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Increasing complexity of Earth's inner core has been revealed in recent decades as the global distribution of seismic stations has improved. The uneven distribution of earthquakes, however, still causes a biased geographical sampling of the inner core. Recent developments in seismic interferometry, which allow for the retrieval of core-sensitive body waves propagating between two receivers, can significantly improve ray path coverage of the inner core. In this study, we apply such earthquake coda interferometry to 1846 USArray stations deployed across the U.S. from 2004 through 2013. Clear inner core phases PKIKP^2 and PKIIKP^2 are observed across the entire array. Spatial analysis of the differential travel time residuals between the two phases reveals significant short-wavelength variation and implies the existence of strong structural variability in the deep Earth. A linear N-S trending anomaly across the middle of the U.S. may reflect an asymmetric quasi-hemispherical structure deep within the inner core with boundaries of 99°W and 88°E.

Water can be present in the mantle in the form of hydrous melts, hydrous phases, or incorporated into the crystal structure of nominally anhydrous minerals of the major mantle mineralogy. The first two forms are likely in the uppermost mantle, where water solubility in major mantle minerals is low, whereas the latter form may be more important deeper in the upper mantle and transition zone. Seismological data contain unique and valuable information on the amount and distribution of water in the Earth's mantle. It is, however, challenging to extract this information because of limitations in the amount and density of available seismic data, multiple interpretations of similar observations, and limited quantification of the effects of water and other parameters on the seismic properties. While increased water content and elevated temperatures both lower seismic velocities, they have opposing effects on the depths of the discontinuities that bound the transition zone. And, while they both increase attenuation, they have opposing effects on the sharpness of these discontinuities. Independent geophysical observations, such a gravity, electrical conductivity, and surface heat flow, can further help to discriminate between temperature, water, and other compositional anomalies as the cause of observed seismic heterogeneity. Various types of observations have been combined to infer water content in the mantle, ranging from a few hundredths of weight percent to several weight percent. Altogether, the seismological literature suggests that the mantle is heterogeneously hydrated. However, with the limited studies available, there does not appear to be an obvious correlation between present tectonic environment and water content, though the literature shows a tendency to interpret inferred anomalously hydrous regions in the mid mantle as being related in one way or another to past subduction of oceanic lithosphere.

Paleomagnetic intensity data from single (ODP 983) and composite (NAPIS-75) oceanic cores are analyzed for evidence of a rotating parametric instability (RPI) in Earth's fluid outer core. RPI, observed in laboratory experiments for over three decades, in Earth's core would be a resonant triad of two inertial modes coupled through straining of the core-mantle boundary. Small deformations in both amplitude, due to the semi-diurnal tide, and direction, from precession, produce growth of the instability on a geomagnetic time scale. Just as in laboratory observations of the RPI, alternating sequences of growth and decay continue at irregular intervals while straining is maintained and as long as the dissipation is sufficiently small. Presented here as e-folding times, decays show much less variance than growths while both are geophysically plausible. Combining growths and decays using linear stability theory yields e-folding times with reduced variance for the single and composite cores.

We determined the equation of state (EOS) of three silicate liquid compositions by shock compression of preheated samples up to 127 GPa. Diopside (Di; Ca_2Mg_2SiO_6) at 1773 K, anorthite (An; CaAl_2Si_2O_8) at 1923 K and the eutectic composition Di_(64)An_(36) at 1673 K were previously studied by shock compression to 38 GPa. The new data extend the EOS of each composition nearly to the Earth’s core-mantle boundary. The previously reported anomaly at 25 GPa for Di_(64)An_(36) eutectic was not reproduced; rather all data for this composition fit within error a straight line Hugoniot in particle velocity vs. shock velocity. Di also displays a linear Hugoniot consistent with ultrasonic data and a third-order finite strain EOS. The full anorthite data set is complex; we examine a model with a transition between two structural states and a fourth‐order finite strain model excluding two points that may not display relaxed behavior. We also report an experiment on room-temperature solid Di_(64)An_(36) aggregate that clearly demonstrates increase upon compression of the Gruneisen parameter of this liquid, much as experiment and theory have shown for forsterite and enstatite liquids. We construct isentropes and isotherms from our Hugoniots using Mie-Gruneisen thermal pressure and evaluate the model of ideal mixing of volumes. Volume may mix almost linearly at high temperature, but deviates strongly when calculated along an isotherm; it remains difficult to reach a firm conclusion. We compare the densities of liquids to lower mantle solids. Our results suggest that basaltic liquids rich in CaO and Al_2O_3 are notably denser than liquids in the MgO-SiO_2 binary and, subject to uncertainties in the behavior of FeO and in corrections for thermal pressure, such liquids may be the most likely candidates for achieving negative buoyancy in the lowermost mantle.

Our understanding of the core-mantle boundary (CMB) region has improved significantly over the past several years due, in part, to the discovery of the post-perovskite phase. Sesimic data suggest that the CMB region is highly heterogeneous, possibly reflecting chemical and physical interaction between outer core material and the lowermost mantle. In this contribution we present the results of a new mechanism of mass transfer across the CMB and comment on possible repercussions that include the initiation of deep, siderophile-enriched mantle plumes. We view the nature of core-mantle interaction, and the geodynamic and geochemical ramifications, as multiscale processes, both spatially and temporally. Three lengthscales are defined. On the microscale (1-50 km), we describe the effect of loading and subsequent shearing of the CMB region and show how this may drive local flow of outer core fluid upwards into D". We propose that larger scale processes operating on a mesoscale (50-300 km) and macroscale regimes (> 300 km) are linked to the microscale, and suggest ways in which these processes may impact on global mantle dynamics.

