ALBERT

Description: The origin of the Bermuda swell and volcanism remains enigmatic. The lack of an associated time-progressive hotspot track and absence of present-day volcanic activity make it difficult to reconcile with a deep mantle plume model. We analyze shear wave splitting measurements to estimate mantle flow direction and receiver function stacks to place constraints on the mantle transition zone thermal structure. *KS phases exhibit well-resolved null arrivals (no splitting) beneath the swell over a range of back azimuths. We find that the 410 and 660 km discontinuities are 49 ± 5 km and 19 ± 5 km deeper than the global average, respectively, leading to a transition zone thickness that is 27 ± 4 km thinner than average. Together, an apparently isotropic upper mantle and a thinned mantle transition zone suggest that mantle flow is primarily vertical beneath the swell, consistent with the presence of hot, buoyant mantle at depth.

Description: 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 sec) delay times, with pronounced lateral variations in δ t . 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 sec) 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.

Description: The Pacific Northwest (PNW) has a complex tectonic history and over the past ∼17 Ma has played host to several major episodes of intraplate volcanism. These events include the Steens/Columbia River flood basalts (CRB) and the striking spatiotemporal trends of the Yellowstone/Snake River Plain (Y/SRP) and High Lava Plains (HLP) regions. Several different models have been proposed to explain these features, which variously invoke the putative Yellowstone plume, rollback and steepening of the Cascadia slab, extensional processes in the lithosphere, or a combination of these. Here we integrate seismologic, geodynamic, geochemical, and petrologic results from the multidisciplinary HLP project and associated analyses of EarthScope USArray seismic data to propose a conceptual model for post-20 Ma mantle dynamics beneath the PNW and the relationships between mantle flow and surface tectonomagmatic activity. This model invokes rollback subduction as the main driver for mantle flow beneath the PNW beginning at ∼20 Ma. A major pulse of upwelling due to slab rollback and upper plate extension and consequent melting produced the Steens/CRB volcanism, and continuing trench migration enabled mantle upwelling and hot, shallow melting beneath the HLP. An additional buoyant mantle upwelling is required to explain the Y/SRP volcanism, but subduction-related processes may well have played a primary role in controlling its timing and location, and this upwelling likely continues today in some form. This conceptual model makes predictions that are broadly consistent with seismic observations, geodynamic modeling experiments, and petrologic and geochemical constraints.

Description: In this study, we utilize data from broadband seismic stations of the Japanese F-net network to investigate anisotropic structure in the lowermost mantle beneath the northwestern Pacific. The comparison of shear wave splitting from phases that have similar paths in the upper mantle but different paths in the lowermost mantle, such as PcS/ScS or SKS/SKKS, can yield constraints on anisotropy in the D″ region. We measured splitting for SKS, SKKS, PcS, and ScS phases at F-net stations and compared the measured splitting parameters to previously published estimates of upper mantle anisotropy beneath the region. We observed many examples of discrepant SKS/SKKS splitting and splitting of ScS, PcS, SKS, and SKKS phases that does not agree well with the known upper mantle anisotropy beneath individual stations. The most likely explanation for these discrepancies is a contribution to splitting from anisotropy in the lowermost mantle. In particular, for SKS/SKKS phases that sample the lowermost mantle just to the south and east of the Kamchatka peninsula, we observed generally N-S fast directions with delay times between 0.5 and 1.5 s. These data suggest the presence of a fairly large, coherent region of deformation in the lowermost mantle beneath the northwestern Pacific. Our preferred model for these observations is that solid-state flow at the base of the mantle induces a lattice-preferred orientation of lowermost mantle minerals, generating a seismically anisotropic fabric.

