Fast Splitting Directions Research Articles

Local-S shear wave splitting along the length of the Alaska–Aleutian subduction zone

SUMMARY The Alaska–Aleutian subduction zone represents an ideal location to study dynamics within a mantle wedge. The subduction system spans several thousand kilometres, is characterized by a slab edge, and has ample seismicity. Additionally, the majority of islands along the arc house broad-band seismic instruments. We examine shear wave splitting of local-S phases originating along the length of the subduction zone. We have dense measurement spacing in two regions, the central Aleutians and beneath Alaska. Beneath Alaska, we observe a rotation in fast splitting directions near the edge of the subducting slab. Fast directions change from roughly trench perpendicular away from the slab edge to trench parallel near the boundary. This is indicative of toroidal flow around the edge of the subducting Alaska slab. In the central Aleutians, local-S splitting is primarily oriented parallel to, or oblique to, the strike of the trench. The local-S measurements, however, exhibit a depth dependence where deeper events show more consistently trench-parallel directions indicating prevalent trench-parallel mantle flow. Our local-S shear wave splitting results suggest trench-parallel orientation are likely present along much of the subduction zone excited by the slab edge, but that additional complexities exist along strike.

Seismic anisotropy indicates organized melt beneath the Mid-Atlantic Ridge aids seafloor spreading

Abstract Lithospheric plates diverge at mid-ocean ridges and asthenospheric mantle material rises in response. The rising material decompresses, which can result in partial melting, potentially impacting the driving forces of the system. Yet the geometry and spatial distribution of the melt as it migrates to the ridge axis are debated. Organized melt fabrics can cause strong seismic anisotropy, which can be diagnostic of melt, although this is typically not found at ridges. We present anisotropic constraints from an array of 39 ocean-bottom seismometers deployed on 0–80 Ma lithosphere from March 2016 to March 2017 near the equatorial Mid-Atlantic Ridge (MAR). Local and SKS measurements show anisotropic fast directions away from the ridge axis, which are consistent with strain and associated fabric caused by plate motions with short delay times, δt (<1.1 s). Near the ridge axis, we find several ridge-parallel fast splitting directions, φ, with SKS δt that are much longer (1.7–3.8 s). This is best explained by ridge-parallel sub-vertical orientations of sheet-like melt pockets. This observation is much different than anisotropic patterns observed at other ridges, which typically reflect fabric related to plate motions. One possibility is that thicker sub-ridge lithosphere with steep sub-ridge topography beneath slower spreading centers focuses melt into vertical, ridge-parallel melt bands, which effectively weakens the plate. Associated buoyancy forces elevate the sub-ridge plate, providing greater potential energy and enhancing the driving forces of the plates.

Variable seismic anisotropy across the Peruvian flat-slab subduction zone with implications for upper plate deformation

We performed shear wave splitting analyses to investigate seismic anisotropy across the northern extent of the Peruvian flat-slab subduction region. We used core-mantle refracted SKS, SKKS and PKS phases from teleseismic events (88° > Δ < 150°) recorded at 45 broadband seismic stations from the Peruvian permanent and portable seismic networks as well as from, international networks (CTBTO, RSBR-Brazil, and RENSIG-Ecuador). The results reveal a complex anisotropy pattern with distinct variations in shear wave splitting along strike. In the northernmost region, the mean delay times range between 1.0 ± 0.2 s and 1.5 ± 0.2 s with fast directions predominantly oriented ENE-WSW approximately perpendicular to the trench, parallel with the motion of the subducting Nazca plate. In the central region of Peru, the predominant fast directions change to a SE-NW orientation that is oblique with the trench. These fast splitting directions are consistent with the pattern seen previously over the southern extent of the flat-slab and correlate well with the current geodetically derived motion of the overriding forearc, the Peruvian Sliver. These characteristics suggest a fundamental change in anisotropic behavior between the northern and central portions of the Peruvian flat-slab and imply that the upper plate deformation is a controlling factor.

