Much progress has been made over the past several decades in delineating the structure of subducting slabs, but several key aspects of their dynamics remain poorly constrained. Major unsolved ...problems in subduction geodynamics include those related to mantle wedge viscosity and rheology, slab hydration and dehydration, mechanical coupling between slabs and the ambient mantle, the geometry of mantle flow above and beneath slabs, and the interactions between slabs and deep discontinuities such as the core‐mantle boundary. Observations of seismic anisotropy can provide relatively direct constraints on mantle dynamics because of the link between deformation and the resulting anisotropy: when mantle rocks are deformed, a preferred orientation of individual mineral crystals or materials such as partial melt often develops, resulting in the directional dependence of seismic wave speeds. Measurements of seismic anisotropy thus represent a powerful tool for probing mantle dynamics in subduction systems. Here I review the observational constraints on seismic anisotropy in subduction zones and discuss how seismic data can place constraints on wedge, slab, and sub‐slab anisotropy. I also discuss constraints from mineral physics investigations and geodynamical modeling studies and how they inform our interpretation of observations. I evaluate different models in light of constraints from seismology, geodynamics, and mineral physics. Finally, I discuss some of the major unsolved problems related to the dynamics of subduction systems and how ongoing and future work on the characterization and interpretation of seismic anisotropy can lead to progress, particularly in frontier areas such as understanding slab dynamics in the deep mantle.
Key PointsSeismic anisotropy in subduction systems reflects complex processes.Different models for anisotropy in subduction systems are evaluated.Major unsolved problems can be addressed with anisotropy observations.
Ultralow velocity zones (ULVZs) and seismic anisotropy are both commonly detected in the lowermost mantle at the edges of the two antipodal large low velocity provinces (LLVPs). The preferential ...occurrences of both ULVZs and anisotropy at LLVP edges are potentially connected to deep mantle dynamics; however, the two phenomena are typically investigated separately. Here we use waveforms from three deep earthquakes to jointly investigate ULVZ structure and lowermost mantle anisotropy near an edge of the Pacific LLVP to the southeast of Hawaii. We model global wave propagation through candidate lowermost mantle structures using AxiSEM3D. Two structures that cause ULVZ‐characteristic postcursors in our data are identified and are modeled as cylindrical ULVZs with radii of ∼1° and ∼3° and velocity reductions of ∼36% and ∼20%. One of these features has not been detected before. The ULVZs are located to the south of Hawaii and are part of the previously detected complex low velocity structure at the base of the mantle in our study region. The waveforms also reveal that, to first order, the base of the mantle in our study region is a broad and thin region of modestly low velocities. Measurements of Sdiff shear wave splitting reveal evidence for lowermost mantle anisotropy that is approximately co‐located with ULVZ material. Our measurements of co‐located anisotropy and ULVZ material suggest plausible geodynamic scenarios for flow in the deep mantle near the Pacific LLVP edge.
Plain Language Summary
Earthquakes cause different types of seismic waves that can be used to create an image of seismically fast and slow regions within Earth's interior. Two large‐scale features with relatively low seismic velocities have been identified at the base of the mantle, one beneath Africa and one beneath the Pacific Ocean, known as large low velocity provinces (LLVPs). Small‐scale, thin features with extremely low velocities, known as ultralow velocity zones (ULVZs), have previously been detected just above the core‐mantle boundary, often located at the edges of the LLVPs. In this study, we investigate a region of the deep mantle at the edge of the Pacific LLVP. We use recordings of earthquake waves that have sampled this region to map two distinct ULVZ regions at this boundary. We also investigate a property known as seismic anisotropy, the directional dependence of seismic wave speeds, which can be used to infer the direction of mantle flow. We outline several potential mantle flow scenarios that are consistent with our data, helping to understand flow processes at the edges of LLVP structures in the deep mantle.
Key Points
We identify and characterize a previously undetected ultralow velocity zone (ULVZ) beneath the central Pacific Ocean
We propose the existence of a thin and broad layer with low seismic velocities in our study region, just above the core‐mantle boundary
Measurements of potentially co‐located seismic anisotropy and ULVZ structure allow the inference of plausible dynamics in the deep mantle
Upper mantle anisotropy has been mapped beneath continents at high spatial resolution. Beneath the oceans, however, shear wave splitting constraints on upper mantle anisotropy are sparse, due to the ...paucity of seismic receivers. A technique that does not require the availability of seismic stations close to the region under study is differential PS‐SKS splitting. Here, we use global wavefield simulations to investigate circumstances under which PS‐SKS splitting can be applied, and then use this technique to measure upper mantle anisotropy beneath the Pacific Ocean basin. Our results demonstrate that upper mantle anisotropy in our study region mostly reflects shearing due to the Pacific plate. North of Fiji, we observe a rotation of fast polarization directions, away from the direction of absolute plate motion of the Pacific plate. This may reflect far‐field mantle flow effects associated with the subduction of the Australian plate beneath the Pacific.
