SKS Waves Research Articles

Crust and Mantle Flow From Central Tibetan Plateau to the Indo‐Burma Subduction Zone

AbstractThe extremely oblique Indo‐Burma subduction zone exhibits dextral strike‐slip faulting along the Sagaing, Kabaw, and Churachandpur‐Mao Faults as well as east‐west shortening between the Sagaing Fault and Bengal Basin. Through regional stress analysis, considering areas from central Tibet, around the eastern Himalaya Syntaxis, to Burma, it has been determined that the principal compressive stress directions align with the principal strain rates. The northeast‐southwest oriented compressive stress direction from the western Shan Plateau continues into Burma. Notably, P axes align with the topographic gradients, and T axes are sub‐parallel to the topographic contours in the Shan Plateau region south of 27°N. These stress patterns are consistent with a gravitational potential energy induced crustal and mantle flow. The alignment of the fast shear wave with the maximum strain rate and the colinear NW‐SE to E‐W fast direction of the SKS wave and T axis determined from focal mechanisms in the Shan Plateau suggest that the mantle lithosphere deforms in concert with the crust. We suggest crust and mantle flow south of the Red River Fault has resulted in widening of the lithosphere in the Shan Plateau in an east‐west direction. Therefore, the Sagaing Fault has bowed approximately 50–100 km westward if we assume that the Sagaing Fault was originally straight. Our results of regional stress inversion are consistent with late Miocene to present E‐W shortening in the Indo‐Burma subduction zone resulting from the release of gravitational potential energy from the central Tibetan Plateau.

Splitting intensity tomography to image depth-dependent seismic anisotropy patterns beneath the Italian Peninsula and surrounding regions

The region between central Europe and the centre of the Mediterranean is characterised by complex tectonics and kinematics. Here, the interaction between thickened crust, subducting lithosphere and surrounding asthenosphere produces strong and pervasive anisotropy in the upper mantle. Shear wave splitting measurements, the most adopted method to image seismic anisotropy so far, when interpreted in a ray-based framework result in little or no depth resolution, hampering a correct image of the anisotropy distribution with depth. In this study, we aim to better constrain the depth-dependent seismic anisotropy beneath Italy and surrounding regions, by isolating for the first time the source region of anisotropy at different depths. To do that, we perform an anisotropy tomography, adopting the splitting intensity inversion method. It is entirely based on the finite-frequency effect in the splitting of SKS waves. We first computed the splitting intensity using SKS waves recorded at all available permanent and temporary stations over the region, obtaining a huge dataset of measurements used as an input for the tomographic inversion. The large-scale 3D model of seismic anisotropy obtained with the inversion shows a clear change of anisotropy properties in terms of fast polarisation direction and intensity for different depths, thus improving the characterization of the main sources of anisotropy in the mantle as a function of depth. Shallower layers (70–100 km depth) are characterised by a complex and variable oriented pattern of anisotropy fast direction and intensity, which becomes progressively more organised with depth (100–300 km). This pattern suggests a strong control exerted by the geometry and motion of the different slab segments and the large-scale asthenospheric flow generated by subduction and roll-back processes. The strength of anisotropy increases with depth, with high values affecting the bulge of the Alps and Apennines chains and the southern Tyrrhenian subduction system. On the contrary, weaker anisotropy characterises the transition zone from the Apennines to Alpine domains beneath the Po plain, and both the Adriatic and European domains.

Teleseismic measurements of Upper Mantle Shear-Wave Anisotropy in Southern Mexico

The Mexican subduction system is an ideal region to study 3-D mantle deformation patterns in response to changes in slab geometry and the presence of tears. Shear-wave splitting measurements were made using SKS, SKKS, and PKS waves in southern Mexico, where the Cocos slab subducts beneath the North American and western Caribbean plates. For most of southern Mexico, the results are consistent with predominantly trench-normal fast polarization directions that can be interpreted as a consequence of sub-slab entrained flow and 2-D corner flow in the mantle wedge in the presence of A-type olivine fabric (or similar). This pattern of trench-perpendicular fast axes extends northward to the region southeast of the Trans-Mexican Volcanic Belt. Beneath its eastern end, fast axes rotate ∼20° clockwise and are likely controlled by the absolute motion of the North American plate. In southeastern Mexico, along the coast and above the mantle wedge tip, the fast axes are trench-normal and the delay times are the shortest. They were interpreted to result from a possibly serpentinized mantle wedge tip. In the same region above the mantle wedge core, the splitting parameters appear to result from different flow patterns in the mantle wedge and the sub-slab mantle.

