the two domains have different physical properties (bulk‐sound versus shear‐wave speed versus density anomalies), the transition in seismic velocities separating them is abrupt, and both domains remain seismically distinct from the ambient mantle. We here report underside reflections from the top of the LLSVP shallow domain, supporting a compositional origin. By exploring a suite of two‐dimensional geodynamic models, we constrain the conditions under which well‐separated “double‐layered” piles with realistic geometry can persist for billions of years. Results show that long‐term separation requires density differences of ∼100 kg/m3 between LLSVP materials, providing a constraint for origin and composition. The models further predict short‐lived “secondary” plumelets to rise from LLSVP roofs and to entrain basaltic material that has evolved in the lower mantle. Long‐lived, vigorous “primary” plumes instead rise from LLSVP margins and entrain a mix of materials, including small fractions of primordial material. These predictions are consistent with the locations of hot spots relative to LLSVPs, and address the geochemical and geochronological record of (oceanic) hot spot volcanism. The study of large‐scale heterogeneity within LLSVPs has important implications for our understanding of the evolution and composition of the mantle.
Key Points
LLSVPs may be compositionally subdivided into a basaltic shallow domain and a primordial deep domain, consistent with seismic observations
Geodynamic simulations predict different types of plumes to rise from the flanks and roofs of such compositionally “double‐layered” piles
Underside reflections from LLSVP roofs support a basaltic origin for LLSVP shallow domains, and thus a chemical origin for the whole LLSVPs
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Knowledge of the amount and distribution of radiogenic heating in the mantle is crucial for understanding the dynamics of the Earth, including its thermal evolution, the style and planform of mantle ...convection, and the energetics of the core. Although the flux of heat from the surface of the planet is robustly estimated, the contributions of radiogenic heating and secular cooling remain poorly defined. Constraining the amount of heat-producing elements in the Earth will provide clues to understanding nebula condensation and planetary formation processes in early Solar System. Mantle radioactivity supplies power for mantle convection and plate tectonics, but estimates of mantle radiogenic heat production vary by a factor of more than 20. Recent experimental results demonstrate the potential for direct assessment of mantle radioactivity through observations of geoneutrinos, which are emitted by naturally occurring radionuclides. Predictions of the geoneutrino signal from the mantle exist for several established estimates of mantle composition. Here we present novel analyses, illustrating surface variations of the mantle geoneutrino signal for models of the deep mantle structure, including those based on seismic tomography. These variations have measurable differences for some models, allowing new and meaningful constraints on the dynamics of the planet. An ocean based geoneutrino detector deployed at several strategic locations will be able to discriminate between competing compositional models of the bulk silicate Earth.
► Abundance estimates for heat producing elements in the Earth vary by factor of three. ► Seismically imaged mantle piles may be enriched in radioactivity. ► Laterally variable geoneutrino fluxes from enriched mantle piles are discernible. ► Models of silicate Earth composition can be tested with geoneutrino data. ► Neutrino tomography can map out variations in deep-seated radioactivity.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The stretching and break-up of tectonic plates by rifting control the evolution of continents and oceans, but the processes by which lithosphere deforms and accommodates strain during rifting remain ...enigmatic. Using scattering of teleseismic shear waves beneath rifted zones and adjacent areas in Southern California, we resolve the lithosphere-asthenosphere boundary and lithospheric thickness variations to directly constrain this deformation. Substantial and laterally abrupt lithospheric thinning beneath rifted regions suggests efficient strain localization. In the Salton Trough, either the mantle lithosphere has experienced more thinning than the crust, or large volumes of new lithosphere have been created. Lack of a systematic offset between surface and deep lithospheric deformation rules out simple shear along throughgoing unidirectional shallow-dipping shear zones, but is consistent with symmetric extension of the lithosphere.
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