Plain Language Summary
Earthquakes cause seismic waves whose speeds sometimes depend on their polarization and propagation direction. This material property, called seismic anisotropy, can be used to infer the direction of flow in Earth's upper mantle. Seismic anisotropy is straightforward to measure directly beneath a seismic station, but harder to study if station coverage is sparse. We use a technique that allows us to infer upper mantle seismic anisotropy beneath the Pacific Ocean in places without nearby seismic stations. Our measurements show that while seismic anisotropy varies laterally beneath the Pacific Ocean, in most cases it can be explained by the movement of the Pacific tectonic plate, leading to horizontal shearing of the underlying mantle. North of Fiji, we can observe the effects that the subduction of the Australian beneath the Pacific tectonic plate has on upper mantle flow.
Key Points
We test the robustness of differential PS‐SKS shear‐wave splitting measurements to characterize anisotropy near the PS bounce point
We use this technique infer seismic anisotropy beneath the Pacific Ocean
A majority of our measurements can be explained by plate motion induced shearing beneath the Pacific plate
Shear wave splitting of SK(K)S phases is often used to examine upper mantle anisotropy. In specific cases, however, splitting of these phases may reflect anisotropy in the lowermost mantle. Here we ...present SKS and SKKS splitting measurements for 233 event‐station pairs at 34 seismic stations that sample D″ beneath Africa. Of these, 36 pairs show significantly different splitting between the two phases, which likely reflects a contribution from lowermost mantle anisotropy. The vast majority of discrepant pairs sample the boundary of the African large low shear velocity province (LLSVP), which dominates the lower mantle structure beneath this region. In general, we observe little or no splitting of phases that have passed through the LLSVP itself and significant splitting for phases that have sampled the boundary of the LLSVP. We infer that the D″ region just outside the LLSVP boundary is strongly deformed, while its interior remains undeformed (or weakly deformed).
Key Points
There is strong anisotropy just outside the boundary of the African LLSVP
The interior of the African LLSVP is (nearly) isotropic
The LLSVP may represent a barrier to ambient mantle flow
Mantle dynamics and seismic anisotropy Long, Maureen D.; Becker, Thorsten W.
Earth and planetary science letters,
09/2010, Letnik:
297, Številka:
3
Journal Article
Recenzirano
Observations of seismic anisotropy yield some of the most direct constraints available on both past and present-day deformation in the Earth's mantle. Insight into the character of mantle flow can ...also be gained from the geodynamical modeling of mantle processes on both global and regional scales. We highlight recent progress toward understanding mantle flow from both observations and modeling and discuss outstanding problems and avenues for progress, particularly in the integration of seismological and geodynamical constraints to understand seismic anisotropy and the deformation that produces it. To first order, the predictions of upper mantle anisotropy made by global mantle circulation models match seismological observations well beneath the ocean basins, but the fit is poorer in regions of greater tectonic complexity, such as beneath continental interiors and within subduction systems. In many regions of the upper mantle, models of anisotropy derived from surface waves are seemingly inconsistent with shear wave splitting observations, which suggests that our understanding of complex anisotropic regions remains incomplete. Observations of anisotropy in the D" layer hold promise for improving our understanding of dynamic processes in the deep Earth but much progress remains to be made in characterizing anisotropic structure and relating it to the geometry of flow, geochemical heterogeneity, or phase transitions. Major outstanding problems related to understanding mantle anisotropy remain, particularly regarding the deformation and evolution of continents, the nature of the asthenosphere, subduction zone geodynamics, and the thermo-chemical state of the lowermost mantle. However, we expect that new seismological deployments and closer integration of observations with geodynamical models will yield rapid progress in these areas.
Although the morphologies of subducting slabs have been relatively well characterized, the character of the mantle flow field that accompanies subduction remains poorly understood. To analyze this ...pattern of flow, we compiled observations of seismic anisotropy, as manifested by shear wave splitting. Data from 13 subduction zones reveal systematic variations in both mantle-wedge and subslab anisotropy with the magnitude of trench migration velocity$|V_{\text{t}}|$. These variations can be explained by flow along the strike of the trench induced by trench motion. This flow dominates beneath the slab, where its magnitude scales with$|V_{\text{t}}|$. In the mantle wedge, this flow interacts with classical corner flow produced by the convergence velocity$V_{\text{c}}$; their relative influence is governed by the relative magnitude of$|V_{\text{t}}|$and$V_{\text{c}}$.
