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1. S ‐to‐ P Receiver Function Imaging of Lithospheric Discontinuities in New Zealand at the Hikurangi Subduction Zone

   https://doi.org/10.1029/2024GC011897  

   Buffett, William A; Rychert, Catherine A; Harmon, Nicholas (March 2025, Geochemistry, Geophysics, Geosystems)

   Abstract Subduction zones are important regions for understanding plate tectonic processes. New Zealand experiences slow slip, volcanism, and back‐arc rifting, and has evidence of large megathrust events and tsunamis. We useS‐to‐Preceiver functions to image lithospheric discontinuities beneath the North Island of New Zealand. A positive discontinuity interpreted as the Moho is imaged at 15–30 ± 3 km depth beneath the overriding Australian Plate. In some locations, near the interface of the Pacific and Australian Plates, we don't image the Pacific Plate Moho, and the Australian Plate Moho is faint or absent. The former could be related to the increasing dip or eclogitization of the Pacific Plate crust, and the latter is likely related to mantle wedge serpentinization. A negative velocity discontinuity associated with the lithosphere‐asthenosphere boundary (LAB) of the Australian Plate is imaged at 63–80 ± 8 km depth across the northwestern side of the island. Negative discontinuities are imaged beneath the southern Pacific Plate at 85–105 ± 10 km and 130 ± 13 km depth, representing either a mid‐lithospheric discontinuity (MLD) and a deeper LAB, or more likely a shallow LAB and a deeper artifact, given that the latter is better aligned with previous work. Beneath the Australian Plate, asthenospheric melt is inferred in the northwest beneath several regions of active volcanism. Beneath the Pacific Plate, asthenospheric melt is inferred near the trench, also corresponding to the transition to where the plates become locked; therefore, plate locking could be related to the buoyancy of the melt. 

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2. Olivine anisotropy suggests Gutenberg discontinuity is not the base of the lithosphere

   https://doi.org/10.1073/pnas.1608269113  

   Hansen, Lars N.; Qi, Chao; Warren, Jessica M. (September 2016, Proceedings of the National Academy of Sciences)

   Tectonic plates are a key feature of Earth’s structure, and their behavior and dynamics are fundamental drivers in a wide range of large-scale processes. The operation of plate tectonics, in general, depends intimately on the manner in which lithospheric plates couple to the convecting interior. Current debate centers on whether the transition from rigid lithosphere to flowing asthenosphere relates to increases in temperature or to changes in composition such as the presence of a small amount of melt or an increase in water content below a specified depth. Thus, the manner in which the rigid lithosphere couples to the flowing asthenosphere is currently unclear. Here we present results from laboratory-based torsion experiments on olivine aggregates with and without melt, yielding an improved database describing the crystallographic alignment of olivine grains. We combine this database with a flow model for oceanic upper mantle to predict the structure of the seismic anisotropy beneath ocean basins. Agreement between our model and seismological observations supports the view that the base of the lithosphere is thermally controlled. This model additionally supports the idea that discontinuities in velocity and anisotropy, often assumed to be the base of the lithosphere, are, instead, intralithospheric features reflecting a compositional boundary established at midocean ridges, not a rheological boundary. 

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3. Reversible subsidence on the North West Shelf of Australia

   https://doi.org/10.1016/j.epsl.2020.116070  

   Gurnis, M. (March 2020, Earth and planetary science letters)

   null (Ed.)

   The Northwest Shelf (NWS) of Australia is characterized by offshore basins associated with Permian and Jurassic rifting and was only slowly subsiding by the Neogene. International Ocean Discovery Program (IODP) Expedition 356 targeted this region by coring four sites in the Northern Carnarvon and Roebuck Basins and two sites in the Perth Basin to the south on the Australian western margin. We use detailed lithological, physical property and age data with paleobathymetric interpretations, to infer tectonic subsidence apparently confined to the NWS that reverses (uplifts) with about the same amplitude and rate as an earlier subsidence event. About 300 m of tectonic subsidence occurred over one million years from 6 to 5 million years ago and then reverses when 300 m of tectonic uplift occurred from 2 to 1 Ma. The along strike extent of this subsidence pattern is ∼ 400 km. The similarity of magnitude and duration of the subsidence and uplift phases suggest that the subsidence is reversible. The results cannot be explained by glacial eustatic variability nor can the uplift event be attributed to sediments filling the accommodation space generated earlier. Reversible subsidence is a key fingerprint of dynamic topography. Although the rates of subsidence and uplift are roughly ∼ 300 m/Myr, a substantial portion of the changes occur over less than 1 Myr and the rates inferred from a detailed least squares analysis can reach up to about 500 m/Myr. These rates are incompatible with dynamic topography associated with motion of Australia over large-scale convection (10 to 40 m/Myr) or that associated with instability of the base of the lithosphere (<15 m/Myr). The vertical motions are too large to be associated with simple flexure of a plate and plate buckling in that the required amplitudes would lead to permanent deformation of the plate. A new geodynamic mechanism is required to fit the observations. 

