Karen M. Fischer

Shear wave splitting, continental keels, and patterns of mantle flow

In this study we investigated the origin of seismic anisotropy in the mantle beneath North America. In particular, we evaluated whether shear wave splitting patterns in eastern North America are better explained by anisotropy caused by lithospheric deformation, anisotropy due to mantle flow beneath the lithosphere, or a combination of both. We examined new measurements of shear wave splitting from the Missouri to Massachusetts broadband seismometer array (MOMA), the North American Mantle Anisotropy and Discontinuity experiment (NOMAD), as well as splitting parameters from several previous studies. We developed a simple finite difference model that approximates mantle flow around a complex, three-dimensional continental lithospheric keel. To evaluate potential anisotropy from mantle flow beneath the lithosphere in eastern North America, we compared shear wave splitting observations to predicted splitting parameters calculated using this mantle flow model. Our results indicate that a significant portion of observed shear wave splitting in eastern North America can be explained by mantle flow around the continental keel. However, shear wave splitting patterns in a few regions of eastern North America indicate that a component of lithospheric anisotropy must exist, particularly in regions containing the largest keel thicknesses. For eastern North America, as well as for splitting observations in Australia, Europe, and South America, we favor a model in which anisotropy is controlled by a combination of both lithospheric deformation and subcontinental mantle flow.

Mantle anisotropy beneath northwest Pacific subduction zones

To assess the location, strength, and orientation of seismic anisotropy in the southern Kuril, Japan, and Izu-Bonin subduction zones, we analyzed shear wave splitting in local S phases and teleseismic SKS phases. Fast directions from these phases are roughly parallel to the direction of absolute Pacific plate motion (∼WNW) in Izu-Bonin, roughly parallel to the strike of the trench (∼NNE) in central Honshu, and roughly N in the southern Kuril back arc in the vicinity of Sakhalin Island. Assuming that the orientation and strength of anisotropy does not vary with depth, modeling of splitting times from local S and teleseismic phases recorded at Sakhalin Island requires that the maximum depth of anisotropy lies between 480 km and 950 km. In contrast, splitting times for local S phases from events in Izu-Bonin rule out anisotropy in the transition zone. All the data are consistent with a model in which the lower transition zone (520–660 km) and lower mantle are largely isotropic and in which anisotropy occurs intermittently in the upper transition zone (410–520 km), possibly due to preferred orientation of β spinel. Assuming that splitting in the upper mantle is produced by preferred orientation of olivine, the observed fast directions indicate that the geometry of back arc strain varies systematically between subduction zones. The relationship of fast directions and plate motions suggests that the subducting slab exerts significant control on back arc flow, but that flow correlated with upper plate deformation must also exist.

Arc-parallel flow in the mantle wedge beneath Costa Rica and Nicaragua

Resolving flow geometry in the mantle wedge is central to understanding the thermal and chemical structure of subduction zones, subducting plate dehydration, and melting that leads to arc volcanism, which can threaten large populations and alter climate through gas and particle emission. Here we show that isotope geochemistry and seismic velocity anisotropy provide strong evidence for trench-parallel flow in the mantle wedge beneath Costa Rica and Nicaragua. This finding contradicts classical models, which predict trench-normal flow owing to the overlying wedge mantle being dragged downwards by the subducting plate. The isotopic signature of central Costa Rican volcanic rocks is not consistent with its derivation from the mantle wedge or eroded fore-arc complexes but instead from seamounts of the Galapagos hotspot track on the subducting Cocos plate. This isotopic signature decreases continuously from central Costa Rica to northwestern Nicaragua. As the age of the isotopic signature beneath Costa Rica can be constrained and its transport distance is known, minimum northwestward flow rates can be estimated (63–190 mm yr-1) and are comparable to the magnitude of subducting Cocos plate motion (∼85 mm yr-1). Trench-parallel flow needs to be taken into account in models evaluating thermal and chemical structure and melt generation in subduction zones.