Shear wave splitting and mantle dynamics of the southern Great Xing'an orogenic belt

We deployed a linear seismic array consisting of 50 broadband stations in the southern part of the Great Xing'an orogenic belt to investigate seismic anisotropy in the Songliao Basin, the orogenic belt and the Erlian Basin by making SKS wave splitting measurements. Different features of anisotropy are revealed on the two sides of the north-south gravity lineament. The anisotropy on the east side of the gravity lineament primarily results from mantle flows in the big mantle wedge above the flat Pacific slab in the mantle transition zone, and the SKS fast polarization direction is generally NW-SE, being consistent with the subduction direction of the western Pacific plate. On the west side of the gravity lineament, the SKS splitting is attributed to freezing anisotropy in the stable lithosphere under the Erlian Basin. The SKS splitting delay times show some changes with azimuths of the teleseismic events, but the fast polarization directions are generally the same, indicating that the anisotropies below these seismic stations have the same origin in the continental lithosphere and active flows in the big mantle wedge.

The lithospheric S-wave velocity structure beneath the NE Tibetan Plateau and its surrounding craton basins

It is essential to investigate the spatial distribution of the lithosphere and asthenosphere in detail, to further obtain the understanding of the effect of plate collision and the process of orogenic movement. From the joint inversion of receiver functions and surface waves, the three-dimensional S-wave velocity structure results down to 200 km depth in the study area were obtained at 1,843 seismic stations. Analysis was performed on the sedimentary thickness, crustal thickness, lower crustal wave velocity, and lithospheric thickness. According to the crustal thickness, we evaluated the distribution of low-velocity zones in the lower crust. The results show that there are low-velocity bodies in the lower crust in the Qinling tectonic belt, but they are not connected, indicating that they may not be able to be used as a channel for material extrusion from the NE Tibetan Plateau at the crustal scale. According to the section results and the depth distribution of the lithosphere-astenosphere boundary, a relatively thick lithosphere exists below the Sichuan Basin and Ordos Basin, and the lithosphere in the east of the study area is relatively thin with a thickness of about 60–80 km, indicating that the lithosphere in the east of the study area has been severely destructed and restructured. The delamination has been observed in the lithosphere under the Songpan-Ganzi Block, showing characteristics of vertical movement of asthenosphere materials. There is a relatively thick low-velocity zone at the top of the mantle lithosphere of the NE plateau; however, it does not exist under the relatively stable Sichuan Basin and the Ordos Block. Compared with the Sichuan Basin and the Ordos Basin at both sides, the Qinling tectonic belt has a low-velocity zone at the depth of 100–160 km, which may be asthenosphere material. In combination with the polarization direction characteristics of the SKS wave, it is clearly observed that asthenospheric material movement exists in an approximate east-west direction beneath the Qinling tectonic belt. Therefore, the asthenosphere beneath the Qinling tectonic belt may serve as an important channel for material extrusion in the NE Tibetan Plateau.

Complex mantle flow beneath the Luxi uplift segment of the Tanlu fault zone inferred from SKS splitting measurements

To better understand the anisotropic structure and mantle dynamics in and around the Luxi uplift segment of the Tanlu fault zone in East China, we collect waveform data recorded at 47 NCISP (Northern China Interior Structure Project) portable seismic stations during November 2000 to August 2001. Using the minimum transverse energy grid-search method of SKS wave splitting measurement, effective splitting results at 35 stations are obtained. Our results show that the fast wave polarization direction (FPD) in the study area is dominated by a nearly E-W direction, with a range of 70°–116° and an average FPD of N98°E, which is generally perpendicular to the trend of the Tanlu fault zone. The splitting time varies from 0.6 to 1.5 s with an average of 0.94 s. Although there are subtle differences in FPDs of different tectonic units, it is still evident that larger splitting times amounting to ∼1.2 s are located north of the Luxi uplift, which is closely associated with the E-W horizontal flow in the big mantle wedge (BMW). In contrast, smaller splitting times are visible between the two branches of the Tanlu fault zone, indicating the existence of upwelling flow in the BMW. These complex mantle flow suggests that the BMW structure has played an important role in the deep dynamic processes beneath East Asia.