SKS Splitting and Upper Mantle Anisotropy Beneath the Southern New England Appalachians: Constraints From the Dense SEISConn Array

AbstractThe geology of southern New England reflects subduction and terrane accretion during the Appalachian Orogeny and rifting during the breakup of Pangea. The presence of a low‐velocity seismic anomaly in the upper mantle beneath New England suggests the possible presence of vertical mantle flow (upwelling). It remains poorly understood how the lithosphere beneath southern New England was deformed by past tectonic events; furthermore, the details of the present‐day mantle flow field remain elusive. Observations of seismic anisotropy have the potential to constrain both past and present upper mantle deformation. Here, we present SK(K)S splitting observations at stations of the Seismic Experiment for Imaging Structure Beneath Connecticut array, a deployment of 15 broadband seismic stations across northern Connecticut. This linear array crosses a number of major terrane boundaries and traverses the Mesozoic Hartford Rift Basin in its central portion; its dense station spacing affords an opportunity to probe anisotropic structure on length scales that are relevant for the complex geology of southern New England. We find evidence for average fast splitting directions that are generally parallel to the absolute motion of the North American Plate, but in a few specific regions they are aligned with local tectonic boundaries. We document a striking decrease in splitting delay times (measured at low frequencies) from 0.9 s at the western end of the array to 0.2 s at the eastern end. We discuss several scenarios that might explain this observation, and explore the implications of our measurements for both present‐day mantle flow and past lithospheric deformation beneath southern New England.

An investigation of seismic anisotropy in the lowermost mantle beneath Iceland

SUMMARYIceland represents one of the most well-known examples of hotspot volcanism, but the details of how surface volcanism connects to geodynamic processes in the deep mantle remain poorly understood. Recent work has identified evidence for an ultra-low velocity zone in the lowermost mantle beneath Iceland and argued for a cylindrically symmetric upwelling at the base of a deep mantle plume. This scenario makes a specific prediction about flow and deformation in the lowermost mantle, which can potentially be tested with observations of seismic anisotropy. Here we present an investigation of seismic anisotropy in the lowermost mantle beneath Iceland, using differential shear wave splitting measurements of S–ScS and SKS–SKKS phases. We apply our techniques to waves propagating at multiple azimuths, with the goal of gaining good geographical and azimuthal coverage of the region. Practical limitations imposed by the suboptimal distribution of global seismicity at the relevant distance ranges resulted in a relatively small data set, particularly for S–ScS. Despite this, however, our measurements of ScS splitting due to lowermost mantle anisotropy clearly show a rotation of the fast splitting direction from nearly horizontal for two sets of paths that sample away from the low velocity region (implying VSH &amp;gt; VSV) to nearly vertical for a set of paths that sample directly beneath Iceland (implying VSV &amp;gt; VSH). We also find evidence for sporadic SKS–SKKS discrepancies beneath our study region; while the geographic distribution of discrepant pairs is scattered, those pairs that sample closest to the base of the Iceland plume tend to be discrepant. Our measurements do not uniquely constrain the pattern of mantle flow. However, we carried out simple ray-theoretical forward modelling for a suite of plausible anisotropy mechanisms, including those based on single-crystal elastic tensors, those obtained via effective medium modelling for partial melt scenarios, and those derived from global or regional models of flow and texture development in the deep mantle. These simplified models do not take into account details such as possible transitions in anisotropy mechanism or deformation regime, and test a simplified flow field (vertical flow beneath the plume and horizontal flow outside it) rather than more detailed flow scenarios. Nevertheless, our modelling results demonstrate that our ScS splitting observations are generally consistent with a flow scenario that invokes nearly vertical flow directly beneath the Iceland hotspot, with horizontal flow just outside this region.

