Kenneth G. Dueker

Mantle discontinuity structure from midpoint stacks of converted P to S waves across the Yellowstone hotspot track

Analysis of a deployment of broadband sensors along a 500-km-long line crossing the Yellowstone hotspot track (YHT) has provided 423 in-plane receiver functions with which to image lateral variations in mantle discontinuity structure. Imaging is accomplished by performing the converted wave equivalent of a common midpoint stack, which significantly improves resolution of mantle discontinuity structure with respect to single-station stacks. Timing corrections are calculated from locally derived tomographic P and S wave velocity images and applied to the Pds (where d is the depth of the conversion) ray set in order to isolate true discontinuity topography. Using the one-dimensional TNA velocity model and a Vp/Vs ratio of 1.82 to map our Pds times to depth, the average depths of the 410- and 660-km discontinuities are 423 and 664 km, respectively, giving an average transition zone thickness of 241 km. Our most robust observation is provided by comparing the stack of all NW back-azimuth arrivals versus all SE back-azimuth arrivals. This shows that the transition zone thickness varies between 261 and 232 km, between the NW and SE portions of our line. More spatially resolved images show that this transition zone thickness variation results from the occurrence of 20–30 km of topography over 200–300 lateral scale lengths on the 410- and 660-km discontinuities. The topography on the 410- and 660-km discontinuities is not correlated either positively or negatively beneath the 600-km-long transect, albeit correlation could be present for wavelengths larger than the length of our transect. If this discontinuity topography is controlled exclusively by thermal effects, then uncorrelated 250° lateral temperature variations are required at the 410- and 660-km discontinuities. However, other sources of discontinuity topography such as the effects of garnet-pyroxene phase transformations, chemical layering, or variations in mantle hydration may contribute. The most obvious correlation between the discontinuity structure and the track of the Yellowstone hotspot is the downward dip of the 410-km discontinuity from 415 km beneath the NW margin of the YHT to 435 km beneath the easternmost extent of Basin and Range faulting. Assuming this topography is thermally controlled, the warmest mantle resides not beneath the Yellowstone hotspot track, but 150 km to the SE along the easternmost edge of the active Basin and Range faulting.

How Laramide-Age Hydration of North American Lithosphere by the Farallon Slab Controlled Subsequent Activity in the Western United States

Starting with the Laramide orogeny and continuing through the Cenozoic, the U.S. Cordilleran orogen is unusual for its width, nature of uplift, and style of tectonic and magmatic activity. We present teleseismic tomography evidence for a thickness of modified North America lithosphere <200 km beneath Colorado and >100 km beneath New Mexico. Existing explanations for uplift or magmatism cannot accommodate lithosphere this thick. Imaged mantle structure is low in seismic velocity roughly beneath the Rocky Mountains of Colorado and New Mexico, and high in velocity to the east and west, beneath the tectonically intact Great Plains and Colorado Plateau. Structure internal to the low-velocity volume has a NE grain suggestive of influence by inherited Precambrian sutures. We conclude that the high-velocity upper mantle is Precambrian lithosphere, and the lowvelocity volume is partially molten Precambrian North America mantle. We suggest, as others have, that the Farallon slab was in contact with the lithosphere beneath most of the western U.S. during the Laramide orogeny. We further suggest that slab de-watering under the increasingly cool conditions of slab contact with North America hydrated the base of the continental lithosphere, causing a steady regional uplift of the western U.S. during the Laramide orogeny. Imaged low-velocity upper mantle is attributed to hydration-induced lithospheric melting beneath much of the southern Rocky Mountains. Laramide-age magmatic ascent heated and weakened the lithosphere, which in turn allowed horizontal shortening to occur in the mantle beneath the region of Laramide thrusting in the southern Rocky Mountains. Subsequent Farallon slab removal resulted in additional uplift through unloading. It also triggered vigorous magmatism, especially where asthenosphere made contact with the hydrated and relatively thin and fertile lithosphere of what now is the Basin and Range. This mantle now is dry, depleted of basaltic components, hot, buoyant, and weak.