Inversion of shear wave waveforms reveal deformation in the lowermost mantle

SUMMARY It is well known that regions of the lowermost mantle—D″—exhibit significant seismic anisotropy. Identifying a unique mechanism for seismic anisotropy in D″ and interpreting results in terms of mantle flow has proved challenging. In an attempt to address this, we outline a method for the direct inversion of shear wave waveform data for the orientation and strength of seismic anisotropy. We demonstrate our method by jointly inverting SKS, SKKS and ScS shear wave data for seismic anisotropy in a fast shear wave velocity anomaly beneath the Eastern Pacific Ocean. Using our inversion method we evaluate four candidate mechanisms for seismic anisotropy in D″: elliptical transverse isotropy (representing layering or inclusions), bridgmanite and post-perovskite (for fabrics dominated by either [100](001) or [100](010) slip). We find that all candidate mechanisms can reasonably explain our input data, with synthetic inversions demonstrating that improved backazimuthal coverage is required to identity a single best-fitting mechanism. By inverting for orientation and anisotropic strength parameters we are able to discount bridgmanite as a candidate mechanism as less plausible solution, as our inversion requires an unreasonable ca. 40 per cent of D″ to consist of aligned bridgmanite crystals. The inversion results for the 4 candidate mechanisms predict two different mantle flow regimes, near vertical upwelling (or downwelling) or predominantly horizontal Southwesterly (or Northwesterly) deformation, both of which are inconsistent with recent mantle flow models. These results show that our new inversion method gives seismologists a powerful new tool to constrain lowermost mantle anisotropy, allowing us to test predictions of lowermost mantle flow.

Mantle flow under the Central Alps: Constraints from shear-wave splitting for non-vertically-incident SKS waves

Shear-wave splitting measurements map the orientation of geodynamically induced fabrics in the upper mantle. However, due to the limited depth resolution of shear-wave splitting investigations from core-mantle refracted phases, it has so far been difficult to differentiate between asthenospheric and lithospheric origins of observed seismic anisotropy. To improve this differentiation, we take advantage of backazimuthal variations of the fast orientation ϕ and delay time Δt that is expected for an olivine-mantle block with vertical b-axis as compared to vertical c-axis. This suggests that shear-wave splitting observations can in principle provide constraints that have so far not been recognized. The two end member cases can occur depending on the type of deformation, for example, a subhorizontal foliation in the case of a Simple Asthenospheric Flow and a subvertical foliation when considering transcurrent/transpressional circumstances during Vertical Coherent Deformation in the lithosphere. In this study we investigate the cause of seismic anisotropy in the Central Alps. Fast orientations ϕ show a spatially coherent and relatively simple mountain-chain-parallel pattern without large azimuthal variations per station, likely related to a single-layer case of upper mantle anisotropy. Considering the measurements of the entire study area, our procedure indicates b-up olivine and thus favors an asthenospheric source of the anisotropy, with a horizontal flow plane of deformation. We also test the influence of position relative to the (European) slab under the Central Alps, distinguishing northern and southern subarea based on vertically-integrated travel times calculated from a tomographic model. Differences in the statistical distribution of splitting parameters ϕ and Δt, and in the backazimuthal variation of their variation δϕ and δΔt, become apparent. While the observed seismic anisotropy in the northern subarea shows indications of planar Poiseuille flow around the Alps, the origin in the southern subarea remains uncertain, but it is likely to be affected by the slab itself.

Transient variation in seismic wave speed points to fast fluid movement in the Earth's outer core

The solid inner core grows through crystallization of the liquid metallic outer core. This process releases latent heat as well as light elements, providing thermal and chemical buoyancy forces to drive the Earth’s geodynamo. Here we investigate temporal changes in the liquid outer core by measuring travel times of core-penetrating SKS waves produced by pairs of large earthquakes at close hypocenters. While the majority of the measurements do not require a change in the outer core, we observe SKS waves that propagate through the upper half of the outer core in the low latitude Pacific travel about one second faster at the time when the second earthquake occurred, about 20 years after the first earthquake. This observation can be explained by 2–3% of density deficit, possibly associated with high-concentration light elements in localized transient flows in the outer core, with a flow speed in the order of 40 km/year.