Spatiotemporal Variations in Crustal Seismic Anisotropy Surrounding Induced Earthquakes Near Fox Creek, Alberta

AbstractThis study analyzes earthquake recordings from four near‐source (&lt;10 km) stations near Fox Creek, Alberta, a region known for hydraulic fracturing‐induced seismicity. We examine the spatiotemporal variations of focal mechanisms and seismic anisotropy in the sedimentary strata. The focal mechanisms of surrounding earthquake swarms are generally consistent with the strike‐slip mechanism of the ML4.6 earthquake, favoring a flower type of fault structure. The NE‐SW‐orientated fast splitting direction, determined from the shear wave splitting measurements, reflects the combined effects of (1) N‐S faults and (2) NE‐SW time‐dependent hydraulically stimulated fractures. The latter effect dominates the apparent anisotropy during the days leading to the mainshock, while its contributions are reduced by 60–70% after the mainshock. Loss of fluid into the fault damage zone, which causes the closure of fractures, is responsible for the observed spatiotemporal variation of seismic anisotropy near the hydraulic fracturing well.

Seismic Anisotropy of the Upper Mantle Below the Western Rift, East Africa

AbstractAlthough the East African rift system formed in cratonic lithosphere above a large‐scale mantle upwelling, some sectors have voluminous magmatism, while others have isolated, small‐volume eruptive centers. We conduct teleseismic shear wave splitting analyses on data from 5 lake‐bottom seismometers and 67 land stations in the Tanganyika‐Rukwa‐Malawi rift zone, including the Rungwe Volcanic Province (RVP), and from 5 seismometers in the Kivu rift and Virunga Volcanic Province, to evaluate rift‐perpendicular strain, rift‐parallel melt intrusion, and regional flow models for seismic anisotropy patterns beneath the largely amagmatic Western rift. Observations from 684 SKS and 305 SKKS phases reveal consistent patterns. Within the Malawi rift south of the RVP, fast splitting directions are oriented northeast with average delays of ~1 s. Directions rotate to N‐S and NNW north of the volcanic province within the reactivated Mesozoic Rukwa and southern Tanganyika rifts. Delay times are largest (~1.25 s) within the Virunga Volcanic Province. Our work combined with earlier studies shows that SKS‐splitting is rift parallel within Western rift magmatic provinces, with a larger percentage of null measurements than in amagmatic areas. The spatial variations in direction and amount of splitting from our results and those of earlier Western rift studies suggest that mantle flow is deflected by the deeply rooted cratons. The resulting flow complexity, and likely stagnation beneath the Rungwe province, may explain the ca. 17 Myr of localized magmatism in the weakly stretched RVP, and it argues against interpretations of a uniform anisotropic layer caused by large‐scale asthenospheric flow or passive rifting.

SKS Splitting and the Scale of Vertical Coherence of the Taiwan Mountain Belt

AbstractMany continental orogens feature a pattern of SKS shear wave splitting with fast polarization directions parallel to the mountain fabrics and delay times of 1–2 s, implying that the crust and lithosphere deform consistently. In the Taiwan arc‐continent collision zone, similar pattern of SKS splitting exists, and thereby lithospheric scale deformation due to collision has been assumed. However, recent dynamic modeling demonstrated that the SKS splitting in Taiwan can be generated by the toroidal flow in the asthenosphere induced by the subduction of the Philippine Sea plate and the Eurasian plate. To further evaluate this hypothesis, we analyzed a new data set using a quantitative approach. The results show that models with slab geometries constrained by seismicity explain the observed fast splitting direction to within 25°, whereas the misfit grows to ~50–60° if the toroidal flow is disrupted by the presence of a sizable aseismic slab beneath central Taiwan as often suggested by tomographic imaging. However, small sized aseismic slab or detached slab fragment can potentially reconcile the splitting observations. We estimated the scale of vertical coherence to be 10–40 km in the lithosphere and 100–150 km in the asthenosphere, making the former unfavorable for accumulating large delay times. The low coherence is caused by the subduction of the Eurasian plate that creates complex deformation different from what characterizes the compressional tectonics above the plate. This suggests that the mountain building in Taiwan is a shallow process, rather than lithospheric in scale.

