Weisen Shen

A synoptic view of the distribution and connectivity of the mid-crustal low velocity zone beneath Tibet

[1] Based on 1–2 years of continuous observations of seismic ambient noise data obtained at more than 600 stations in and around Tibet, Rayleigh wave phase velocity maps are constructed from 10 s to 60 s period. A 3-D Vsv model of the crust and uppermost mantle is derived from these maps. The 3-D model exhibits significant apparently inter-connected low shear velocity features across most of the Tibetan middle crust at depths between 20 and 40 km. These low velocity zones (LVZs) do not conform to surface faults and, significantly, are most prominent near the periphery of Tibet. The observations support the internal deformation model in which strain is dispersed in the deeper crust into broad ductile shear zones, rather than being localized horizontally near the edges of rigid blocks. The prominent LVZs are coincident with strong mid-crustal radial anisotropy in western and central Tibet and probably result at least partially from anisotropic minerals aligned by deformation, which mitigates the need to invoke partial melt to explain the observations. Irrespective of their cause in partial melt or mineral alignment, mid-crustal LVZs reflect deformation and their amplification near the periphery of Tibet provides new information about the mode of deformation across Tibet.

Crustal and uppermost mantle structure beneath the United States

This paper presents a new model of the shear velocity structure of the crust and uppermost mantle beneath the contiguous U.S. The model is based on more than a decade of USArray Transportable Array (TA) data across the U.S. and derives from a joint Bayesian Monte Carlo inversion of Rayleigh wave group and phase speeds determined from ambient noise and earthquakes, receiver functions, and Rayleigh wave ellipticity (H/V) measurements. Within the Bayesian inverse theoretic framework, a prior distribution of models is posited and a posterior distribution is inferred beneath all of the more than 1800 TA stations across the U.S. The resulting mean and standard deviation of the mean of the posterior distribution at each station summarize the inversion results, which are then interpolated onto a regular 0.25°×0.25° grid across the U.S. to define the final 3-D model. We present arguments that show that the standard deviation of the posterior distribution overestimates the effect of nonsystematic errors in the final model by a factor of 4–5 and identify uncertainties in density and mantle Q as primary potential sources of remaining systematic error in the final model. The model presents a great many newly resolved structural features across the U.S. that require further analysis and dedicated explication. We highlight here low-velocity anomalies in the upper mantle that underlie the Appalachians with centers of anomalies in northern Georgia, western Virginia, and, most prominently, New England.

Origins of topography in the western U.S.: Mapping crustal and upper mantle density variations using a uniform seismic velocity model

To investigate the physical basis for support of topography in the western U.S., we construct a subcontinent scale, 3-D density model using ~1000 estimated crustal thicknesses and S velocity profiles to 150 km depth at each of 947 seismic stations. Crustal temperature and composition are considered, but we assume that mantle velocity variations are thermal in origin. From these densities, we calculate crustal and mantle topographic contributions. Typical 2σ uncertainty of topography is ~500 m, and elevations in 84% of the region are reproduced within error. Remaining deviations from observed elevations are attributed to melt, variations in crustal quartz content, and dynamic topography; compositional variations in the mantle, while plausible, are not necessary to reproduce topography. Support for western U.S. topography is heterogeneous, with each province having a unique combination of mechanisms. Topography due to mantle buoyancy is nearly constant (within ~250 m) across the Cordillera; relief there (>2 km) results from variations in crustal chemistry and thickness. Cold mantle provides ~1.5 km of ballast to the thick crust of the Great Plains and Wyoming craton. Crustal temperature variations and dynamic pressures have smaller magnitude and/or more localized impacts. Positive gravitational potential energy (GPE) anomalies (~2 × 1012N/m) calculated from our model promote extension in the northern Basin and Range and near the Sierra Nevada. Negative GPE anomalies (−3 × 1012N/m) along the western North American margin and Yakima fold and thrust belt add compressive stresses. Stresses derived from lithospheric density variations may strongly modulate tectonic stresses in the western U.S. continental interior.

A one-dimensional seismic model for Uturuncu volcano, Bolivia, and its impact on full moment tensor inversions

Using receiver functions, Rayleigh wave phase velocity dispersion determined from ambient noise and teleseismic earthquakes, and Rayleigh wave horizontal to vertical ground motion amplitude ratios from earthquakes observed across the PLUTONS seismic array, we construct a one-dimensional (1‑D) S-wave velocity (Vs) seismic model with uncertainties for Uturuncu volcano, Bolivia, located in the central Andes and overlying the eastward-subducting Nazca plate. We find a fast upper crustal lid placed upon a low-velocity zone (LVZ) in the mid-crust. By incorporating all three types of measurements with complimentary sensitivity, we also explore the average density and Vp/Vs (ratio of P-wave to S-wave velocity) structures beneath the young silicic volcanic field. We observe slightly higher Vp/Vs and a decrease in density near the LVZ, which implies a dacitic source of the partially molten magma body. We exploit the impact of the 1-D model on full moment tensor inversion for the two largest local earthquakes recorded (both magnitude ∼3), demonstrating that the 1-D model influences the waveform fits and the estimated source type for the full moment tensor. Our 1-D model can serve as a robust starting point for future efforts to determine a three-dimensional velocity model for Uturuncu volcano.