Mantle Flow Deflected by Arc–Continent Collision and Continental Subduction in Eastern Indonesia

Abstract We investigate seismic anisotropy across southeastern Indonesia where the Indo-Australian plate subducts beneath and collides with the Sunda-Banda arc. Geochemical, geodetic, and tomographic studies reveal an along-strike transition from oceanic subduction to continental subduction and collision near central Flores that is due to a change of lithospheric composition in the subducting plate. To investigate the anisotropic fabric and dynamics of the upper mantle surrounding this young (∼5 Ma) arc–continent collision, we perform shear wave splitting analysis on local and teleseismic S waves recorded by an array of broadband seismometers that crosses the subduction–collision boundary. Seismic anisotropy inferred from our local S dataset shows that anisotropic sources above the slab extend to depths exceeding 100 km. Analysis of teleseismic SKS and SKKS waves reveal a shift in subslab fast axes from trench-parallel to trench-perpendicular near the ocean–continent boundary in the lower plate, which we relate to regional subslab mantle flow being deflected around the subducted continental lithosphere. Along-strike variations in anisotropic fast axes from teleseismic phases overlap with distinct structural and tectonic boundaries that divide distinct regions of the collision, implying the effects of the collision transcend any one dataset and highlighting the complexity of collisional boundaries. These results shed light on the interaction between tectonic structure and mantle dynamics in an emergent collision, and help constrain the nature of upper mantle deformation in the early stages of collision.

Deformation and seismic anisotropy in the SE Tibetan Plateau lithospheric mantle inferred from Maguan peridotite xenoliths

SKS wave splitting measurements revealed strong and complex seismic anisotropy in the upper mantle beneath the southeastern Tibetan Plateau. To better understand lithospheric deformation and upper mantle anisotropy observed in this region, we performed studies on microstructures, seismic anisotropy and chemical compositions of the mantle peridotite xenoliths collected from Maguan, Yunnan Province, in the SE Tibetan Plateau. The mantle xenoliths show two types of olivine crystal preferred orientation (CPO), AG–type and A–type. AG–type fabric is characterized by a concentration of [010]-axis normal to the foliation and a large circle girdle of the [100] and [001] axes in foliation. A–type fabric shows point concentrations of [100] and [010] axes subparallel to lineation and subnormal to foliation, respectively. Orthopyroxene and clinopyroxene align their [001]-axis sub-parallel to the [100]-axis of olivine. Seismic anisotropy calculated from the fabric of xenoliths displays moderate to strong polarization anisotropy (AVs) ranging from 3.1% to 7.0%, with 4.6% on average. Based on the seismic properties of xenoliths, we propose that, in SE Tibetan Plateau (south of 26°N), the SKS wave splitting is attributed to melt-enhanced anisotropy with a vertical structural frame lithosphere. Under this structural configuration, the large delay time could be expected with the fast wave polarization direction (FPD) in E–W direction. Our study also supports the model of double anisotropic layers in this area. The fast orientation of lower layer caused by lithospheric mantle anisotropy is in E–W direction, while the upper one with the NNW–SSE FPD is related to the frozen anisotropy in the middle-lower crust.

Asthenospheric Flow of Plume Material Beneath Arabia Inferred From S Wave Traveltime Tomography