The discovery of post-perovskite (pPv), the high pressure polymorph of (Mg x ,Fe 1-x )SiO 3 perovskite (Pv), may have profound implications for the thermal, chemical and dynamical structure of the deep Earth, if it can be demonstrated that pPv currently exists in the lower mantle. The requisite basis for such a demonstration is seismological observation of elastic velocity and density structures diagnostic of or at least compatible with the expected properties of pPv occurrence in a realistic lower mantle chemical and thermal environment. A critical assessment of seismological observations versus predictions for a lower mantle model with pPv is undertaken, with the limitations and robust attributes of the seismological data being summarized. The existence of a seismic velocity discontinuity several hundred kilometers above the core-mantle boundary is a primary line of evidence for the presence of a Pv-pPv phase change. However, some attributes of the discontinuity, such as localized P wave reflections from a large P velocity increase and the sharpness of observed P and S velocity increases, reveal inconsistencies with expected properties of a Pv-pPv phase transition. Lateral variations in temperature can produce complex phase boundary structure that explains variable S wave observations, but such elastic heterogeneity intrinsically complicates testing the pPv hypothesis. The combination of rapidly expanding seismological data sets and new high resolution data analysis procedures reveal multiple seismic discontinuities near the base of the mantle; in some cases these may be consistent with forward and reverse Pv-pPv transformations, bounding a pPv layer or lens in D" above the CMB. Other phase changes or compositional contrasts could also be involved. At present, existence of pPv in the deep Earth is quite plausible, but not yet conclusively demonstrated. Future seismological and mineral physics research directions for further testing the hypothesis that pPv is present in the lower mantle are suggested.

The discovery of post-perovskite phase transition near the core-mantle bound-ary(CMB) has turned our heads to the potentially important role played by the increasing complexity of the physical properties in the lower-mantle models. In this study we have investigated the influences on lower mantle dynamics by the strongly depth-dependent coefficient of thermal expansion and radiative thermal conductivity together with the post-perovskite transition within the framework of isochemical models. We have carried out the simulations in both 2-D and 3-D Cartesian geometries. First, we review the basic connection between the temperature profile and the Clapeyron slope, calling attention to the special relationship between the temperature intercept of the post-perovskite phase change and the temperature at the core-mantle boundary. Double-crossing of the post-perovskite boundary takes place only, when the temperature of the CMB is greater than the temperature intercept of the phase change. We find that mantle plumes become multiscale in nature because of the combined effects exerted by variable mantle viscosity, strongly depth-dependent thermal expansivity, radiative thermal conductivity at the bottom of the mantle, the spinel to perovskite phase transition and the perovskite to post-perovskite phase change in the deep mantle. Both radiative thermal conductivity and strongly decreasing thermal expansivity in the lower mantle can help to induce partially layered convection with slabs stagnating in the transition zone. In our isochemical models a second low viscosity zone is created under the transition zone accompanied by intense shear heating. Secondary mantle plumes emerge from this region at the base of the transition zone. Large-scale upwellings in the lower mantle are induced mainly by both the style of lower-mantle stratification and the decrease in thermal expansivity. They control the location and the local dynamics of the upper-mantle plumes. In these models with variable thermal conductivity and viscosity, an increase in the temperature of the CMB causes a greater tendency for layered convection. From the same depth-dependent thermal expansivity, we can deduce the 3-D density anomalies from the seismic velocity anomalies inferred from seismic tomographic inversion. Using these density distributions, we can calculate the viscous responses of the Earth due to these density anomalies for a given viscosity structure. We then focus on the lateral viscosity variations of the deep mantle on the solution of the inverse problem involving the inferences of the viscosity from the long-wavelength geoid. Our solution for the large-scale lateral viscosity structure in the lowermost mantle shows that the region underneath hot spots have significantly higher viscosity in the deep mantle than the region below subduction regions. Recent inferences from first-principles calculations and laboratory experiments on analogue post-perovskite material also surmise the rheology of post-perovskite would be dominated by dislocation mechanism and be softer than perovskite. We put forth a hypothetical scenario in which the bottom portions of the superplumes in the deep mantle are stiffer than the adjacent post-perovskite mantle and are held fixed by the surrounding horizontal flow of post-perovskite.

Perturbations in the Earth's rotation rate at decadal time periods strongly favor the existence of dissipative coupling at the Core–Mantle Boundary (CMB). Here, we explored the plausibility of maintaining a conducting layer on the mantle-side of the CMB, which can couple the outer core and mantle through Lorentz torques. We propose a suction mechanism that maintains a porous medium on the mantle side of the CMB, with the interconnected pore-space partly or entirely filled with liquid iron up to a thickness of ∼1 km. The suction arises from the deviatoric stresses supported by the mantle-solid in regions of mantle downwelling. Infiltration of liquid iron occurs by percolation, but is inhibited by the rate of viscous dilation of the solid mantle. Our model enables core-mantle material exchange, and maintains a thin conducting layer that has seismic detection potential. Our model is only marginally satisfactory in explaining the inferred CMB coupling.