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4. A Thick, Sheared Zone at the Base of an Oceanic Plate: The Lithosphere-Asthenosphere Boundary of the Subducting Nazca Plate in Colombia as Revealed through P-to-S Receiver Functions

   Bishop, Brandon; Fernandez, Ademar; Warren, Linda M.; Pedraza, Patricia; Prieto, German A.; Dionicio, Viviana (December 2022, AGU Fall Meeting)

   The structure of the lithosphere-asthenosphere boundary (LAB) beneath oceanic plates is key to understanding how plates interact with the underlying mantle. Prior contradictory geophysical observations have been used to argue for a thin, melt-rich boundary that decouples the plate from the rest of the mantle, or for a much broader anisotropic and thermally controlled boundary that indicates significant coupling with the rest of the mantle. The predictions of models based on these interpretations can be tested most easily in a subduction zone setting where the steady increase in pressure at the base of the subducting plate’s LAB will have differing effects on melt and anisotropy. Melt remains stable within the mantle to ~150-250 km (for carbonate melt) or to ~330 km (for silicate melt), while anisotropy induced by different processes should have no significant change until ~250 km to ~440 km depth. We calculate P-to-S receiver functions (PRFs) using varying frequency bands at broadband seismic stations with >4 years of data from the Servicio Geológico Colombiano’s Red Sismológica Nacional de Colombia to investigate the characteristics of the LAB of the subducting Nazca oceanic plate from the coast to the Andean foreland (corresponding to slab LAB depths of ~50 km to >400 km). The use of PRFs permits identification and analysis of anisotropy across the boundary while calculation at a range of frequency bands permits tuning of the PRFs to differing spatial scales to determine the size and abruptness of the boundary. We find that the P-to-S converted phase of the subducted Nazca plate’s LAB is detectable 4-5 seconds after the converted phase of the plate’s Moho to at least ~150 km depth. Assuming the slab has an average Vp/Vs of 1.75 to 1.78 and Vp of 8.2 km/s (+2.5% dVp), this corresponds to a plate thickness of ~50 km, matching the expected thickness given the Nazca plate’s age in the region (~10-20 Myrs). We find that the Nazca plate’s LAB is most consistently detectable in the <0.24 Hz band and largely undetectable in the <2.4 Hz band, indicating the LAB is gradational and between 10 and 30 km in thickness. Amplitude variations and complexities in the LAB converted phases further indicate that the boundary marks a change in anisotropy most consistent with the LAB representing a sheared zone between the plate and underlying mantle. 

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5. Rayleigh Waves From OHANA OBSs in the Northeast Pacific Ocean Reveal Low Deep Shear Velocities and Pervasive Azimuthal Anisotropy

   https://doi.org/10.1029/2025JB032104  

   Laske, Gabi; Atkisson, Grace; Collins, John A; Blackman, Donna K (May 2026, Journal of Geophysical Research: Solid Earth)

   Abstract The 2021–2023 OHANA ocean‐bottom seismometer deployment in the northeast Pacific Ocean provides a rich data set for seismic studies to explore the crust, lithosphere and asthenosphere in a 600 km wide region about 1,500 km northeast of Hawaii, west of the Moonless Mountains. The study area covers mainly 40‐to‐55 Myr‐old Pacific lithosphere. A fundamental question to be addressed is whether this particular area has the signature of typical oceanic lithosphere that has a normal plate cooling history or if there is evidence for plate rejuvenation from a secondary mantle‐dynamic process. We present the analysis of path‐averaged low‐frequency (10–60 mHz) Rayleigh‐wave phase‐velocity curves from earthquake records retrieved for 210 two‐station pairs, and its inversion for depth‐dependent shear velocity,Vs. Forward modeling and the inversion of the average phase velocity indicate lower shear velocities than anticipated for 50‐Myr old lithosphere. Velocities in the mid‐to‐lower lithosphere and in the asthenosphere are 2%–3% lower than expected. We also observe significant and internally consistent azimuthal anisotropy of up to 5%, where the “fast direction” places between the fossil and modern plate motion directions. A preliminary analysis of ambient‐noise cross‐correlation functions reveals similar patterns for the fundamental mode as well as the first overtone. 

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