Seismic evidence for lithospheric foundering beneath the southern Transantarctic Mountains, Antarctica

The 3000-km-long Transantarctic Mountains (TAMs), which separate cratonic East Antarctica from tectonically active West Antarctica, remain one of the least understood of Earth’s major mountain ranges. The tectonic mechanism that generates the high elevation, as well as the processes that produce major differences between various sectors of the TAMs, are still uncertain. Here we present newly constructed seismic images of the crust and uppermost mantle beneath central Antarctica derived from recently acquired seismic data, indicating ongoing lithospheric foundering beneath the southern TAMs. These images reveal an absence of thick, cold cratonic lithosphere beneath the southern TAMs. Instead, an uppermost-mantle slow seismic anomaly extends across the mountain front and 350 km into East Antarctica, beneath a high plateau near the South Pole. Under the slow anomaly, a relatively high-wavespeed root is found at ~200 km depth, connected with the East Antarctic lithosphere, suggesting that sinking lithosphere has been replaced at shallow depths by warm, slow-velocity asthenosphere. A mantle lithosphere foundering model is proposed to interpret these images, which best explains the present large area of high elevation and the uplift of the TAMs, as well as Miocene-age volcanism in the Mount Early region.

Upper mantle structure of the Tonga-Lau-Fiji region from Rayleigh wave tomography

We investigate the upper mantle seismic structure in the Tonga-Lau-Fiji region by jointly fitting the phase velocities of Rayleigh waves from ambient-noise and two-plane-wave tomography. The results suggest a wide low-velocity zone beneath the Lau Basin, with a minimum SV-velocity of about 3.7 ± 0.1 km/s, indicating upwelling hot asthenosphere with extensive partial melting. The variations of velocity anomalies along the Central and Eastern Lau Spreading Centers suggest varying mantle porosity filled with melt. In the north where the spreading centers are distant from the Tonga slab, the inferred melting commences at about 70 km depth, and forms an inclined zone in the mantle, dipping to the west away from the arc. This pattern suggests a passive decompression melting process supplied by the Australian plate mantle from the west. In the south, as the supply from the Australian mantle is impeded by the Lau Ridge lithosphere, flux melting controlled by water from the nearby slab dominates in the back-arc. This source change results in the rapid transition in geochemistry and axial morphology along the spreading centers. The remnant Lau Ridge and the Fiji Plateau are characterized by 60–80 km thick lithosphere underlain by a low-velocity asthenosphere. Our results suggest the removal of the lithosphere of the northeastern Fiji Plateau-Lau Ridge beneath the active Taveuni Volcano. Azimuthal anisotropy shows that the mantle flow direction rotates from trench-perpendicular beneath Fiji to spreading-perpendicular beneath the Lau Basin, which provides evidence for the southward flow of the mantle wedge and the Samoan plume. This article is protected by copyright. All rights reserved.

The distribution and composition of high‐velocity lower crust across the continental U.S.: Comparison of seismic and xenolith data and implications for lithospheric dynamics and history

The composition of the continental lower crust is not well known. High seismic wavespeeds may indicate mafic or garnet-bearing material, with implications for emplacement history, evolution, and rheological and dynamic behavior. In this contribution, we use recent seismic results from the EarthScope Transportable Array, compilations of active source studies, and selected xenolith studies to attempt to map the distribution of high-velocity lower crust across the continental U.S. and assess its relationship to proposed emplacement and destruction-related mechanisms such as under-and intraplating, collision, extension, heating, cooling, hydration, and delamination. Thin layers of high-velocity lower crust related to regional processes are found scattered throughout the continent. Thicker layers in large areas are found in the central and eastern U.S. in areas with thick crust, bounded roughly by the Rocky Mountain Front. Emplacement processes likely originally spanned this boundary, and the difference between the two domains may reflect garnet growth with cooling and aging of continental crust in much of the central and eastern U.S., while crustal thickness and lithospheric temperatures in the western U.S. are unfavorable for growth and maintenance of thick layers of high-velocity garnet-bearing lower crust.