Mantle flow through a tear in the Nazca slab inferred from shear wave splitting

AbstractA tear in the subducting Nazca slab is located between the end of the Pampean flat slab and normally subducting oceanic lithosphere. Tomographic studies suggest mantle material flows through this opening. The best way to probe this hypothesis is through observations of seismic anisotropy, such as shear wave splitting. We examine patterns of shear wave splitting using data from two seismic deployments in Argentina that lay updip of the slab tear. We observe a simple pattern of plate‐motion‐parallel fast splitting directions, indicative of plate‐motion‐parallel mantle flow, beneath the majority of the stations. Our observed splitting contrasts previous observations to the north and south of the flat slab region. Since plate‐motion‐parallel splitting occurs only coincidentally with the slab tear, we propose mantle material flows through the opening resulting in Nazca plate‐motion‐parallel flow in both the subslab mantle and mantle wedge.

Heterogeneous seismic anisotropy in the transition zone and uppermost lower mantle: evidence from South America, Izu-Bonin and Japan

SUMMARY Measurements of seismic anisotropy are commonly used to constrain deformation in the upper mantle. Observations of anisotropy at mid-mantle depths are, however, relatively sparse. In this study we probe the anisotropic structure of the mid-mantle (transition zone and uppermost lower mantle) beneath the Japan, Izu-Bonin, and South America subduction systems. We present source-side shear wave splitting measurements for direct teleseismic S phases from earthquakes deeper than 300km that have been corrected for the effects of upper mantle anisotropy beneath the receiver. In each region, we observe consistent splitting with delay times as large as 1 s, indicating the presence of anisotropy at mid-mantle depths. Clear splitting of phases originating from depths as great as ∼600km argues for a contribution from anisotropy in the uppermost lower mantle as well as the transition zone. Beneath Japan, fast splitting directions are perpendicular or oblique to the slab strike and do not appear to depend on the propagation direction of the waves. Beneath South America and Izu-Bonin, splitting directions vary from trench-parallel to trench-perpendicular and have an azimuthal dependence, indicating lateral heterogeneity. Our results provide evidence for the presence of laterally variable anisotropy and are indicative of variable deformation and dynamics at mid-mantle depths in the vicinity of subducting slabs.

Testing models of sub‐slab anisotropy using a global compilation of source‐side shear wave splitting data

AbstractRecently, a number of source‐side shear wave splitting measurements that directly constrain anisotropy in the upper mantle beneath subducting slabs have been published. Such measurements have yielded an observational foundation on which to base our understanding of the dynamics of the sub‐slab mantle. Here we compile measurements from recent studies of source‐side splitting beneath slabs that employ identical measurement and processing techniques. We use this compilation to test the predictions of a number of recently proposed conceptual models for the dynamics of the sub‐slab mantle, including those that invoke three‐dimensional return flow beneath slabs, strong radial anisotropy in suboceanic asthenosphere that is entrained via subduction, and a model based on the correlation between sub‐slab splitting behavior and the age of the subducting lithosphere. We find that a model in which fast splitting directions are determined by slab age matches the observations better than either the 3‐D return flow or radial anisotropic models. Based on this observation, we propose that the sub‐slab mantle is characterized by two distinct anisotropic and mantle flow regimes. Beneath younger lithosphere (&lt;95 Ma), we propose that the sub‐slab mantle is characterized by 2‐D entrained flow resulting in an entrained mantle layer. Beneath older lithosphere (&gt;95 Ma), the entrained layer is thin and effectively serves as decoupling layer; the dynamics of the sub‐slab region beneath old lithosphere is therefore dominated by three‐dimensional return flow. This variation in the amount of mechanical coupling may be facilitated by the onset of small‐scale convection beneath older lithosphere.