AbstractWidespread Cenozoic volcanism in the Arabian Peninsula has been attributed to mantle plume activity and/or lithospheric thinning due to rift‐related extension. However, there is discrepancy between geochemical and geophysical studies about which mechanism is dominant over the other for post‐12 Ma volcanism. Plume signals in some volcanic fields in the Arabian shield are not evident in isotope analyses, but low‐velocity anomalies connected to Afar are found beneath the Arabian shield in tomographic studies and interpreted as asthenospheric flow from the Afar plume. To resolve this contradiction, we investigate the upper mantle beneath the Arabian Peninsula and northeastern Africa by inverting relative S and SKS wave arrival times recorded at dense seismic networks to derive a high‐resolution S wave velocity model. Our results clearly show a narrow, elongated low‐velocity anomaly along the Makkah‐Madinah‐Nafud (MMN) volcanic line beneath the Arabian shield at 100–300 km depth which extends northward to Harrat Ithnayn and Harrat Lunayyir, but most likely not further north. The limited extent of the low‐velocity anomaly and variations in lithospheric thickness of the Arabian shield may explain why geochemical studies did not find plume signals in some harrats. Therefore, the timing and plume signals of volcanism in western Arabia may not be age progressive from Afar. We also find a possible connection between the low‐velocity anomalies beneath Harrat Lunayyir and the MMN line, suggesting that the 2007–2009 seismic swarm may be associated with northward asthenospheric flow of plume material from Afar.

Flow plane orientation in the upper mantle under the Western/Central United States from SKS shear-wave splitting observations

SUMMARY The causes of seismic anisotropy are still under debate. In particular, it is important to understand the extent to which seismic anisotropy is due to more recent geodynamic activities in the asthenosphere, or to frozen-in deformation in the lithosphere. We show that these two endmember cases can in principle be distinguished using shear-wave splitting observations from SKS waves. This is illustrated by the simple example of pure olivine with horizontal a-axis, and differing orientations of the other two axes, namely, vertical b and vertical c. The azimuthal dependence of shear-wave splitting measurements is described by two parameters, which can provide additional information about subsurface deformation. In particular, the oscillation parameter d1 constrains the orientation of foliation. We demonstrate that shear-wave splitting in the Western and Central United States indeed shows the predicted azimuthal dependence, related to a mainly subhorizontally oriented flow plane of deformation in the upper mantle. This has important implications for asthenospheric flow.

A model space search approach to finite-frequency SKS splitting intensity tomography in a reduced parameter space

Here we develop a theoretical and practical framework for the tomographic inversion of shear wave splitting intensity measurements for anisotropic structure in the upper mantle using a model space search approach. Treating the anisotropic scatterers as a first order perturbation to the background isotropic state, we implement the Born approximation to compute the integral sensitivity kernels in a finite frequency framework. We implement a parametrization of the anisotropy based on insights from olivine elasticity and fabric development that involves three parameters (corresponding to the azimuth and dip of the anisotropic symmetry axis, plus a strength parameter). Previous work on finite-frequency shear wave splitting tomography has implemented a linearization technique to invert splitting intensity data for the spatial distribution of anisotropic scatterers. The inverse problem, however, is strongly non-linear in terms of several of the involved parameters (those that describe the orientation of the symmetry axis), and their variation is not of first order. Therefore, in the case of a realistic upper mantle where anisotropic structure varies in a complicated manner, a linearization technique may not be adequate. To ameliorate these problems, we implement a model space search approach (specifically, a Markov chain Monte Carlo with Gibbs sampling algorithm) to the tomographic inversion of splitting intensity data. This approach allows for the visualization of posterior probability distributions for anisotropic parameters in the inversion. We perform a suite of synthetic resolution tests to demonstrate the reliability of our method, using a station distribution from an actual deployment of a dense seismic network. These resolution tests show that anisotropic structure may be resolved up to a length scale of roughly 50 km with teleseismic SKS waves for station spacing of 10–15 km.

Evaluating the Resolution of Deep Mantle Plumes in Teleseismic Traveltime Tomography

AbstractThe strongest evidence to support the classical plume hypothesis comes from seismic imaging of the mantle beneath hot spots. However, imaging results are often ambiguous and it is questionable whether narrow plume tails can be detected by present‐day seismological techniques. Here we carry out synthetic tomography experiments based on spectral element method simulations of seismic waves with period T > 10 s propagating through geodynamically derived plume structures. We vary the source‐receiver geometry in order to explore the conditions under which lower mantle plume tails may be detected seismically. We determine that wide‐aperture (4,000–6,000 km) networks with dense station coverage (<100–200 km station spacing) are necessary to image narrow (<500 km wide) thermal plume tails. We find that if uncertainties on traveltime measurements exceed delay times imparted by plume tails (typically <1 s), the plume tails are concealed in seismic images. Vertically propagating SKS waves enhance plume tail recovery but lack vertical resolution in regions that are not independently constrained by direct S paths. We demonstrate how vertical smearing of an upper mantle low‐velocity anomaly can appear as a plume originating in the deep mantle. Our results are useful for interpreting previous plume imaging experiments and guide the design of future experiments.