Mantle discontinuity structure beneath the Colorado Rocky Mountains and High Plains

Analysis of mantle discontinuity structure using converted P to S (PaS) phases beneath Colorado from the Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) Rocky Mountain Front (RMF) experiment reveals significant topography at the 410 and 660 km depth discontinuities and corresponding transition zone thickness variations. A stack of all radial receiver functions resolves the 410 and 660 km discontinuities at average depths of 419 and 677 km, respectively. Imaging of lateral variations in mantle discontinuity structure is accomplished by geographically binning the Pds conversion points and then stacking the receiver functions in each bin to form spatial images, analogous to common depth point stacking. Corrections for lateral velocity heterogeneity are calculated using the local S wave tomographic model of Lee and Grand [1996] and a constant ∂lnVs/∂lnVP scaling of 1.3. This scaling value is determined from the relative scaling between teleseismic P and S wave travel time residuals measured from the Rocky Mountain Front deployment. Mantle discontinuity images using 150 km square bins show 20 km of 410 km discontinuity topography, 30 km of 660 km discontinuity topography, and up to 40 km of transition zone thickness variation. Features of the discontinuity structure include a 20 km depression of the 660 km discontinuity beneath western Colorado and a gradual 10 km dip of the 410 km discontinuity beneath the High Plains. The thickening of the transition zone beneath southwest Colorado is consistent with the presence of the subducted Farallon slab in this region as imaged by Van der Lee and Nolet [1997]. In general, our results show that the transition zone discontinuity structure is more complex than that predicted by the simple model of olivine phase boundaries modulated by vertically coherent thermal anomalies.

Physical state of the western U.S. upper mantle

Using observed P wave images of the western U.S. upper mantle, which show lateral variations of up to 8%, and existing scaling relations, we infer that the low-velocity mantle is hot and partially molten to depths of 100–200 km, and that the high-velocity upper mantle is subsolidus. Most the high-velocity upper mantle within a few hundred kilometers of the coastline appears to be relatively dense, suggesting that it is relatively cool (i.e., a thermal lithosphere). This is expected for features associated with the subducting Juan de Fuca and Gorda slabs, and the high velocity upper mantle beneath the Transverse Ranges has been attributed to the sinking of negatively buoyant mantle lithosphere. Other high-velocity mantle structures near the continental margin are consistent with this interpretation. In contrast, the generally high elevations of the continental interior imply a buoyant upper mantle there, an inference that holds for both the high- and the low-velocity upper mantle. The only reasonable way to produce the high-velocity low-density upper mantle is through basalt depletion, thereby creating mantle of increased solidus temperature and decreased density. We distinguish a marginal domain, within ∼250 km of the Pacific coast, from an interior domain. This is based on the inferred upper mantle compositional difference and regional associations: beneath the marginal domain, upper mantle structures trend parallel to the surface physiography and young tectonic structures, whereas upper mantle structures beneath the continental interior trend northeasterly. This northeast orientation is discordant with the young tectonic structures, but aligns with young volcanic activity. The high lateral gradients in observed upper mantle seismic structure found throughout the western United States imply high lateral gradients in the: associated temperature or partial melt fields. Because these fields diffuse on time scales of less than a few tens of millions of years, the imaged upper mantle structure is young. The following upper mantle processes are hypothesized to account for these findings and inferences. Away from the plate margin, small-scale upper mantle convection driven by partial melt-induced buoyancy of hot upper mantle leads to the production and segregation of melt and the creation of compositional variations. The heterogeneous upper mantle P wave structure of the elevated continental interior is largely a consequence of partial meltvariations that are modulated by the compositional variations, and throughout this region we infer high temperatures and low densities. Near the plate margin, relative plate motions force upper mantle flow, although upper mantle flow driven by the positive buoyancy of melt and the negative buoyancy lithosphere is important locally.