Teleseismic shear wave splitting is used to estimate mantle anisotropy beneath Precambrian province boundaries in the southern Rocky Mountains of Wyoming, Colorado, and New Mexico. Data from the passive seismic experiment of the Continental Dynamics of the Rocky Mountains (CD-ROM) project and Laramie Seismic Array are used. Data from the south CD-ROM line in New Mexico show consistent northeast fast directed shear-wave splitting, correlating with absolute plate motion and parallel to the fabric of Proterozoic accretion structures. In Wyoming, a clear change in shear-wave splitting parameters is seen across the Cheyenne Belt Archean/Proterozoic suture zone. South of the suture zone, northeast directed fast directions are observed. North of the Cheyenne Belt, shear wave splitting parameters vary with backazimuth, with the same backazimuthal patterns observed with the CD-ROM and Laramie arrays, despite the arrays being separated by over 100 km distance. This similarity in backazimuthal variation suggests a common anisotropic structure beneath both regions rather than lateral variations in anisotropy. The azimuthal variations of both the north CD-ROM and Laramie arrays are best modeled with a plunging axis of anisotropy. Our preferred model of anisotropy has an axis of symmetry that plunges steeply to the northwest. CD-ROM tomography has shown a steeply plunging fast velocity anomaly that has been forward modeled as an anisotropic slab. This feature may also be the source of the observed pattern of shear wave splitting.

We report recent findings in the field of high-pressure mineral physics with important implications for Earth's lower mantle. We show that the two main constituents of the lower mantle, namely, (Mg,Fe)SiO 3 -magnesium silicate perovskite-and (Mg,Fe)O-ferropericlase-undergo electronic transitions at lower mantle pressures (70 and 120 GPa), in which iron transforms from the high-spin state to the low-spin state. The transformations modify the thermochemical state of Earth's lower mantle. Minerals bearing high-spin iron have characteristic absorption lines in the near-infrared, hindering radiative conductivity at lower-mantle temperatures. These absorption lines shift to the visible (green to violet) range in the low-spin state, and their intrinsic intensities decrease; the minerals thus become increasingly transparent in the near-infrared and their radiative and total thermal conductivities rise. Thus, the heat conductivity of the lowermost mantle could be higher than previously thought. Moreover, the spin-driven partitioning of iron between the two mineral phases can explain large-scale chemical heterogeneities in the mantle that are driven by regional temperature variations. It is noteworthy that the transition pressures correspond to the bottom third of the lower mantle (70 GPa, 1700-km depth), and to the last 300 km above the core-mantle boundary (120 GPa, 2600-km depth); these regions have very special geophysical signatures, as chemical heterogeneities have been reported by seismology in the first case and the bottom 300 km of Earth's mantle constitutes the D layer. Our observations provide a mineral physics basis for these features in Earth's lower mantle.

The results of seismology and geochemistry for mantle structure are widely believed to be discordant, the former favoring whole-mantle convection and the latter favoring layered convection with a boundary near 650 km. However, a different view arises from recognizing effects usually ignored in the construction of these models, including physical plausibility and dimensionality. Self-compression and expansion affect material properties that are important in all aspects of mantle geochemistry and dynamics, including the interpretation of tomographic images. Pressure compresses a solid and changes physical properties that depend on volume and does so in a highly nonlinear way. Intrinsic, anelastic, compositional, and crystal structure effects control seismic velocities; temperature is not the only parameter, even though tomographic images are often treated as temperature maps. Shear velocity is not a good proxy for density, temperature, and composition or for other elastic constants. Scaling concepts are important in mantle dynamics, equations of state, and wherever it is necessary to extend laboratory experiments to the parameter range of the Earth's mantle. Simple volume-scaling relations that permit extrapolation of laboratory experiments, in a thermodynamically self-consistent way, to deep mantle conditions include the quasiharmonic approximation but not the Boussinesq formalisms. Whereas slabs, plates, and the upper thermal boundary layer of the mantle have characteristic thicknesses of hundreds of kilometers and lifetimes on the order of 100 million years, volume-scaling predicts values an order of magnitude higher for deep-mantle thermal boundary layers. This implies that deep-mantle features are sluggish and ancient. Irreversible chemical stratification is consistent with these results; plausible temperature variations in the deep mantle cause density variations that are smaller than the probable density contrasts across chemical interfaces created by accretional differentiation and magmatic processes. Deep-mantle features may be convectively isolated from upper-mantle processes. Plate tectonics and surface geochemical cycles appear to be entirely restricted to the upper ~1,000 km. The 650-km discontinuity is mainly an isochemical phase change but major-element chemical boundaries may occur at other depths. Recycling laminates the upper mantle and also makes it statistically heterogeneous, in agreement with high-frequency scattering studies. In contrast to standard geochemical models and recent modifications, the deeper layers need not be accessible to surface volcanoes. There is no conflict between geophysical and geochemical data, but a physical basis for standard geochemical and geodynamic mantle models, including the two-layer and whole-mantle versions, and qualitative tomographic interpretations has been lacking.

The Colorado Mineral Belt (CMB) is delineated as a belt of mostly Laramide mineralization within a broader zone of Laramide and older magmatism in central Colorado. This paper focuses on the profound negative Bouguer gravity anomaly that coincides with this broader zone of protracted magmatic activity, and is among the largest gravity anomalies in North America. Seismic studies suggest that parts of the CMB region are underlain by anomalously low-density crust and that the CMB lies within a broad zone of low seismic velocities in the uppermost mantle. This study explores simple distributions of subsurface mass deficits that can explain the CMB negative Bouguer gravity anomaly, and are constrained by geologic estimates of the extent of crustal plutonic bodies and by seismically-inferred crustal and upper mantle velocity anomalies. Specifically, we consider forward models that include (1) a low-density batholithic body in a 20-km thick upper crust with a density contrast of 150 kg/m 3 , (2) a 25-km thick lower crust with a potential low-density body with a density contrast of 150 kg/m 3 , and (3) an upper mantle with a low-density body with variable density contrast placed at variable depth. We discuss the viability of our first-order forward models and their consistency with seismic observations, and suggest refinements that can improve our understanding of the structure of the CMB.