Investigation of mantle kinematics beneath the Hellenic-subduction zone with teleseismic direct shear waves

The subduction and roll-back of the African plate beneath the Eurasian plate along the arcuate Hellenic trench is the dominant geodynamic process in the Aegean and western Anatolia. Mantle flow and lithospheric kinematics in this region can potentially be understood better by mapping seismic anisotropy. This study uses direct shear-wave splitting measurements based on the Reference Station Technique in the southern Aegean Sea to reveal seismic anisotropy in the mantle. The technique overcomes possible contamination from source-side anisotropy on direct S-wave signals recorded at a station pair by maximizing the correlation between the seismic traces at reference and target stations after correcting the reference stations for known receiver-side anisotropy and the target stations for arbitrary splitting parameters probed via a grid search. We obtained splitting parameters at 35 stations with good-quality S-wave signals extracted from 81 teleseismic events. Employing direct S-waves enabled more stable and reliable splitting measurements than previously possible, based on sparse SKS data at temporary stations, with one to five events for local SKS studies, compared with an average of 12 events for each station in this study. The fast polarization directions mostly show NNE-SSW orientation with splitting time delays between 1.15s and 1.62s. Two stations in the west close to the Hellenic Trench and one in the east show N-S oriented fast polarizations. In the back-arc region three stations exhibit NE-SW orientation. The overall fast polarization variations tend to be similar to those obtained from previous SKS splitting studies in the region but indicate a more consistent pattern, most likely due to the usage of a larger number of individual observations in direct S-wave derived splitting measurements. Splitting analysis on direct shear waves typically resulted in larger split time delays compared to previous studies, possibly because S-waves travel along a longer path in the same anisotropic structure. Considering the S-derived splitting measurements of this study together with earlier SKS and Rayleigh wave anisotropy modelling results we suggest that the very consistent direct S-derived fast shear wave directions can be explained by the lattice-preferred orientation of olivine in the asthenospheric mantle due to mantle flow induced by the roll-back of the slab. It is possible that a small contribution originated in the lower crust beneath the study region where anisotropic fabric might have formed in response to extension in the Miocene.

Seismic anisotropy beneath the southern Ordos block and the Qinling-Dabie orogen, China: Eastward Tibetan asthenospheric flow around the southern Ordos

SKS wave splitting analysis is performed to estimate the seismic anisotropy in the upper mantle using teleseismic data recorded by a temporary seismic array of 180 stations called SOSArray deployed in the southern Ordos block and the Qinling-Dabie orogen. The most important finding is that large delay times with NW-SE fast polarization directions in the northeastern Tibet are continuous across the boundary into the southwestern part of the Ordos block, where the SKS wave splitting results are significantly different from those in the rest of the Ordos block. Based on our SKS wave splitting results in addition to the results from previous studies, we propose an asthenospheric flow model for the eastward extrusion of the Tibetan upper mantle. The model consists of two corner flows around the southwestern corner and the southeastern corner of the Ordos block and the eastward flow along the Weihe graben and the Qinling-Dabie orogen for the escaping Tibetan upper mantle. Finally, the clockwise turning flow of the asthenosphere around the southwestern corner of Ordos block has currently extended laterally into the interior of the Ordos block, suggesting that the thick cold lithospheric root of the southwestern Ordos block there is currently being replaced with hot Tibetan asthenosphere at depths, that is, we observed an on-going process of thermal erosion of a cratonic lithosphere by lateral hot asthenospheric flow.