Dehydration melting at the top of the lower mantle

Cycling water through the transition zone The water cycle involves more than just the water that circulates between the atmosphere, oceans, and surface waters. It extends deep into Earths interior as the oceanic crust subducts, or slides, under adjoining plates of crust and sinks into the mantle, carrying water with it. Schmandt et al. combined seismological observations beneath North America with geodynamical modeling and high-pressure and -temperature melting experiments. They conclude that the mantle transition zone—410 to 660 km below Earths surface—acts as a large reservoir of water. Science, this issue p. 1265 Downwelling of hydrous minerals may cause partial melting of Earth’s lower mantle. The high water storage capacity of minerals in Earth’s mantle transition zone (410- to 660-kilometer depth) implies the possibility of a deep H2O reservoir, which could cause dehydration melting of vertically flowing mantle. We examined the effects of downwelling from the transition zone into the lower mantle with high-pressure laboratory experiments, numerical modeling, and seismic P-to-S conversions recorded by a dense seismic array in North America. In experiments, the transition of hydrous ringwoodite to perovskite and (Mg,Fe)O produces intergranular melt. Detections of abrupt decreases in seismic velocity where downwelling mantle is inferred are consistent with partial melt below 660 kilometers. These results suggest hydration of a large region of the transition zone and that dehydration melting may act to trap H2O in the transition zone.

Western U.S. upper mantle structure

Regional three-dimensional inversions of upper mande P wave velocity structure are created from teleseismic P wave travel time residuals recorded by many of the high-density high-frequency regional arrays operated within the western United States. These inversions are adjusted to a global (International Seismic Centre) reference and merged to obtain an image of the upper mantle beneath western United States. The P wave velocities in the upper mantle are slow on average, and the structure is very heterogeneous. Where resolution is good, coherent upper mantle structures are imaged that extend as deep as ∼200 km (the Juan de Fuca and Gorda slabs, which penetrate to greater depths, are exceptions) and deviate from the average velocity (at a given depth) by as much as ±4%. Lateral resolution of these structures usually is very good, although the magnitude of the actual seismic variations is probably greater than that imaged. The long wavelength part of the imaged mantle structure defines coherent elongate features with wavelengths of 200–500 km. Within ∼250 km of the Pacific Coast, these structures have a wavelength of ∼250 km and trend parallel to the surface physiography and young tectonic structures. Beneath the continental interior, where use is made of additional seismic studies to infer average structure in regions of poor teleseismic data coverage, structures have a wavelength of ∼500 km and trend northeasterly. This northeast orientation is discordant with young tectonic structures but aligns with young volcanic activity.

Mantle-driven dynamic uplift of the Rocky Mountains and Colorado Plateau and its surface response: Toward a unified hypothesis

The correspondence between seismic velocity anomalies in the crust and mantle and the differential incision of the continental-scale Colorado River system suggests that significant mantle-to-surface interactions can take place deep within continental interiors. The Colorado Rocky Mountain region exhibits low-seismic-velocity crust and mantle associated with atypically high (and rough) topography, steep normalized river segments, and areas of greatest differential river incision. Thermochronologic and geologic data show that regional exhumation accelerated starting ca. 6–10 Ma, especially in regions underlain by low-velocity mantle. Integration and synthesis of diverse geologic and geophysical data sets support the provocative hypothesis that Neogene mantle convection has driven long-wavelength surface deformation and tilting over the past 10 Ma. Attendant surface uplift on the order of 500–1000 m may account for ∼25%–50% of the current elevation of the region, with the rest achieved during Laramide and mid-Tertiary uplift episodes. This hypothesis highlights the importance of continued multidisciplinary tests of the nature and magnitude of surface responses to mantle dynamics in intraplate settings.

Seismic migration processing of P-SV converted phases for mantle discontinuity structure beneath the Snake River Plain, western United States