Description: Three-dimensional models of mantle flow at subduction zones make it possible to explain the common occurrence of trench-parallel sub-slab seismic anisotropy. Sub-slab flow becomes inherently three-dimensional when slab-driven flow interacts with a wide variety of ambient background mantle flow conditions. This interaction depends on slab geometries, mechanical coupling parameters, and lower mantle viscosities. Deflection of sub-slab mantle flow is a robust feature for all model parameters and geometries as the slab acts as an obstruction to the ambient, background mantle flow. Background mantle flow can become trench-perpendicular or trench-parallel sub-slab flow depending on whether the ambient background mantle flow is deflected beneath the bottom of the slab or towards the edge of the slab. The first case is especially prominent in models with short slabs that do not penetrate into the lower mantle. The second case is especially prominent in models with long, steep slabs. The results are also highly sensitive to the amount of mechanical coupling between the subducting plate and the mantle beneath it. High levels of coupling create a boundary layer of trench-perpendicular entrained flow, pushing the deflection due to the obstructing slab away from the slab. We compare our sub-slab flow model predictions with a global set of seismic anisotropy fast directions in the sub-slab mantle, and find generally good agreement between the anisotropy observations (dominantly trench-parallel or trench-perpendicular) and the mantle flow directions predicted for decoupled systems.

Description: Combined analyses of deep tow magnetic anomalies and International Ocean Discovery Program Expedition 349 cores show that initial seafloor spreading started around 33 Ma in the northeastern South China Sea (SCS), but varied slightly by 1-2 myr along the northern continent-ocean boundary (COB). A southward ridge jump of ∼ 20km occurred around 23.6 Ma in the East Subbasin; this timing also slightly varied along the ridge and was coeval to the onset of seafloor spreading in the Southwest Subbasin, which propagated for about 400km southwestward from ∼23.6 Ma to ∼ 21.5 Ma. The terminal age of seafloor spreading is ∼15 Ma in the East Subbasin and ∼16 Ma in the Southwest Subbasin. The full spreading rate in the East Subbasin varied largely from ∼20 to ∼80km/myr, but mostly decreased with time except for the period between ∼26.0 Ma and the ridge jump (∼23.6 Ma), within which the rate was the fastest at ∼70km/myr on average. The spreading rates are not correlated, in most cases, to magnetic anomaly amplitudes that reflect basement magnetization contrasts. Shipboard magnetic measurements reveal at least one magnetic reversal in the top 100m of basaltic layers, in addition to large vertical intensity variations. These complexities are caused by late-stage lava flows that are magnetized in a different polarity from the primary basaltic layer emplaced during the main phase of crustal accretion. Deep tow magnetic modeling also reveals this smearing in basement magnetizations by incorporating a contamination coefficient of 0.5, which partly alleviates the problem of assuming a magnetic blocking model of constant thickness and uniform magnetization. The primary contribution to magnetic anomalies of the SCS is not in the top 100m of the igneous basement. This article is protected by copyright. All rights reserved.

Description: Understanding 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 (〉 95 Ma) is associated with trench parallel splitting while younger lithosphere (〈 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 mid-mantle depths beneath these regions.

Description: A complete characterization of seismic anisotropy can yield powerful constraints on mantle flow and deformation. This is particularly important for the mantle wedge above subducting slabs, where the geometry of mantle flow remains poorly understood. We seek to better characterize the geometry and strength of anisotropy in the mantle wedge beneath northeast Honshu and Hokkaido, both of which overlie the subducting Pacific plate. Previous studies indicate that upper mantle anisotropy in the Japan subduction zone is highly complex and exhibits dramatic spatial variations. To provide complementary constraints on the along strike variations in anisotropy, we analyze teleseismic receiver functions from stations of the broadband F-net array using the multitaper correlation receiver function estimator. Backazimuthal variations in P-to-SH converted energy provide clear evidence for complicated anisotropic structure in the mantle wedge beneath northeast Honshu and Hokkaido. In northeast Honshu, forward modeling of receiver functions using synthetic seismograms suggests the presence of an anisotropic layer in the forearc mantle wedge above the subducting slab and a second anisotropic layer beneath the crust of the overriding plate. We also see evidence for a region of low (isotropic) velocity in the central part of the wedge beneath NE Honshu. Comparisons between transverse component receiver functions at stations located in NE Honshu and Hokkaido reveal striking differences, providing further evidence for along strike variation in anisotropic structure in the mantle wedge beneath Japan.