Sub‐slab anisotropy beneath the Sumatra and circum‐Pacific subduction zones from source‐side shear wave splitting observations

AbstractUnderstanding the dynamics of subduction is critical to our overall understanding of plate tectonics and the solid Earth system. Observations of seismic anisotropy can yield constraints on deformation patterns in the mantle surrounding subducting slabs, providing a tool for studying subduction dynamics. While many observations of seismic anisotropy have been made in subduction systems, our understanding of the mantle beneath subducting slabs remains tenuous due to the difficulty of constraining anisotropy in the sub‐slab region. Recently, the source‐side shear wave splitting technique has been refined and applied to several subduction systems worldwide, making accurate and direct measurements of sub‐slab anisotropy feasible and offering unprecedented spatial and depth coverage in the sub‐slab mantle. Here we present source‐side shear wave splitting measurements for the Central America, Alaska‐Aleutians, Sumatra, Ryukyu, and Izu‐Bonin‐Japan‐Kurile subduction systems. We find that measured fast splitting directions in these regions generally fall into two broad categories, aligning either with the strike of the trench or with the motion of the subducting slab relative to the overriding plate. Trench parallel fast splitting directions dominate beneath the Izu‐Bonin, Japan, and southern Kurile slabs and part of the Sumatra system, while fast directions that parallel the motion of the downgoing plate dominate in the Ryukyu, Central America, northern Kurile, western Sumatra, and Alaska‐Aleutian regions. We find that plate motion parallel fast splitting directions in the sub‐slab mantle are more common than previously thought. We observe a correlation between fast direction and age of the subducting lithosphere; older lithosphere (&gt;95 Ma) is associated with trench parallel splitting while younger lithosphere (&lt;95 Ma) is associated with plate motion parallel fast splitting directions. Finally, we observe source‐side splitting for deep earthquakes (transition zone depths) beneath Japan and Sumatra, suggesting the presence of anisotropy at midmantle depths beneath these regions.

Pronounced zonation of seismic anisotropy in the Western Hellenic subduction zone and its geodynamic significance

Many subduction zones exhibit significant retrograde motion of their arc and trench. The observation of fast shear-wave velocities parallel to the trench in such settings has been inferred to represent trench-parallel mantle flow beneath a retreating slab. Here, we investigate this process by measuring seismic anisotropy in the shallow Aegean mantle. We carry out shear-wave splitting analysis on a dense array of seismometers across the Western Hellenic Subduction Zone, and find a pronounced zonation of anisotropy at the scale of the subduction zone. Fast SKS splitting directions subparallel to the trench-retreat direction dominate the region nearest to the trench. Fast splitting directions abruptly transition to trench-parallel above the corner of the mantle wedge, and rotate back to trench-normal over the back-arc. We argue that the trench-normal anisotropy near the trench is explained by entrainment of an asthenospheric layer beneath the shallow-dipping portion of the slab. Toward the volcanic arc this signature is overprinted by trench-parallel anisotropy in the mantle wedge, likely caused by a layer of strained serpentine immediately above the slab. Arcward steepening of the slab and horizontal divergence of mantle flow due to rollback may generate an additional component of sub-slab trench-parallel anisotropy in this region. Poloidal flow above the retreating slab is likely the dominant source of back-arc trench-normal anisotropy. We hypothesize that trench-normal anisotropy associated with significant entrainment of the asthenospheric mantle near the trench may be widespread but only observable at shallow-dipping subduction zones where stations nearest the trench do not overlie the mantle wedge.

Distinctive upper mantle anisotropy beneath the High Lava Plains and Eastern Snake River Plain, Pacific Northwest, USA

The Pacific Northwest (PNW) has experienced voluminous intraplate volcanism over the past ∼17 Ma, beginning with the Steens/Columbia River flood basalts and continuing with the still‐ongoing volcanism in the High Lava Plains (HLP) and eastern Snake River Plain (SRP). Here we present two complementary datasets (SKS splitting and Rayleigh wave phase velocity anisotropy) that place constraints on the anisotropic structure of the upper mantle beneath the HLP and SRP regions. Beneath the HLP, SKS phases reveal dominantly E‐W fast splitting directions and large (up to ∼2.7 s) delay times, with pronounced lateral variations in δt. Lateral and depth variability in the strength of anisotropy beneath the HLP is also evident from Rayleigh wave dispersion. Beneath the SRP, SKS splitting delay times are much smaller (∼0.5 s), and surface wave observations suggest a region of upper mantle anisotropy (∼50–150 km depth) with a geometry that deviates significantly from the generally plate motion parallel fast directions observed just outside of the SRP. Beneath the HLP, the geometry of the anomalously strong anisotropy is similar to the anisotropy in the deeper parts of the upper mantle, resulting in constructive interference and large SKS splitting delay times. Beneath the SRP, the geometry of the anomalous anisotropic region in the shallow mantle is different, resulting in destructive interference and reduced SKS splitting delay times. We discuss several possible explanations for these observations, including variations in olivine lattice‐preferred orientation (LPO) strength, transitions in olivine fabric type, and a contribution from aligned partial melt.