The influence of plate motions on three-dimensional back arc mantle flow and shear wave splitting

Both the polarization direction of the fast shear waves and the types of deformation within overriding plates vary between the back arc basins of western Pacific subduction zones. The goal of this study is to test the possibility that motions of the overriding plates may control the patterns of seismic anisotropy and therefore the observed shear wave splitting. We calculated three-dimensional models of viscous asthenospheric flow driven by the motions of the subducting slab and overriding plates. Shear wave splitting was calculated for polymineralic anisotropy within the back arc mantle wedge assuming that the anisotropy was created by flow-induced strain. Predicted splitting may differ substantially depending on whether anisotropy is computed directly using polycrystalline plasticity models or is based on the orientation of finite strain. A parameter study shows that both finite strain and textural anisotropy developed within three-dimensional, plate-coupled asthenospheric flow models are very heterogeneous when back arc shearing occurs within the overriding plate. Therefore predicted shear wave splitting varies strongly as a function of plate motion boundary conditions and with ray path through the back arc asthenosphere. Flow models incorporating plate motion boundary conditions for the Tonga, southern Kuril, and eastern Aleutian subduction zones produce splitting parameters consistent with observations from each region. Trench-parallel flow generated by small variations in the dip of the subducting plate may be important in explaining observed fast directions of anisotropy sampled within the innermost corner of the mantle wedge.

A sharp lithosphere–asthenosphere boundary imaged beneath eastern North America

Plate tectonic theory hinges on the concept of a relatively rigid lithosphere moving over a weaker asthenosphere, yet the nature of the lithosphere–asthenosphere boundary remains poorly understood. The gradient in seismic velocity that occurs at this boundary is central to constraining the physical and chemical properties that create differences in mechanical strength between the two layers. For example, if the lithosphere is simply a thermal boundary layer that is more rigid owing to colder temperatures, mantle flow models indicate that the velocity gradient at its base would occur over tens of kilometres. In contrast, if the asthenosphere is weak owing to volatile enrichment or the presence of partial melt, the lithosphere–asthenosphere boundary could occur over a much smaller depth range. Here we use converted seismic phases in eastern North America to image a very sharp seismic velocity gradient at the base of the lithosphere—a 3–11 per cent drop in shear-wave velocity over a depth range of 11 km or less at 90–110 km depth. Such a strong, sharp boundary cannot be reconciled with a purely thermal gradient, but could be explained by an asthenosphere that contains a few per cent partial melt or that is enriched in volatiles relative to the lithosphere.

P‐to‐S and S‐to‐P imaging of a sharp lithosphere‐asthenosphere boundary beneath eastern North America

[1] S-to-P (Sp) scattered energy independently confirms the existence of a seismic velocity discontinuity at the lithosphere-asthenosphere boundary that was previously imaged using P-to-S (Ps) scattered energy in eastern North America. Exploration of the different sensitivities of Ps and Sp scattered energy suggests that the phases contain independent yet complementary high-resolution information regarding velocity contrasts. Combined inversions of Ps and Sp energy have the potential to tightly constrain associated velocity gradients. In eastern North America, inversions of Sp and Ps data require a strong, 5–10% velocity contrast that is also sharp, occurring over less than 11 km at 87–105 km depth. Thermal gradients alone are insufficient to create such a sharp boundary, and therefore another mechanism is required. A boundary in composition, hydration, or a change in anisotropic signature could easily produce a sufficiently localized velocity gradient. Taken separately, the magnitudes of the effects of these mechanisms are too small to match our observed velocity gradients. However, our observations may be explained by a boundary in hydration coupled with a boundary in depletion and/or anisotropy. Alternatively, a small amount of melt in the asthenosphere could explain the velocity gradient. The tight constraints on velocity gradients achieved by combined modeling of Ps and Sp energy offer promise for defining the character of the lithosphere-asthenosphere boundary globally.

The depth distribution of mantle anisotropy beneath the Tonga subduction zone

Abstract Shear-wave splitting recorded by a temporary deployment of broadband seismometers located above the Tonga subduction zone yields strong constraints on the depth distribution of anisotropy in the mantle beneath the Tonga back-arc region. Splitting parameters obtained for local S and teleseismic SKS phases were modeled using a ray-based method that predicts shear-wave splitting on individual phase paths and determines the best-fitting values of anisotropic strength, orientation and depth extent. Splitting in teleseismic SKS phases is identical to that in S phases from local earthquakes that occur at the base of the transition zone, demonstrating that there is no significant splitting due to anisotropy in the lower mantle. Splitting times for local S phases do not vary significantly with depth, and for models in which the strength of anisotropy is laterally uniform, they rule out anisotropy in the transition zone. Finally, the observed splitting indicates that anisotropy of more than 0.8% exists in the upper mantle, with a fast symmetry axis at 60°W, roughly parallel to the absolute plate motion vector of the subducting Pacific lithosphere.