We have interpreted the refraction/wide-angle reflection seismic profile acquired as part of the Continental Dynamics of the Rocky Mountains project. The profile extends ∼955km from northern New Mexico to southern Wyoming, crossing the Tertiary-Quaternary volcanics of the Jemez lineament, the Paleoproterozoic Mazatzal and Yavapai terranes, the Cheyenne belt, and the southern Archean Wyoming Province. We inverted the travel-time data from the 10 shot profile with both a layer based inversion method and a tomographic method. The two techniques yield comparable upper and middle crustal velocity structures. Lower crustal velocities are well constrained in the layer based model but are not in the tomographic model. From the layer based model, velocities in the crystalline crust and the upper mantle are lower than typical for continents and for modern orogens. Lower crustal velocities rarely exceed 7.00 km/s, likely due to the regionally high heat flow. We infer that the low upper mantle velocities beneath the Jemez lineament (7.70-7.76 km/s) are indicative of upper mantle partial melt. Crustal thickness increases from south to north, with thinner crust under the Jemez lineament (40-42 km), and thicker crust under northern Colorado, the Cheyenne belt, and southern Wyoming (51-53 km). Although the Cheyenne belt outcrops as a narrow zone separating Paleoproterozoic and Archean terranes, the seismic model shows broad lateral variation in crustal velocity near the suture, and a thick crust in the northern half of the profile. Part or all of the crustal thickening is likely to have occurred subsequent to continental accretion.

Two recent passive source (earthquake) seismic experiments have produced a teleseismic and regional event data set which provides constraints on the structure of the crust and upper mantle beneath the Colorado Rocky Mountains and two major Precambrian province boundaries. The passive source component of the Continental Dynamics of the Rocky Mountains (CD-ROM) experiment included two dense north-south linear arrays of broad-band seismometers straddling Precambrian province boundaries in Colorado, Wyoming, and New Mexico. The Rocky Mountain Front (RMF) experiment included thirty broadband seismometers spaced uniformly throughout Colorado. Results from a spectrum of seismological imaging and inversion techniques indicate that the Cheyenne Belt Archean-Proterozoic boundary in southern Wyoming has signatures in both the crust and upper mantle, while the Yavapai-Mazatzal province boundary is less clearly defined. Studies of data from the RMF experiment show pronounced low seismic velocities in the crust and upper mantle beneath the Rocky Mountains, and high attenuation (low Q) in the mantle. Techniques of passive source seismology used with RMF and CD-ROM experiment data are described here, along with reference to corresponding studies.

Upper mantle velocities in the regions of the Rio Grande rift and the Southern Rockies were investigated using Pn waves from the broadband seismic data obtained from the Continental Dynamics - Rocky Mountain (CDROM) experiment. A velocity of 7.8 +/- 0.1 km/s on the eastern flank of the Rio Grande rift in New Mexico was measured from epicentral distance vs. travel time data for ten Pn measurements from a south-western New Mexico earthquake that was in-plane with the southern CDROM line. Thirty-two interstation Pn measurements were made using eight western United States earthquakes. Using these measurements another bulk velocity estimate was made by solving for the best-fit velocity for interstation distances and travel times; this method also shows that the upper mantle beneath the Rio Grande rift has a slow velocity of 7.8 +/- 0.1 km/s. This low velocity is consistent with the high heat flow in the Rio Grande rift area and the evidence of modem rifting. Individual measurements made in the north end of the southern Rocky Mountains are near the global average Pn value of 8.1 km/s.

Using single-crystal elasticity data we constrain the effect of chemical substitutions on the elastic properties of mantle minerals and estimate the consequences for the seismic properties of mantle rocks. At ambient conditions the calculated relative variation of compressional and shear velocities ∂lnv P /∂X Fe and ∂Inv S /∂X Fe due to Fe-Mg substitution, range between -0.05 and -0.46 and between -0.08 and -0.74 respectively in the main mantle minerals. The corresponding heterogeneity ratios R = ∂lnv S /∂lnv P for Fe-Mg substitution range between 0.9 and 1.7 suggesting that the effect of this substitution is very different in different solid solutions systems. More limited experimental results for Ca-Mg substitution and Al enrichment in pyroxenes and garnets were also evaluated. Only Ca-Mg substitution in garnets is found to produce large (>2.0) values of R. Heterogeneity parameters at upper mantle and transition zone conditions can be substantially different from ambient P-T values in some cases. Using a first-order approximation of the effect of Fe-Mg substitution on the elastic properties of the most relevant upper mantle rocks, we find that the sensitivities of seismic velocities to Fe enrichment can vary as much as 2-3 times between the different rock types. We estimate that in the upper mantle the value of ∂lnv S /∂lnv P for pyrolite, piclogite and harzburgite decreases from 1.5 to 1.0 at the base of the transition zone, while it only decreases from 1.5 to 1.3 in mid ocean ridge basalt eclogite, which is enriched in garnet. We also estimate that the seismic effect of lateral changes in lithology from average mantle to subducted slab rocks decreases in intensity at upper mantle and transition zone depths, in agreement with seismic tomographic models. Information about the effects of Ca and Al enrichment are still too incomplete to make predictions of their effects on whole rocks, but they could be relevant based on our limited information.