Complex anisotropy beneath the Peruvian flat slab from frequency‐dependent, multiple‐phase shear wave splitting analysis

AbstractFlat or shallow subduction is a relatively widespread global occurrence, but the dynamics remain poorly understood. In particular, the interaction between flat slabs and the surrounding mantle flow has yet to be studied in detail. Here we present measurements of seismic anisotropy to investigate mantle flow beneath the Peruvian flat‐slab segment, the largest present‐day region of flat subduction. We conduct a detailed shear wave splitting analysis at a long‐running seismic station (NNA) located near Lima, Peru. We present measurements of apparent splitting parameters (fast direction φ and delay time δt) for SKS, ScS, and local S phases from 80 events. We observe well‐defined frequency dependence and backazimuthal variability, indicating the likely presence of complex anisotropy. Forward modeling the observations with two or three layers of anisotropy reveals a likely layer with a trench‐normal fast direction underlying a layer with a more trench‐oblique (to trench‐subparallel) fast direction. In order to further constrain the anisotropic geometry, we analyzed the source‐side splitting from events originating within the slab measured at distant stations. Beneath the flat‐slab segment, we found trench‐normal fast splitting directions in the subslab mantle, while within the dipping portion of the slab further to the east, likely trench‐subparallel anisotropy within the slab itself. This subslab pattern contradicts observations from elsewhere in South America for “normal” (i.e., more steeply dipping) slab conditions. It is similar, however, to inferences from other shallowly dipping subduction zones around the world. While there is an apparent link between slab dip and the surrounding mantle flow, at least beneath Peru, the precise nature of the relationship remains to be clarified.

Subduction of oceanic asthenosphere: A critical appraisal in central Alaska

Song and Kawakatsu (2012) have shown that the sub-slab fast splitting pattern observed in most subduction zones can be a direct consequence of subduction of the oceanic asthenosphere that has strong radial anisotropy. This model not only explains the non-intuitive trench-parallel splitting pattern in most of subduction zones, but also predicts the trench-normal behavior (fast polarization direction sub-parallel to the absolute plate motion of the incoming plate) observed in several shallow subduction zones. The general validity of such a scenario is crucial to fundamental understandings of the development of mantle anisotropy in sub-lithosphere or/and sub-slab conditions, the nature and formation of oceanic asthenosphere as well as the flow pattern and mass transport near subduction zones. To validate this scenario, we examine SKS splitting patterns observed across the fore-arc in central Alaska. Here the fast splitting direction varies from plate motion sub-parallel near the trench to mostly trench-parallel beyond the 100km slab-isodepth contour, while being strongly variable in between. After taking into account the rotation of anisotropy symmetry in the oceanic asthenosphere with respect to the local plate motion obliquity and down-dip variations in the slab dip, we reproduce a general 90-degree switch in fast splitting direction as well as the back azimuth dependent splitting pattern across the entire fore-arc. The current validation further augments the idea that, apart from anisotropy in the mantle wedge and the subducting slab, subduction of the oceanic asthenosphere is likely to be the dominant source of seismic anisotropy in central Alaska and potentially in many subduction zones. Furthermore, this result also provides alternative views to models of seismic anisotropy in the mantle wedge and on the length scale in which the 3D mantle flow may be important.

