Daoyuan Sun

Mushy magma beneath Yellowstone

A recent prospective on the Yellowstone Caldera discounts its explosive potential based on inferences from tomographic studies which suggests a high degree of crystallization of the underlying magma body. In this study, we show that many of the first teleseismic P-wave arrivals observed at seismic stations on the edge of the caldera did not travel through the magma body but have taken longer but faster paths around the edge. After applying a number of waveform modeling tools, we obtain much lower seismic velocities than previous studies, 2.3 km/sec (V_p) and 1.1 km/sec (V_s). We estimate the physical state of the magma body by assuming a fluid-saturated porous material consisting of granite and a mixture of rhyolite melt and water and CO_2 at a temperature of 800°C and pressure at 5 km (0.1 GPa). We found that this relatively shallow magma body has a volume of over 4,300 km^3 and is about 32% melt saturated with about 8% water plus CO_2 by volume.

Direct measures of lateral velocity variation in the deep Earth

Current tomographic models of the Earth display perturbations to a radial stratified reference model. However, structures in the deep mantle that are chemically dense with low Rayleigh numbers can develop enormous relief, perhaps with boundaries closer to vertical than to radial. Such features are hard to detect with present tomographic modeling techniques because the timing anomalies are based on long-period filtered waveforms with complexity removed. Here we develop a new tool for processing array data on the basis of a decomposition referred to as a multipath detector, which can be used to distinguish between horizontal structure (in-plane multipathing) and vertical (out-of-plane multipathing) directly from processing array waveforms. A lateral gradient coefficient based on this detector provides a direct constraint on the sharpness of the boundaries and material properties. We demonstrate the usefulness of this approach by processing samples of both P and S data from the Kaapvaal array in southern Africa, which are compared with synthetic predictions from a metastable dynamic model containing sharp edges. Both data and simulations produce timing gradients larger than 2 s/deg in azimuthal changes for S waves, where only minor effects are obtained for P waves. These results further validate the case for distinct chemistry inside the African Low Shear Velocity Province.

Seismological support for the metastable superplume model, sharp features, and phase changes within the lower mantle

Recently, a metastable thermal-chemical convection model was proposed to explain the African Superplume. Its bulk tabular shape remains relatively stable while its interior undergoes significant stirring with low-velocity conduits along its edges and down-welling near the middle. Here, we perform a mapping of chemistry and temperature into P and S velocity variations and replace a seismically derived structure with this hybrid model. Synthetic seismogram sections generated for this 2D model are then compared directly with corresponding seismic observations of P (P, PCP, and PKP) and S (S, SCS, and SKS) phases. These results explain the anticorrelation between the bulk velocity and shear velocity and the sharpness and level of SKS travel time delays. In addition, we present evidence for the existence of a D” triplication (a putative phase change) beneath the down-welling structure.

A narrow, mid-mantle plume below southern Africa

New waveform tomographic evidence displays a narrow plume-like feature emitting from the top of the large African low-velocity structure in the lower mantle. A detailed SKS wavefield is assembled for a segment along the structures southern edge by combining multiple events recorded by a seismic array in the Kaapvaal region of southern Africa. With a new processing technique that emphases multi-pathing, we locate a relatively jagged, sloping wall 1000 km high with low velocities near its basal edge. Forward modeling indicates that the plumes diameter is less than 150 km and consistent with an iso-chemical, low-viscosity plume conduit.

Hot upwelling conduit beneath the Atlas Mountains, Morocco

The Atlas Mountains of Morocco display high topography, no deep crustal root, and regions of localized Cenozoic alkaline volcanism. Previous seismic imaging and geophysical studies have implied a hot mantle upwelling as the source of the volcanism and high elevation. However, the existence, shape, and physical properties of an associated mantle anomaly are debated. Here we use seismic waveform analysis from a broadband deployment and geodynamic modeling to define the physical properties and morphology of the anomaly. The imaged low-velocity structure extends to ~200 km beneath the Atlas and appears ~350 K hotter than the ambient mantle with possible partial melting. It includes a lateral conduit, which suggests that the Quaternary volcanism arises from the upper mantle. Moreover, the shape and temperature of the imaged anomaly indicate that the unusually high topography of the Atlas Mountains is due to active mantle support.

Predicting a Global Perovskite and Post-Perovskite Phase Boundary

A lower mantle S-wave triplication with a S cd branch occurring between S and ScS has been recognized for many years with a variety of explanations. It is particularly strong when sampling regions beneath the circum-Pacific lower mantle fast velocity belt seen in global tomographic models where it has been modeled with a 2-3% jump in S-velocity. General properties are (1) it tends to arrive earlier beneath the fastest anomalies and (2) its rapid changes in strength and timing relative to S. The recently discovered Perovskite (Pv) to Post-perovskite (PPv) phase transition [Murakami et al., 2004] is expected to show this change in timing assuming a positive Clapeyron slope (γ) between 3 to 9 MPa/K; however, the predicted velocity jump is about half of the above ID modeling results. Here we model the phase boundary height by mapping S-wave tomography into temperature assuming uniform chemistry, or a Mono-Phase-Transition (MPT), and a more complex mapping procedure involving possible changes in Chemistry (CPT). A few adjustable parameters involving reference phase boundary height and velocity jump are determined from comparing synthetic seismogram predictions with densely sampled observations. Particularly strong S cd data are explained by focusing effects caused by small zones of enhanced velocities (slab buckling) seen in some tomographic models. These sharp features in S-wave tomography are confirmed by the behavior of differential PKP (AB-DF) which shows 4 s changes over a distance of 300 km correlating well with the corresponding S-wave models beneath Central America. Thus adding 3D propagational effects caused by these structures to the Pv to PPv velocity jump predicted from mineral physics appears to generate results compatible with S cd waveform observations.