As it is expected to be the dominant phase in the lower mantle, any pressure-induced phase changes in MgSiO 3 could require significant modifications in current models of the dynamics and structures of Earth's deep mantle. Studies to date have yielded discrepant results regarding the high-pressure stability of MgSiO 3 perovskite. Understanding the source of discrepancy is essential, both to resolving the stability of perovskite and to developing more reliable techniques for understanding the Earth's deep interior. In this report, we give an overview of previous studies on the stability of MgSiO 3 perovskite and recent observations on the post-perovskite transition. We also summarize our measurements on MgSiO 3 perovskite to core-mantle boundary pressure-temperature (P-T) conditions using in-situ X-ray diffraction. Major peaks in our diffraction patterns are best explained by those of MgSiO 3 perovskite at 1200-2500-km depth conditions. No evidence of dissociation to MgO + SiO, and a transition to a cubic perovskite structure has been found to core-mantle boundary P-T conditions. We have also observed a new peak at 2.62-2.57 A at 88-145 GPa, the existence of which may be relevant to a modification in perovskite crystal structure. However, the possibility that this peak may be from a chemical reaction among gasket, anvil materials, and sample cannot be ruled out. More significant changes are observed during heating above 2500 K at 135-145 GPa: appearance of new peaks, splitting of a peak, and intensity changes of some diffraction peaks. The recently proposed post-perovskite phase explains dominant new diffraction features. Based on the results available as of May 2005, the post-perovskite transition appears to be relevant to the D seismic discontinuity. Furthermore, depth of the post-perovskite transition may be very sensitive to variations in chemical composition as well as temperature.

Utilizing the Rocky Mountain Front (RMF) broadband seismic dataset acquired in 1992, this study has derived the seismic attenuation structure underlying part of the Southern Rocky Mountains and surrounding areas through measurements of differential t* of S-phase waveforms. Previous studies of the area include P, S and surface wave travel time tomography, and all indicate low upper mantle velocities below the Rocky Mountain region. Calculations of intrinsic attenuation coupled with current velocity models aid in the determination of temperature, partial melt distributions, and compositional variation. A N-S zone of high shear wave attenuation (Q s 30) is found in the mantle beneath the Rocky Mountains and lies east of the region of lowest shear wave velocity. Relationships between shear wave attenuation and shear wave velocity are consistent with both thermal and compositional variability. Along the eastern Colorado Rockies and due north of the Rio Grande Rift, the relationships are consistent with an interpretation of elevated temperatures by up to 50 K at 125 km depth. West of this region low velocities and low attenuation suggest either unusual composition or very high temperatures. The low density mantle material beneath the Colorado Rocky Mountains in addition to increased crustal thickness and low density crustal intrusions provides a density contrast sufficient to support its overburden.

We inverted Rayleigh wave data recorded in the Rocky Mountain Front Broadband Seismic Experiment for shear-wave velocity structure and azimuthal anisotropy. Distinctive structures are imaged beneath the southern Rocky Mountains, the western Great Plains, and the eastern Colorado Plateau. Beneath the southern Rockies, shear velocities are anomalously low from the Moho to depths of 150 km or more, suggesting replacement or delamination of the mantle lithosphere. The lowest velocities are beneath the extension of the Rio Grande rift into southern Colorado and are probably associated with partial melt. Beneath the Colorado Plateau, a thin, high-velocity lid is underlain by a low velocity layer to a depth of at least 160 km. Under the high plains, the velocities are above average down to ∼150 km depth, but not as fast as beneath the cratonic core of the continent. A crustal, low-velocity anomaly is observed beneath the high elevations of central Colorado. Elsewhere, inferred crustal thickness correlates with elevation, with the thickest crust beneath the San Juan Mountains in southwestern Colorado. These crustal anomalies suggest that much of the isostatic compensation for the high topography takes place within the crust. We observe a simple pattern of azimuthal anisotropy in the Rocky Mountain region with fast directions rotated slightly counterclockwise from the absolute plate motion of the North America plate and strength increasing with period. The observed anisotropy can be explained by deep asthenospheric flow dominated by current plate motion and shallower and perhaps laterally variable anisotropy in the upper lithosphere.

During the field experiments of the Continental Dynamics Rocky Mountains Project (CD-ROM) in 1999, a 950 km long refraction / wide-angle reflection seismic profile was acquired in the southern Rocky Mountains. This 950 km long seismic line extended from northern New Mexico to central Wyoming. The sedimentary thickness along the line varies between 3 km in the North Park basin, Colorado. The Precambrian basement in New Mexico is ∼2 km deep. In Colorado and Wyoming, portions of the line are located on basement exposures intervened by basins up to 2.5 km in depth. The velocities at the top of the upper crust vary between 5.75 km/s and 6.15 km/s and increase grad-ually to (v max = 6.35 km/s) at 25 km depth. The data suggest that the uppermost Precambrian basement consists of late Proterozoic volcanic and sedimentary rocks or felsic intrusions. The northern Wet Mountains are underlain by high velocities in the upper crust, and Cambrian mafic rocks crop out in these mountains. The Colorado Mineral belt correlates with a broad zone of relatively low velocities in the Precambrian crust indicating the presence of felsic intrusions. The average thickness of the upper crust is 20 km in the Great Plains and 25 km in the Rocky Mountains region. Strong lateral velocity variations occur in the upper crust. Major vertical velocity steps are prominent at the upper to middle crust. The middle crust velocities increase from 6.60 to 6.75 km/s and its average thickness is ∼10 km.

