Stephen P. Grand

Mantle shear structure beneath the Americas and surrounding oceans

Maps of lateral variation in shear velocity within the mantle beneath North and South America, their surrounding oceans, and parts of Africa and Eurasia are produced from inversion of travel times of horizontally polarized shear body waves. The data consist of S and ScS waves as well as multibounce phases SS, SSS and SSSS. Waves that bottom within the upper mantle are modeled using synthetic seismograms in order to estimate travel times for each of the multiple arrivals caused by velocity discontinuities near 400 and 660 km depth. The model consists of blocks with uniform slowness anomalies relative to a one-dimensional starting model and extends from the surface to the core-mantle boundary. The blocks have horizontal dimensions of roughly 275 by 275 km and vary in the vertical dimension from 75 to 150 km. The data are inverted using a simultaneous iterative reconstruction technique algorithm. The upper 400 km of the model is dominated by lateral variations that correspond to surface tectonic environments. Three shields on three separate continents have higher than average velocities down to between 320 and 400 km depth. Young tectonically active regions are very slow in the upper 250 km. The transition zone from 400 to 660 km depth is the most poorly resolved region. High velocity beneath western South America in the transition zone is probably associated with subducting slab. The transition zone velocity beneath the western and central part of North America also appears to be slightly faster than average. The lower mantle is dominated by large-scale sheets of higher than average velocity and more equidimensional regions of slow velocity. From South America to Siberia, sheet-like high-velocity anomalies are observed from 750 km depth to the core-mantle boundary. Another lower mantle high-velocity anomaly is seen beneath southern Eurasia. The high-velocity lower mantle anomalies seem to be associated with subduction during the last 150 Ma. Comparing the location of past subduction with the location of lower mantle anomalies, the identification of lower mantle anomalies with old subducted slabs suggests slow sinking of slabs in the lower mantle (about 1 to 2 cm/yr). If this interpretation is correct then high velocity in the deepest mantle off the west coast of South America through the western United States requires significant subduction from 120 to 150 Ma a few thousand kilometers off the coast of the Americas. The slowest deep region found in this study is at the base of the mantle beneath the eastern Atlantic Ocean and may be associated with hotspots in that region. Other hotspots do not appear to be associated with slow lower mantle velocity.

Low-velocity zone atop the 410-km seismic discontinuity in the northwestern United States

The seismic discontinuity at 410 km depth in the Earths mantle is generally attributed to the phase transition of (Mg,Fe)2SiO4 (refs 1, 2) from the olivine to wadsleyite structure. Variation in the depth of this discontinuity is often taken as a proxy for mantle temperature owing to its response to thermal perturbations. For example, a cold anomaly would elevate the 410-km discontinuity, because of its positive Clapeyron slope, whereas a warm anomaly would depress the discontinuity. But trade-offs between seismic wave-speed heterogeneity and discontinuity topography often inhibit detailed analysis of these discontinuities, and structure often appears very complicated. Here we simultaneously model seismic refracted waves and scattered waves from the 410-km discontinuity in the western United States to constrain structure in the region. We find a low-velocity zone, with a shear-wave velocity drop of 5%, on top of the 410-km discontinuity beneath the northwestern United States, extending from southwestern Oregon to the northern Basin and Range province. This low-velocity zone has a thickness that varies from 20 to 90 km with rapid lateral variations. Its spatial extent coincides with both an anomalous composition of overlying volcanism and seismic ‘receiver-function’ observations observed above the region. We interpret the low-velocity zone as a compositional anomaly, possibly due to a dense partial-melt layer, which may be linked to prior subduction of the Farallon plate and back-arc extension. The existence of such a layer could be indicative of high water content in the Earths transition zone.

Thermochemical structure and dynamics of the African superplume

[1] We present a new three-dimensional (3-D) model of the thermochemical structure of the African superplume region obtained from simultaneous inversion of global seismic and convection-related data. Convection-related observations include the global free-air gravity field, tectonic plate motions, dynamic surface topography and the excess ellipticity of the core-mantle boundary. A 3-D image of the chemically-induced density perturbations provides direct evidence that intrinsically-dense material is entrained within the superplume and concentrated into a rounded structure � 1000 km above the core-mantle boundary. The thermally-induced density perturbations are greater in magnitude than the chemically-induced implying overall positive buoyancy throughout the superplume. The observed morphology and density signatures are consistent with a thermochemical plume that has risen from a compositionally-distinct ‘pile’ at the base of the mantle and may be currently deforming under the influence of its intrinsic negative chemical buoyancy. Citation: Simmons, N. A., A. M. Forte, and S. P. Grand (2007), Thermochemical structure and dynamics of the African superplume, Geophys. Res. Lett., 34, L02301, doi:10.1029/2006GL028009.