Anisotropic zonation in the lithosphere of Central North America: Influence of a strong cratonic lithosphere on the Mid-Continent Rift

We present shear-wave splitting analyses of SKS and SKKS waves recorded at sixteen Superior Province Rifting Earthscope Experiment (SPREE) seismic stations on the north shore of Lake Superior, as well as fifteen selected Earthscope Transportable Array instruments south of the lake. These instruments bracket the Mid-Continent Rift (MCR) and sample the Superior, Penokean, Yavapai and Mazatzal tectonic provinces. The data set can be explained by a single layer of anisotropic fabric, which we interpret to be dominated by a lithospheric contribution. The fast S polarization directions are consistently ENE-WSW, but the split time varies greatly across the study area, showing strong anisotropy (up to 1.48s) in the western Superior, moderate anisotropy in the eastern Superior, and moderate to low anisotropy in the terranes south of Lake Superior. We locate two localized zones of very low split time (<0.6s) adjacent to the MCR: one in the Nipigon Embayment, an MCR-related magmatic feature immediately north of Lake Superior, and the other adjacent to the eastern end of the lake, at the southern end of the Kapuskasing Structural Zone (KSZ). Both low-splitting zones are adjacent to sharp bends in the MCR axis. We interpret these two zones, along with a low-velocity linear feature imaged by a previous tomographic study beneath Minnesota and the Dakotas, as failed lithospheric branches of the MCR. Given that all three of these branches failed to propagate into the Superior Province lithosphere, we propose that the sharp bend of the MCR through Lake Superior is a consequence of the high mechanical strength of the Superior lithosphere ca. 1.1Ga.

Ultralow-velocity zone geometries resolved by multidimensional waveform modelling

Ultra-low velocity zones (ULVZs) are thin patches of material with strongly reduced seismic wave speeds situated on top of the core-mantle boundary (CMB). A common phase used to detect ULVZs is SPdKS (SKPdS), an SKS wave with a short diffracted P leg along the CMB. Most previous efforts have examined ULVZ properties using 1D waveform modeling approaches. We present waveform modeling results using the 2.5D finite difference algorithm PSVaxi allowing us better insight into ULVZ structure and location. We characterize ULVZ waveforms based on ULVZ elastic properties, shape, and position along the SPdKS raypath. In particular, we vary the ULVZ location (e.g. source or receiver side), ULVZ topographical profiles (e.g. boxcar, trapezoidal, or Gaussian) and ULVZ lateral scale along great circle path (2.5o, 5o, 10o). We observe several waveform effects absent in 1D ULVZ models and show evidence for waveform effects allowing the differentiation between source and receiver side ULVZs. Early inception of the SPdKS/SKPdS phase is difficult to detect for receiver-side ULVZs with maximum shifts in SKPdS initiation of ∼3o in epicentral distance, whereas source-side ULVZs produce maximum shifts of SPdKS initiation of ∼5o, allowing clear separation of source- versus receiver-side structure. We present a case study using data from up to 300 broadband stations in Turkey recorded between 2005 and 2010. We observe a previously undetected ULVZ in the southern Atlantic Ocean region centered near 45o S, 12.5oW, with a lateral scale of ∼3°, VP reduction of 10%, VS reduction of 30%, and density increase of 10% relative to PREM.

Seismic Anisotropy and Mantle Flow Driven by the Cocos Slab Under Southern Mexico

Shear wave splitting measurements were made using SKS and SKKS waves recorded by the Meso-American Subduction Experiment, which was deployed in southern Mexico starting at the coast of the Pacific Ocean and running north toward the Gulf of Mexico. In this segment of the Middle America Trench the oceanic Cocos plate subducts under the continental North American plate. The active volcanic arc is located at the southern end of the Trans-Mexican Volcanic Belt. Unlike most subduction zones, however, the volcanic arc is not subparallel to the trench. In the fore-arc, between the trench and the Trans-Mexican Volcanic Belt, the Cocos slab subducts subhorizontally. Beneath the volcanic belt, however, the slab dives steeply into the mantle. A marked difference in the orientation of the fast polarization directions is observed between the fore-arc and the back-arc. In the fore-arc the fast axes determined using SKS phases are oriented NE–SW, in the same direction as the relative motion between the Cocos and North American plates, and are approximately perpendicular to the trench. Physical conditions in the subslab mantle are consistent with the existence of A-type olivine and consequently entrained mantle flow is inferred. Strong coupling between the slab and the surrounding mantle is observed. In the back-arc SKS fast polarization directions are oriented N–S and are perpendicular to the strike of the slab. Given the high temperatures in the mantle wedge tip, the development of A-type, or similar, olivine fabric throughout the mantle wedge is expected. The orientation of the fast axes is consistent with corner flow in the mantle wedge.