Upper mantle tomographic body wave images from the CD-ROM deployment reveal two major lithospheric anomalies across two primary structural boundaries in the southern Rocky Mountains: a ∼200 km deep high velocity north-dipping Cheyenne slab beneath the Archean-Proterozoic Cheyenne belt, and a 100 km deep low velocity Jemez body beneath the Proterozoic-Proterozoic Jemez suture. The Cheyenne slab is most likely a slab fragment accreted against the Archean Wyoming during the Proterozoic arc collision processes. This interpretation suggests that the ancient slab's thermal signature has been diffused away and non-thermal explanations for the high velocity slab are required. Tomographic modeling of possible chemical and anisotropic velocity variations associated with the slab shows that our isotropic velocity images can be explained via non-thermal models. In addition, the de-correlation of the P- and S-velocity images and the CD-ROM shear-wave splitting modeling are consistent with a dipping slab. The Jemez body plausibly results from the combination of low-solidus materials in the suture lithosphere and the late Cenozoic regional heating of the lithosphere. The 100 km deep lithospheric layering and the uniform shear-wave splitting measurements support our contention that the Jemez body is a lithospheric anomaly. A third low velocity structure extends beneath the middle Rio Grande Rift to 300 km depth. This anomaly may manifest a thermal upwelling that could increase heat flow into the lithosphere. Our results suggest that lithospheric heterogeneities related to fossil accretionary processes have been preserved in the Precambrian sutures, and are preferentially affecting the subsequent tectonism in this region.

Controlled source seismic techniques offer useful tools to study the structure of the earth's crust and upper mantle in considerable detail. The seismic reflection technique uses source-receiver offsets that are relatively small in comparison to the intended maximum depth of penetration, and the goal is to form an image of the subsurface structure through sophisticated data processing. The seismic refraction technique uses source-receiver offsets that are large in comparison to the intended maximum depth of penetration. Sophisticated interpretation methods have been developed to derive velocity models from these data. Reflection and refraction data are traditionally gathered along profiles and produce 2-D results, but new instrumentation is making surveys with an element of 3-D coverage possible at a crustal scale. From the standpoint of seismic refraction and reflection surveys, coverage for the Southern Rocky Mountains and surrounding areas before the 1999 CD-ROM project was relatively poor. Reflection surveys targeting the deep crust were particularly sparse. Of the pre-1999 refraction data, several profiles are at least partly unreversed, most have only a few widely spaced shotpoints, and for most profiles, the interval between recording stations is typically >10 km. The thickest crust in the region (∼50 km) does not correlate directly with the highest topography. The Southern Rocky Mountains are bisected by the Rio Grande rift whose crust thins from north to south and is at least 5 km thinner than that of adjacent regions. There is also evidence for crustal thinning across southern Wyoming.

The Continental Dynamics - Rocky Mountains project (CD-ROM) seismic refraction data illuminate the architecture of the lithosphere along a profile that traverses the Southern Rocky Mountains. Velocity models derived using several techniques indicate that crustal thickness increases from ∼45 under New Mexico to 55 km in central Colorado and thins to 40 to 45 km under southern Wyoming. A high-velocity lowermost crustal layer (Vp = 7.0 to 7.4 km s -1 ) with a thickness ranging from 5 to 10 km is evident in the Southern Rocky Mountains - Great Plains portion of the model. A very prominent mid-crustal interface lies at depths of ∼25 to 30 km within the crust and correlates with a similar discontinuity reported in the mid-continent region. Velocities increase to about 6.8 km s -1 below this discontinuity so it can be thought of as the Conrad discontinuity. This observation, together with the CD-ROM deep reflection data, suggests that the main elements of crustal structure developed during initial assembly of the continent (ca. 1.76-1.6 Ga) and during extensive magmatism at 1.4 Ga. Subsequent modification of the crust was primarily local nature. For example, the areas of thickest crust do correlate well with the occurrence of Rocky Mountain uplifts, suggesting that mechanical thickening of Laramide age plays a role where crustal thickness exceeds a value of ∼45 km. Gravity modeling based on the velocity model shows that the isostatic compensation of the high Southern Rocky Mountains results from a complex interplay of low-density material in the crust, crustal thickening, and a transition from high to low-density mantle beneath the range.

Seismological and geodynamical observations have established the presence of a major thermo-chemical boundary layer (TCBL) in the lowermost mantle. This boundary layer plays a critical role in regulating heat flow through the core-mantle boundary, thereby influencing the dynamo-generating core flow regime. It also plays an important role in the mantle convection system, possibly serving as a source of boundary-layer instabilities and as a reservoir for long-lived geochemical heterogeneities. Two end-member conceptual models for the TCBL have emerged, both reconcilable with current observational constraints: a global, stably-stratified, chemically distinct layer may exist in the lowermost 250 km of the mantle (the global TCBL model), or this region may be a partially mixed boundary layer involving a composite of downwelling thermo-chemical anomalies such as oceanic lithospheric slabs or eclogitic oceanic crustal components and ancient dense chemical anomalies dynamically concentrated into large agglomerations beneath upwellings (the hybrid TCBL model). For the global TCBL model, laterally varying partial melt fractions within the layer are required to account for various seismological observations, and large dynamic topography on the upper boundary of this layer is expected: there is evidence for both of these attributes of the TCBL. The hybrid TCBL model requires additional complexity such as a phase transition or structural fabric transition to account for various seismological observations: some mineralogical candidates have been proposed. The outstanding challenge, requiring multi-disciplinary advances, is to discriminate between these competing conceptual models, as they differ in implications for thermal history, chemical processing, and dynamical behavior of the TCBL.