The group of Insect-specific viruses (ISVs) includes viruses apparently restricted to insects based on their inability to replicate in the vertebrates. Increasing numbers of ISVs have been discovered ...and characterized representing a diverse number of viral families. However, most studies have focused on those ISVs belonging to the family Flaviviridae, which highlights the importance of ISV study from other viral families, which allow a better understanding for the mechanisms of transmission and evolution used for this diverse group of viruses. Some ISVs have shown the potential to modulate arboviruses replication and vector competence of mosquitoes. Based on this, ISVs may be used as an alternative tool for biological control, development of vaccines, and diagnostic platforms for arboviruses. In this review, we provide an update of the general characteristics of ISVs and their interaction with arboviruses that infect vertebrates.
•ISVs includes viruses apparently restricted to insects based on their inability to replicate in the vertebrates.•Some ISVs are phylogenetically closer related to arboviruses, while others are more related to plant viruses.•Some ISVs are phylogenetically closer related to arboviruses, while others are more related to plant viruses.
SUMMARY
Seismic anisotropy has been detected at many depths of the Earth, including its upper layers, the lowermost mantle and the inner core. While upper mantle seismic anisotropy is relatively ...straightforward to resolve, lowermost mantle anisotropy has proven to be more complicated to measure. Due to their long, horizontal ray paths along the core–mantle boundary (CMB), S waves diffracted along the CMB (Sdiff) are potentially strongly influenced by lowermost mantle anisotropy. Sdiff waves can be recorded over a large epicentral distance range and thus sample the lowermost mantle everywhere around the globe. Sdiff therefore represents a promising phase for studying lowermost mantle anisotropy; however, previous studies have pointed out some difficulties with the interpretation of differential SHdiff–SVdiff traveltimes in terms of seismic anisotropy. Here, we provide a new, comprehensive assessment of the usability of Sdiff waves to infer lowermost mantle anisotropy. Using both axisymmetric and fully 3-D global wavefield simulations, we show that there are cases in which Sdiff can reliably detect and characterize deep mantle anisotropy when measuring traditional splitting parameters (as opposed to differential traveltimes). First, we analyze isotropic effects on Sdiff polarizations, including the influence of realistic velocity structure (such as 3-D velocity heterogeneity and ultra-low velocity zones), the character of the lowermost mantle velocity gradient, mantle attenuation structure, and Earth’s Coriolis force. Secondly, we evaluate effects of seismic anisotropy in both the upper and the lowermost mantle on SHdiff waves. In particular, we investigate how SHdiff waves are split by seismic anisotropy in the upper mantle near the source and how this anisotropic signature propagates to the receiver for a variety of lowermost mantle models. We demonstrate that, in particular and predictable cases, anisotropy leads to Sdiff splitting that can be clearly distinguished from other waveform effects. These results enable us to lay out a strategy for the analysis of Sdiff splitting due to anisotropy at the base of the mantle, which includes steps to help avoid potential pitfalls, with attention paid to the initial polarization of Sdiff and the influence of source-side anisotropy. We demonstrate our Sdiff splitting method using three earthquakes that occurred beneath the Celebes Sea, measured at many transportable array stations at a suitable epicentral distance. We resolve consistent and well-constrained Sdiff splitting parameters due to lowermost mantle anisotropy beneath the northeastern Pacific Ocean.
The lower mantle is dominated by two large structures with anomalously low shear wave velocities, known as Large Low‐Shear Velocity Provinces (LLSVPs). Several studies have documented evidence for ...strong seismic anisotropy at the base of the mantle near the edges of the African LLSVP. Recent work has identified a smaller structure with similar low‐shear wave velocities beneath Eurasia, dubbed the Perm Anomaly. Here we probe lowermost mantle anisotropy near the Perm Anomaly using the differential splitting of SKS and SKKS phases measured at stations in Europe. We find evidence for lowermost mantle anisotropy in the vicinity of the Perm Anomaly, with geographic trends hinting at lateral variations in anisotropy across the boundaries of the Perm Anomaly as well as across a previously unsampled portion of the African LLSVP border. Our observations suggest that deformation is concentrated at the boundaries of both the Perm Anomaly and the African LLSVP.
Key Points
We identify seismic anisotropy in the lowermost mantle near the Perm Anomaly
Anisotropy of the Perm Anomaly is similar to that of the African LLSVP
Both structures may interact with mantle flow in a similar way
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