Huaiyu Yuan

Lithospheric layering in the North American craton

How cratons—extremely stable continental areas of the Earth’s crust—formed and remained largely unchanged for more than 2,500 million years is much debated. Recent studies of seismic-wave receiver function data have detected a structural boundary under continental cratons at depths too shallow to be consistent with the lithosphere–asthenosphere boundary, as inferred from seismic tomography and other geophysical studies. Here we show that changes in the direction of azimuthal anisotropy with depth reveal the presence of two distinct lithospheric layers throughout the stable part of the North American continent. The top layer is thick (∼150 km) under the Archaean core and tapers out on the surrounding Palaeozoic borders. Its thickness variations follow those of a highly depleted layer inferred from thermo-barometric analysis of xenoliths. The lithosphere–asthenosphere boundary is relatively flat (ranging from 180 to 240 km in depth), in agreement with the presence of a thermal conductive root that subsequently formed around the depleted chemical layer. Our findings tie together seismological, geochemical and geodynamical studies of the cratonic lithosphere in North America. They also suggest that the horizon detected in receiver function studies probably corresponds to the sharp mid-lithospheric boundary rather than to the more gradual lithosphere–asthenosphere boundary.

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.

Stratified seismic anisotropy and the lithosphere‐asthenosphere boundary beneath eastern North America

Long records of teleseismic observations accumulated at permanent seismic stations Harvard, MA; Palisades, NY; and Standing Stone, PA, in eastern North America are inverted for vertical distribution of anisotropic parameters. High-resolution anisotropy-aware P wave receiver function analysis and multiple-layer core-refracted SKS waveform modeling favor more than one layer of anisotropy beneath all sites. Our analyses suggest that the depth sensitivity to stratified anisotropic seismic velocity in converted phases and the SKS waveforms are complementary and confirm that these two approaches yield consistent lithospheric anisotropic fast axis directions. We illustrate the feasibility of the lithosphere-asthenosphere boundary detection on a regional scale through anisotropy-aware receiver functions. Joint interpretation of receiver functions and SKS waveforms beneath eastern North America suggests a thin (~100 km) anisotropic lithosphere with fast axis orientation nearly orthogonal to the strike of major tectonic units and an underlying anisotropic asthenosphere with fast axis directions that favor the HS3-NUVEL 1A plate motion model. Consistent lithospheric anisotropy inferred from both techniques suggests broad presence of coherent fabric in the lower lithosphere, possibly developed in a regional scale delamination event after the assembly of Appalachians.

Upper mantle tomographic Vp and Vs images of the rocky mountains in wyoming, colorado and new Mexico : Evidence for a thick heterogeneous chemical lithosphere

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 slabs 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.

A Probabilistic Shear Wave Velocity Model of the Crust in the Central West Australian Craton Constrained by Transdimensional Inversion of Ambient Noise Dispersion

The Capricorn Orogen in central Western Australia played important roles in initializing and finalizing the West Australian craton. Surface geological mapping and isotopic studies show that the crust has recorded over a billion years of tectonic history spanning from its crustal formation in the Archean to episodes of tectonothermal events during the Proterozoic cratonization processes. The region therefore provides us with an ideal laboratory to characterize the seismic signature associated with tectonic processes. We constructed a crustal shear wave velocity model of the core region of the orogen, the Glenburgh Terrane and its north boundary, by inverting the array group velocity dispersion data measured from a high-density temporary array. A modified Bayesian transdimensional tomography technique, which incorporates a smooth-varying regional reference velocity model and Moho topography, was used to invert for the crustal velocity variations. The inverted velocity model adds great detail to the intracrustal structure and provides complementary seismic velocity information to refine the regional tectonic processes. Distinct patterns in the velocity structure support that the Glenburgh Terrane is an Archean microcontinent and favor the role of Paleoproterozoic subductions/accretions during the assembly of the West Australian Craton.