Angular momentum exchange between the Earth's mantle and core influences the Earth's rotation on time scales of decades and longer, in particular in the length of day (LOD) which have been measured with progressively increasing accuracy for the last two centuries. There are four possible coupling mechanisms for transferring the axial angular momentum across the core-mantle boundary (CMB): viscous, magnetic, topography, and gravitational torques. Here we use our scalable, modularized, fully dynamic geodynamo model for the core to assess the importance of these torques. This numerical model, as an extension of the Kuang-Bloxham model that has successfully simulated the generation of the Earth's magnetic field, is used to obtain numerical results in various physical conditions in terms of specific parameterization consistent with the dynamical processes in the fluid outer core. The results show that depending on the electrical conductivity of the lower mantle and the amplitude of the boundary topography at CMB, both magnetic and topographic couplings can contribute significantly to the angular momentum exchange. This implies that the core-mantle interactions are far more complex than has been assumed and that there is unlikely a single dominant coupling mechanism for the observed decadal LOD variation.

several structures is placed in the context of the Earth's history. We review the heat flow across the core-mantle boundary, and show a simple model of the inner core growth. We present a model of the initial chemical stable stratification of the core and its subsequent disruption. Model calculations show that viscous compaction efficiently expels liquid from the inner core, and also causes a crust-like structure to form beneath the inner-core boundary because the inner core growth rate gradually decreases. Next, we consider how the radial structure is modified as a result of the lateral variation of

Recent studies of the core-mantle boundary (CMB) have revealed some very anomalous structures interpreted in terms of ultra low velocity zones (ULVZ). However, there remains considerable uncertainties about their physical descriptions or even if they occur above or below the CMB. They have only been detected in isolated situations using rather special techniques; these includes: distortions in SKS with the development of SKPdS and SPdKS, broadband PKP precursors, distinct ScS and S beyond 100 degree, and rapid changes in differential travel times of neighboring phases. Here we report on a situation where ray paths associated with PKP precursors and SKPdS sample the same ULVZ structure. The structure lies beneath central Africa and has been detected from WWSSN analog data (SKPdS) discussed previously. This data set has been enhanced with a collection of digital records sampling an elongated North-South zone roughly 800 km long. The entire SKPdS data set can be modeled with a ridge-shaped cross section with widths of 250 to 400 km and drops in P and S velocity of 10 and 30 percent. Fortunately, a new IRIS station (MSKU) located in Western Africa provided excellent PKP data from the New Britain Region events sampling the above structure. The PKP and strong precursors can be modeled by 2D synthetics generated from the same structure (used in modeling SKP dS) which provides a strong constraint on the definition characteristics of this particular ULVZ.

Layered viscosity, temperature-dependent viscosity, and surface plates have an important effect on the scale and morphology of structure in spherical models of mantle convection. We find that long-wavelength structures can be produced either by a layered viscosity with a weak upper mantle or temperature-dependent viscosity even in the absence of surface plates, corroborating earlier studies, However,combining the layered viscosity structure with a temperature-dependent viscosity results in structure with significantly shorter wavelengths. Our models show that the scale of convection is mainly controlled by the surface plates, supporting the previous two-dimensional studies. Our models with surface plates: layered and temperature-dependent viscosity, and internal heating explain mantle structures inferred from seismic tomography. The models show that hot upwellings initiate at the core-mantle boundary (CMB) with linear structures, and as they depart from CMB, the linear upwellings quickly change into quasi-cylindrical plumes that dynamically interact with the ambient mantle and surface plates while ascending through the mantle. A linear upwelling structure is generated again at shallow depths (

We review theoretical approaches and results in the study of the elasticity of mantle materials under extreme conditions. First principles methods for calculating the elastic constants of minerals with complex structures are described and are compared with alternative approaches. These include models at the ab initio, semi-empirical and continuum levels. Special attention is payed to the continuum description as embodied in the extension of finite strain theory to anisotropic strains, and to the description of the variation of the elastic constants with pressure and temperature. First principles results on a number of important mantle phases are reviewed. Comparison of theory and experiment is featured including an account of predictions of first principles calculations that have subsequently been confirmed by experiment. A discussion of the effect of pressure on elastic constants is focused around three issues: 1) acoustic anisotropy, which is shown to change qualitatively (in sign and magnitude) over the mantle pressure regime, 2) elastic instabilities, which occur in silica in the vicinity of the phase transition from stishovite to the CaCl 2 -structure near 47 GPa, and 3) an evaluation of the anisotropic generalization of finite-strain theory, which is shown to converge rapidly for the elastic constants of MgSiO 3 perovskite. The possible effect of high temperature on elastic properties is explored by extending the first principles calculations with a semi-empirical thermal correction based on finite strain theory. In the case of the elastic anisotropy of periclase, temperature is found to have an effect secondary to that of pressure over the mantle regime.

Resolving anelastic attenuation, its spatial and depth variations, can provide important constraints on the composition and dynamics of the earth. However, measuring Q is difficult because of interfering non-linear elastic effects which contribute to the amplitudes of seismic waves. We review recent progress in the determination of global Q-structure in the earth using seismic data. The 1D profile of Qμ appears to be reasonably well constrained in the upper-mantle and transition zone, less so in the lower mantle and D. Some discrepancies remain, in the inner core, between body wave and free oscillation estimates of average Qμ and its depth variation and it is not yet clear whether most of the bulk attenuation is in the core or in the mantle, although recent normal mode studies favor the latter. Large lateral variations in Q are now well established in the crust and the uppermost mantle, where they correlate with past and present tectonic activity. Only few studies to date have proposed 3D tomographic images of the deeper parts of the mantle. While significant lateral variations are well documented, and appear to correlate with those of elastic heterogeneities, large uncertainties remain in the amplitudes of the lateral variations and their details. While there may be some indication for lateral variations in D, it is not clear yet whether such variations are required at all in the bulk of the lower mantle. An interesting recent finding has been that of anisotropy in attenuation in the inner core. More work is needed to document and understand the latter, constrain Q (1D and 3D) in the lower mantle, and resolve lateral variations in the upper mantle more quantitatively. A particular challenge for the future is to be able to accurately quantify the perturbing effects due to elastic structure in global measurements of surface wave and body wave amplitudes.