Subduction of oceanic asthenosphere: Evidence from sub‐slab seismic anisotropy

The oceanic asthenosphere is characterized as a low viscosity channel down to 200–300 km depth separating the cold lithosphere from above, and it is intimately linked to a layer of low seismic velocity and prominent seismic anisotropy observed globally beneath ocean basins. While subduction of tectonic plates in convergent margins is well recognized, the fate of oceanic asthenosphere remains enigmatic. We demonstrate that subduction of the oceanic asthenosphere characterized by weak azimuthal anisotropy and strong radial anisotropy explains the essence of sub‐slab shear‐wave splitting patterns, where the fast splitting direction changes from predominantly trench‐parallel (or sub‐parallel) under relatively steep subduction zones to frequently trench‐normal under shallow subduction zones. To explain the observed splitting time, the thickness of the subducted asthenosphere is estimated to be 100 ± 50 km.

Crust-mantle mechanical coupling in Eastern Mediterranean and Eastern Turkey

Present-day crust-mantle coupling in the Eastern Mediterranean and eastern Turkey is studied using the Global Positioning System (GPS) and seismic anisotropy data. The general trend of the shear wave fast-splitting directions in NE Turkey and Lesser Caucaus align well with the geodetic velocities in an absolute plate motion frame of reference pointing to an effective coupling in this part of the region of weak surface deformation. Farther south, underneath the Bitlis Suture, however, there are significant Pn delays with E-W anisotropy axes indicating significant lateral escape. Meanwhile, the GPS reveals very little surface deformation. This mismatch possibly suggests a decoupling along the suture. In the Aegean, the shear wave anisotropy and the Pn anisotropy directions agree with the extensional component of the right-lateral shear strains except under the Crete Basin and other parts of the southern Aegean Sea. This extensional direction matches perfectly also with the southward pulling force vectors across the Hellenic trench; however, the maximum right-lateral shear directions obtained from the GPS data in the Aegean do not match either of these anisotropies. Seismic anisotropy from Rayleigh waves sampled at 15s, corresponding to the lower crust, match the maximum right-lateral maximum shear directions from the GPS indicating decoupling between the crust and the mantle. This decoupling most likely results from the lateral variations of the gravitational potential energies and the slab-pull forces.

Mantle flow in subduction systems: The subslab flow field and implications for mantle dynamics

The character of the mantle flow field in subduction zones remains poorly understood, despite its importance for our understanding of subduction dynamics. In particular, little attention has been paid to mantle flow beneath subducting slabs. In order to identify processes that make first‐order contributions to the global pattern of subslab mantle flow, we have compiled shear wave splitting measurements from subduction zones worldwide from previously published studies and estimated average splitting parameters for the subslab region. Globally, the subslab region is overwhelmingly dominated by trench‐parallel fast splitting directions. We tested for relationships between splitting delay time, a measure of the strength of anisotropy, and parameters that are indicators of tectonic processes, such as trench migration velocity, convergence velocity and obliquity, age of subducting lithosphere, and slab dip, curvature, seismicity, and thickness. We used several different plate motion models to describe plate and trench motions and evaluated the differences among the models. We find that only one parameter, namely, the trench migration velocity in a Pacific hot spot reference frame, appears to correlate well with the strength of the subslab splitting signal. This finding supports a model in which the mantle beneath subducting slabs is dominated by three‐dimensional flow induced by trench migration. We explore several implications of our model for various aspects of mantle dynamics, including the choice of a suitable reference frame(s) for mantle flow, the existence of a thin decoupling zone between slabs and the subslab mantle, and consequences for mass transfer between the upper and lower mantle.

From mantle to crust: Stretching the Mediterranean

The origin of forces driving the deformation of the continental crust near subduction zones and especially in backarc regions is debated. This work is based on a compilation of SKS fast splitting directions that give an image of flow lines in the mantle around the Mediterranean subduction zones and a comparison with stretching and shear directions in metamorphic core complexes that show the pattern of deformation at the scale of the middle and lower crusts. We find that : (1) the two sets of directions are parallel in the three main backarc regions, namely the Alboran Sea, the Tyrrhenian Sea and the Aegean Sea showing that the lithosphere deforms with the same direction of stretching in the crust and the mantle, suggesting that (2) crustal deformation is mainly driven from below by slab retreat, and (3) the lithospheric fabric is reset within a few millions of years in backarc environments.