Mesozoic plate-motion history below the northeast Pacific Ocean from seismic images of the subducted Farallon slab

The high-resolution seismic imaging of subducted oceanic slabs has become a powerful tool for reconstructing palaeogeography. The images can now be interpreted quantitatively by comparison with models of the general circulation of the Earths mantle. Here we use a three-dimensional spherical computer model of mantle convection to show that seismic images of the subducted Farallon plate provide strong evidence for a Mesozoic period of low-angle subduction under North America. Such a period of low-angle subduction has been invoked independently to explain Rocky Mountain uplift far inland from the plate boundary during the Laramide orogeny. The computer simulations also allow us to locate the largely unknown Kula–Farallon spreading plate boundary, the location of which is important for inferring the trajectories of ‘suspect’ terrain across the Pacific basin.

GRACE detects coseismic and postseismic deformation from the Sumatra-Andaman earthquake

[1] We show that spherical harmonic (SH) solutions of the Gravity Recovery and Climate Experiment (GRACE) are now of sufficient quality to observe effects of co-seismic and post-seismic deformation due to the rupture from the Mw = 9.3 Sumatra-Andaman earthquake on December 26, 2004, and its companion Nias earthquake (Mw = 8.7) on March 28, 2005. The improved GGM 03 SH (Level 2) solutions, and improved filtering methods provide estimates with spatial resolution comparable to earlier estimates from range-rate (Level 1) GRACE data.The gravityfield disturbance extends over 1800 km along Andaman and Sunda subduction zones, and changes with time following events. Gravity changes may be due to afterslip, viscoelastic relaxation, or other processes associated with dilatation. Satellite gravity measurements from GRACE provide a unique new measure of deformation and post-seismic processes associated with major earthquakes, especially in areas which are primarily oceanic. Citation: Chen, J. L., C. R. Wilson, B. D. Tapley, and S. Grand (2007), GRACE detects coseismic and postseismic deformation from the SumatraAndaman earthquake, Geophys. Res. Lett., 34, L13302,

Upper mantle convection beneath the central Rio Grande rift imaged by P and S wave tomography

[1] We present models for upper mantle P and S velocity structure beneath a southwestern United States transect extending from near the center of the Colorado Plateau across the Rio Grande rift to the Great Plains. The models were derived from travel times of compressional and shear seismic phases recorded by the La Ristra passive seismic array deployed from July 1999 to May 2001. Large variations in seismic velocity (up to 8% in S and 5% in P) are seen across the transect in the upper 200 km of the mantle. Seismically slow mantle underlies the Rio Grande rift and Jemez lineament and relatively slow mantle is seen beneath the Navajo volcanic field within the Colorado Plateau. The relative variations of P and S velocity imply that high temperatures are the primary cause of the slow mantle although a small amount of partial melting or hydration cannot be ruled out. Sharp boundaries in mantle seismic velocity are coincident with boundaries of Proterozoic structural trends implying that ancient lithospheric structure exerts a control on the tectonic and magmatic activity in the region. Weaker seismic variations are imaged below 200 km depth with three southeastward dipping structures in our images. Two of the structures have fast seismic anomalies, beneath the central Colorado Plateau and the Great Plains respectively, and the third has anomalously slow seismic wave speed. We interpret the western deep seismic anomaly to be foundering Farallon slab and the slow anomaly just to the east as upwelling mantle possibly associated with volatile release from the sinking Farallon slab. Beneath the Great Plains, there is also downwelling in the upper mantle. The combination of upwelling in the west and downwelling in the east are likely part of an upper mantle convection cell that may include entrained lithosphere from beneath the rift. INDEX TERMS: 7218 Seismology: Lithosphere and upper mantle; 8121 Tectonophysics: Dynamics, convection currents and mantle plumes; 8180 Tectonophysics: Evolution of the Earth: Tomography; KEYWORDS: convection, Rio Grande rift, Colorado Plateau

Dynamic Topography Change of the Eastern United States Since 3 Million Years Ago

By the Sea Side The Atlantic coastal plain of North America has been thought of as a passive margin, responding mostly to the weight of deposited sediments. As a result, the fine-scale stratigraphy of the sediments has been used to infer changes in global sea level through the Cenozoic. However, recent work has shown that the coastal plain has deformed in response to flow in Earths mantle. Rowley et al. (p. 1560, published online 16 May) used a model of flow in the mantle to show that the topography of the mid-Atlantic and Southern United States coast varied by 60 meters or more during the past 5 million years. Mantle flow has deformed the presumed passive eastern margin of North America by up to 60 meters during the past 5 million years. Sedimentary rocks from Virginia through Florida record marine flooding during the mid-Pliocene. Several wave-cut scarps that at the time of deposition would have been horizontal are now draped over a warped surface with a maximum variation of 60 meters. We modeled dynamic topography by using mantle convection simulations that predict the amplitude and broad spatial distribution of this distortion. The results imply that dynamic topography and, to a lesser extent, glacial isostatic adjustment account for the current architecture of the coastal plain and proximal shelf. This confounds attempts to use regional stratigraphic relations as references for longer-term sea-level determinations. Inferences of Pliocene global sea-level heights or stability of Antarctic ice sheets therefore cannot be deciphered in the absence of an appropriate mantle dynamic reference frame.