This chapter discusses seismic anisotropy in the mantle, highlighting mechanisms, observations and interpretations. Although evidence of anisotropy in the Earth is now commonplace, it can be difficult to uniquely identify its underlying cause. Part of the difficulty is due to the nature of wave propagation in anisotropic media; intuition based on wave propagation in isotropic media can be misleading. Observations suggest that anisotropy is not uniform throughout the mantle, but appears to be concentrated in the boundary layers of the convecting mantle where deformation is largest. Studies of surface waves and shear-wave splitting in phases like SKS have provided abundant evidence of anisotropy in the uppermost boundary-layer. In continental settings, the magnitude of the anisotropy is quite variable and its orientation mimics trends in surface geology, suggesting the crust and mantle are coupled during orogenic deformation. In contrast, upper-mantle anisotropy in oceanic settings appears to be quite consistent over large regions and reflects large-scale flow patterns in the mantle. Deeper in the mantle, there is some evidence of anisotropy in the upper-mantle transition zone suggesting that in places a boundary layer may exist between the upper and lower mantle. The bulk of the lower mantle seems to be isotropic except for the D region, the lowermost boundary layer. Here the anisotropy exhibits regional variations on scales similar to those in the lithosphere. As it appears that there are regions of azimuthal anisotropy in D , there is a potential problem separating the effects of upper-mantle anisotropy from lower-mantle anisotropy in data interpretations. At present, observations of seismic anisotropy lack global coverage, but as this improves we are faced with the challenging task of including anisotropy in global seismic models.

We present here a new test of the layered convection hypothesis based on mantle-flow calculations which employ three-dimensional seismic models of mantle structure. The mantle-flow predictions are obtained using a newly reformulated viscous flow theory which also incorporates the constraints arising from the observed geometry of rigid surface plates. The motions of the tectonic plates are predicted, rather than imposed. We derive, through a formal inversion procedure, the mantle viscosity profile that provides a best match to the observed tectonic plate motions and to the global free-air gravity anomalies. Two distinct end-member models are considered in these inversions: a model that yields a whole-mantle style of flow and a strictly layered-flow model that includes an explicit mechanical barrier to vertical flow at 670 km depth. We find, after considering the uncertainties in the seismic tomographic model, that the convection-related surface observables predicted on the basis of the layered and whole-mantle flow models are indistinguishable. We consider here the implications of layered convection for the flow-induced deformation at the bottom of the upper mantle. We compare this deformation field with that predicted by a model which yields a whole-mantle style of flow. Significant differences are found in the predicted directions of maximum extension rate which may be related to seismic inferences of azimuthal anisotropy at the bottom of the upper mantle. We propose that a systematic study of deep-mantle anisotropy, particularly in the mantle transition zone, may provide the clearest constraints on the degree of vertical layering in the mantle convective flow.

We present data which indicate that the broad, low shear velocity anomaly beneath southern Africa is stronger and more extensive than previously thought. Recordings of earthquakes in the southwestern Atlantic Ocean at an array of broadband seismic stations in eastern Africa show anomalously large propagation time delays of the shear phases S, ScS, and SKS which vary rapidly with epicentral distance. By forward modeling, we estimate that the low velocity anomaly extends from the core-mantle boundary about 1500 km up into the mantle and that the average shear velocity within this structure is 3% lower than in standard models such as PREM. Strong velocity contrasts exist at its margins (2% over about 300 km). These seismic characteristic are consistent with recent numerical simulations of lower mantle mega-plume formation.

A laser-heating system was constructed that can stably heat through a diamond-anvil cell, probe regions as small as 6.7 μm in diameter, and accurately measure visible spectral radiation at frequencies up to 30 Hz. Spectra can be analyzed to measure temperature and to identify phenomena of interest such as melting. Reflecting optics were used where dispersion is critical. Temperature is stabilized by feedback from the thermal emissions of laser-heated samples and by attenuating the laser beam using a liquid-crystal variable waveplate and a fixed polarizer. The response time of the stabilizer is estimated to be 1.25 ms. The laser-heating system is suitable for experiments at the pressures and temperatures of the Earth's lower mantle and core. With this apparatus, a magnesium-iron-silicate perovskite, (Mg .88 Fe .12 ) SiO 3 , was melted in a diamond-anvil cell at pressures between 25 GPa and 85 GPa. Natural bronzite and synthetic enstatite were the starting materials. Polycrystalline samples with no pressure medium and single crystals in a NaCl pressure medium were melted. Melting was determined in situ by thermal analysis and corroborated by the appearance of glass in temperature-quenched samples. Corrections for radial temperature gradients in the samples were obviated by aperturing the collecting optics of the spectrometer. Corrections for axial temperature gradients were estimated from a simple model. The slope of the melting curve between 35 GPa and 85 GPa is 5.2 ± 0.8 K/GPa and sub-adiabatic. The melting temperature extrapolated to the core-mantle boundary is 4502 ± 176 K. Both are significantly less than recent estimates.