We experiment with backprojection migration processing of teleseismic receiver functions from the Snake River Plain (SRP) broadband seismic experiment. Previous analyses of data from this experiment have used a common midpoint (CMP) stacking approach, a method widely applied for analysis of P-SV converted phases (receiver functions) to obtain high-resolution imaging of upper mantle discontinuities. The CMP technique assumes that all P-SV conversions are produced by flat-lying structures and may not properly image dipping, curved, or laterally discontinuous interfaces. In this paper we adopt a backprojection migration scheme to solve for an array of point scatterers that best produces the large suite of observed receiver functions. We first perform synthetic experiments that illustrate the potential improvement of migration processing over CMP stacks. Application of the migration processing to the SRP data set shows most of the major features as in the original CMP work, but with a weaker 410-km discontinuity and a more intermittent discontinuity at 250 km apparent depth. Random resampling tests are also performed to assess the robustness of subtle features in our discontinuity images. These tests show that a 20-km elevation of the 660-km discontinuity directly beneath the Snake River Plain is robust, but that the variations in 410-km discontinuity topography that we observe are not stable upon resampling. “Bright spots” near 250 km apparent depth are robust upon resampling, but interpretation of these features is complicated by possible sidelobe artifacts from topside Moho reverberations.

Crust and upper mantle velocity structure of the Yellowstone hot spot and surroundings

[1] The Yellowstone hot spot has recently been shown to be a plume that extends into the transition zone. At roughly 60–120 km depth, the plume material rising beneath Yellowstone Park is sheared SW by North America Plate motion, producing a profound low velocity layer emplaced beneath the thin lithosphere. To constrain the absolute seismic velocity of the plate-sheared plume layer, fundamental mode Rayleigh wave observations have been inverted for phase velocity using the two plane wave technique. The resulting phase velocity models are inverted with Moho-converted P to S arrival times to better constrain crustal thickness and absolute S wave velocity structure to � 120 km depth. A regionalized S wave velocity model has an extremely low velocity minimum of 3.8 ± 0.1 km/s at 80 km depth beneath the hot spot track. Nonregionalized 3-D velocity models find a velocity minimum of 3.9 km/s beneath the hot spot track. Below 120 km depth, our resolution diminishes such that the lateral spreading of the plume track is not resolved. The volume of the low velocity plume layer is small and the estimated buoyancy flux for the Yellowstone plume is <0.1 Mg/s which contrasts with the � 9 Mg/s value for Hawaii. In addition, a notable region of thick crust and high lower crustal velocities is found around Billings, Montana, consistent with previous refraction and receiver function studies that interpret this as evidence for a massive Precambrian underplating event. Citation: Schutt, D. L., K. Dueker, and H. Yuan (2008), Crust and upper mantle velocity structure of the Yellowstone hot spot and surroundings, J. Geophys. Res., 113, B03310, doi:10.1029/2007JB005109.

Ubiquitous low‐velocity layer atop the 410‐km discontinuity in the northern Rocky Mountains

Receiver functions from three 30-station IRIS-PASSCAL small-aperture arrays (2–15 km station spacing) operated for 10 months each in the northern Rocky Mountains show a ubiquitous negative polarity P to S conversion just preceding the 410-km discontinuity arrival. Data from the three arrays were sorted into NW, SE, and SW back-azimuth quadrants and stacked to form nine quadrant stacks. Remarkably, the negative polarity arrival (NPA) is apparent in 8 of the 9 quadrant stacks, with 7 of the 8 having well-correlated waveforms. Each quadrant stack also contains clear P to S conversions from the 410- and 660-km discontinuities. Moveout analysis shows that all the major phases display the correct moveout for forward scattered P-S phases. The waveshapes for the seven similar NPA-410 km discontinuity arrivals are modeled with a five-parameter “double gradient slab” model that is parameterized as follows: a top gradient thickness and shear velocity decrease; a constant velocity layer; bottom gradient thickness; and shear velocity increase. Model misfit is assessed via a grid search over the model space using a reflectivity code to calculate synthetic seismograms. Model likelihood is determined by calculating 1- and 2-D marginal probability density functions (PDF) for the five parameters. The 1-D marginals display a range of peak values, although significant overlap is observed for the top gradient thickness and its associated velocity decrement. From the peak value of the summary PDF, we find the top velocity gradient to be sharp (<6.4 km) and the shear velocity decrement to be large (8.9% Vs). Defining an effective thickness of the low-velocity layer as the mean layer thickness plus half the mean gradient thicknesses, the 410 low-velocity layer thickness is found to be 22 km. A review of changes in the physical state required to match our new 410-LVL constraints suggests that the water-filter model remains an operative hypothesis to test.