Imaging the seismic structure of the crust and upper mantle beneath the Great Plains, Rio Grande Rift, and Colorado Plateau using receiver functions

Received 20 October 2004; accepted 2 March 2005; published 28 May 2005. [1] The seismic structure of the crust and upper mantle of the southwestern United States is examined using receiver functions calculated from teleseismic arrivals recorded in the Colorado Plateau–Rio Grande Rift–Great Plains Seismic Transect (LA RISTRA) experiment. We apply receiver function estimation and filtering methods developed by Wilson and Aster (2005) to produce receiver functions with decreased sensitivity to noise and deconvolutional instability. Crustal thickness and Vp/Vs ratios are estimated using both direct and reverberated P-to-S receiver function modes. We apply regularized receiver function migration methods to produce a multiple-suppressed image of the velocity discontinuity structure of the subsurface. Our results show that crustal thickness averages 44.1 ± 2.3 km beneath the Great Plains (GP) and 45.6 ± 1.1 km beneath the Colorado Plateau (CP). Crustal thinning beneath the Rio Grande Rift (RGR) is broadly symmetric about the rift axis, with the thinnest crust (35 km) located directly beneath the rift axis, suggesting a pure shear stretched lithosphere beneath the RGR. We also observe a prominent northwest dipping discontinuity, ranging from 65 to 85 km deep beneath the CP, and possible subcrustal discontinuities beneath the GP. These discontinuities, along with recent xenolith data, are consistent with preserved ancient lithospheric structures such as relict suture zones associated with Proterozoic subduction. We observe an upper mantle discontinuity at 220–300 km depth that may correlate with similar discontinuities observed beneath eastern North America. We also observe relatively flat discontinuities at 410 and 660 km depth, indicating there is not a large-scale thermal anomaly beneath the RGR at these depths.

Crust and upper mantle shear wave structure of the southwest United States: Implications for rifting and support for high elevation

[1] Surface wave phase velocities from 29 earthquakes are used to map the shear velocity structure to � 350 km depth across the 950-km-long Rio Grande Rift Seismic Transect Experiment (LA RISTRA) seismic array in the southwest United States. Events from a range of back azimuths minimize the effects of multipathing. The resulting velocity model reveals a transition in lithospheric thickness from 200 km in the Great Plains to 45–55 km beneath the Rio Grande Rift, thickening beneath the Colorado Plateau to 120–150 km. The upper mantle low-velocity signature of the rift is roughly twice the width of its surface morphology. An asthenospheric low-velocity channel underlies the region west of the Great Plains and extends to 300 km depth. This channel is likely the result of warm mantle infill behind the sinking Farallon plate. Buoyant forces within this channel are sufficient to support much of the high elevation of the rift and plateau. No evidence for a deep mantle source is found beneath the rift, implying that present rifting is not driven by deep mantle upwelling. Velocities from 55 to 90 km beneath the rift axis are 10% slower than beneath the Great Plains, consistent with small amounts of partial melt. Low velocities extend to 200–300 km depth on either side of the rift but not directly beneath it, forming an inverted-U shape. This feature may reflect mantle that has cooled through vertical advection in a subadiabatic environment. Upwelling may be reinforced by small-scale convection caused by variations in lithospheric thickness and shallow mantle temperatures. INDEX TERMS: 7205 Seismology: Continental crust (1242); 7255 Seismology: Surface waves and free oscillations; 8109 Tectonophysics: Continental tectonics—extensional (0905); 8120 Tectonophysics: Dynamics of lithosphere and mantle—general; KEYWORDS: surface waves, continental rifting, upper mantle structure

Evidence for anisotropy in the deep mantle beneath Alaska

We consider the possibility that the velocity structure of D″ is anisotropic. The data we examined consist of seismograms from 9 deep Japanese earthquakes recorded at WWSSN receiver stations in North America. The source-receiver combinations span distances of 70°–106° with associated S waves passing through D″ beneath Alaska. Differential travel times of the S, Scd, ScS and SKS phases are used to constrain the velocity structure in D″. Shear waves refracted by D″ are observed beyond 72.2° and provide a sensitive measurement of the velocity structure in D″. Beyond 93°, the vertically polarized (SV) and horizontally polarized (SH) shear waves often appear distinctly split, although, at distances less than 89° the components are more nearly synchronous. Near 94°, SH occurs as a double arrival. SV in this range, however, remains a single arrival roughly synchronous with the second SH arrival. We have been unable to reproduce these effects in isotropic model synthetics. Synthetics for transversely isotropic models have been computed that do match these waveforms. The anisotropy was constrained to be only within D″, with a vertical symmetry axis. We conclude that these observations may be explained by an anisotropic D″ layer. The D″ discontinuity may be due to a transition to anisotropic mantle a few hundred kilometers above the core-mantle boundary.