The global seismographic network surpasses its design goal

This year, the Global Seismographic Network (GSN) surpassed its 128-station design goal for uniform worldwide coverage of the Earth. A total of 136 GSN stations are now sited from the South Pole to Siberia, and from the Amazon Basin to the sea floor of the northeast Pacific Ocean—in cooperation with over 100 host organizations and seismic networks in 59 countries worldwide (Figure 1). Established in 1986 by the Incorporated Research Institutions for Seismology (IRIS) to replace the obsolete, analog Worldwide Standardized Seismograph Network (WWSSN),the GSN continues a tradition in global seismology that dates back more than a century to the network of Milne seismographs that initially spanned the globe. The GSN is a permanent network of state-of-the-art seismological and geophysical sensors connected by available telecommunications to serve as a multi-use scientific facility and societal resource for scientific research, environmental monitoring, and education for our national and international community.

Strong along-arc variations in attenuation in the mantle wedge beneath Costa Rica and Nicaragua

Attenuation structure in the Central American subduction zone was imaged using local events recorded by the Tomography Under Costa Rica and Nicaragua array, a 20-month-long deployment (July 2004 until March 2006) of 48 seismometers that spanned the fore-arc, arc, and back-arc regions of Nicaragua and Costa Rica. P and S waveforms were inverted separately for the corner frequency and moment of each event and for the path-averaged attenuation operator (t*) of each event-station pair, assuming attenuation is slightly frequency-dependent ( = 0.27). Then, tomographic inversions were performed for S and P attenuation (Q S ?1 and Q P ?1). Since P wave amplitudes reflect both shear and the bulk moduli, tomographic inversions were also performed to determine shear and bulk attenuation (Q S ?1 and Q ?1), the loss of energy per cycle owing to shearing and uniform compression, respectively. Damping and other inversion tomographic parameters were systematically varied. As is typical in subduction zone attenuation studies, a less attenuating slab, upper plate, and wedge corner and a more attenuating mantle wedge were imaged. In addition, first-order differences between the mantles beneath Nicaragua and Costa Rica were observed. The slab in Nicaragua is more attenuating than the slab in Costa Rica. A larger zone of higher shear attenuation also characterizes the Nicaraguan mantle wedge. Within the wedge, maximum attenuation values at 1 Hz correspond to Qs = 38–73 beneath Nicaragua and Qs = 62–84 beneath Costa Rica, and average values are Qs = 76–78 and Qs = 84–88, respectively. Attenuation variations correlate with along-arc trends in geochemical indicators that suggest that melting beneath Nicaragua occurs at more hydrated conditions, and possibly to greater extents and depths, relative to northern Costa Rica. Shear attenuation dominates over bulk attenuation in the well-resolved regions of the wedge. The more extensive zones of greater shear attenuation observed in the Nicaraguan wedge could be explained by higher temperatures and/or greater hydration, but comparison with petrological data suggests that hydration variations play a larger role. Average wedge attenuation values are comparable to estimates for the Andes and Japan, greater than those for Alaska, and less than those for Tonga-Lau.

Waning buoyancy in the crustal roots of old mountains

When mountains form through the collision of lithospheric plates, uplift of the Earths surface is accompanied by thickening of the crust, and the buoyancy of these deep crustal roots (relative to the surrounding mantle) is thought to contribute to the support of mountain topography. Once active tectonism ceases, continuing erosion will progressively wear away surface relief. Here I provide new constraints on how crustal roots respond to erosional unloading over very long timescales. In old collisional mountain belts, ratios of surface relief to the thickness of the underlying crustal root are observed to be smaller than in young mountains. On the basis of gravity data, this trend is best explained by a decrease in the buoyancy of the crustal root with greater age since the most recent mountain-building episode—which is consistent with metamorphic reactions produced by long-term cooling. An approximate balance between mountain and root mass anomalies suggests that the continental lithosphere remains weak enough to permit exhumation of crustal roots in response to surface erosion for hundreds of millions of years. The amount of such uplift, however, appears to be significantly reduced by progressive loss of root buoyancy.