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1. Multigenetic Origin of the X‐Discontinuity Below Continents: Insights From African Receiver Functions

   https://doi.org/10.1029/2022GC010782  

   Pugh, Stephen; Boyce, Alistair; Bastow, Ian D.; Ebinger, C. J.; Cottaar, Sanne (March 2023, Geochemistry, Geophysics, Geosystems)

   Abstract Constraints on chemical heterogeneities in the upper mantle may be derived from studying the seismically observable impedance contrasts that they produce. Away from subduction zones, several causal mechanisms are possible to explain the intermittently observed X‐discontinuity (X) at 230–350 km depth: the coesite‐stishovite phase transition, the enstatite to clinoenstatite phase transition, and/or carbonated silicate melting, all requiring a local enrichment of basalt. Africa hosts a broad range of terranes, from Precambrian cores to Cenozoic hotspots with or without lowermost mantle origins. With the absence of subduction below the margins of the African plate for >0.5 Ga, Africa presents an ideal study locale to explore the origins of the X. Traditional receiver function (RF) approaches used to map seismic discontinuities, such as common conversion‐point stacking, ignore slowness information crucial for discriminating converted upper mantle phases from surface multiples. By manually assessing depth and slowness stacks for 1° radius overlapping bins, normalized vote mapping of RF stacks is used to robustly assess the spatial distribution of converted upper mantle phases. The X is mapped beneath Africa at 233–340 km depth, revealing patches of heterogeneity proximal to mantle upwellings in Afar, Canaries, Cape Verde, East Africa, Hoggar, and Réunion with further observations beneath Cameroon, Madagascar, and Morocco. There is a lack of an X beneath southern Africa and strikingly, the magmatic eastern rift branch of the southern East African Rift. With no relationships existing between depth and amplitudes of observed X and estimated mantle temperatures, multiple causal mechanisms are required across a range of continental geodynamic settings. 

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2. New Insights Into Lithospheric Structure and Melting Beneath the Colorado Plateau

   https://doi.org/10.1029/2021GC010252  

   Golos, E_M; Fischer, K_M (March 2022, Geochemistry, Geophysics, Geosystems)

   Abstract The Colorado Plateau and its surroundings serve as an archetypal case to investigate the interaction of mantle melting processes and lithospheric structure. It has been hypothesized that widespread Cenozoic volcanism indicates the encroachment of the convective upwelling of asthenosphere toward the Plateau center. In this study, we generate a Common Conversion Point (CCP) stack of S‐to‐p (Sp) receiver functions to image the locations of lithospheric discontinuities in the southwestern United States. Our results are broadly similar to prior work, showing a strong and continuous Negative Velocity Gradient (NVG) consistent with the Lithosphere‐Asthenosphere Boundary (LAB) over much of the study area. However, with several methodological improvements, we are able to obtain more reliable NVG depth picks below the Colorado Plateau where the LAB becomes weaker, deeper, and broader. We compare the inferred topography of NVGs with the locations of volcanoes, and find that the majority of recent volcanoes are co‐located with lithosphere that is ∼80 km thick. This appears to be the critical depth at which partial melt from upwelling asthenosphere pooling at the base of (or within) the lithosphere may percolate to the surface. We compare our CCP profiles with magma equilibration conditions determined from petrologic analysis and find good agreement between the depth of NVGs and depth of magma equilibration. This analysis provides insight into the progression of magmatism and lithospheric loss toward the center of the Colorado Plateau, and demonstrates how small‐scale processes like melting influence lithosphere‐asthenosphere interactions that persist over large temporal and spatial scales. 

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3. Mantle transition zone beneath West Antarctica: Expanded mapping in previously unstudied region

   Horton, A.C.; Emry, E.; Hansen, S.E.; van Wijk, J.; O'Donnell, J.P.; Brisbourne, A.; Stuart, G.W.; Anandakrishnan, S.; Aster, R.C.; Huerta, A.D.; et al (December 2018, American Geophysical Union)

   With the ongoing discussion of Earth structure under West Antarctica and how it relates to the extension and volcanism of the area, we explore the possibility of a hydrated or thermally perturbed mantle underneath the region. Using P-wave receiver functions, we focus on the Mantle Transition Zone (MTZ) and how its thickness fluctuates from the global average (240-260 km). Prior studies have explored the West Antarctic regions of Marie Byrd Land and the West Antarctic Rift, but we expand this to include ~3-5 years of recent, additional seismic data from the Amundsen Sea and Pine Island Bay regions. Several years of additional data from the Ronne-Fichtner Ice Shelf, Ellsworth Land, and Marie Byrd Land regions will help provide a more complete picture of the mantle transition zone. Data for this study was obtained from IRIS for earthquakes of a 5.5 magnitude or greater. We use an iterative, time domain deconvolution method, filtered with Gaussian widths of 0.5, 0.75, and 1.0. All events within their respective Gaussian filter have undergone quality check by removing waveforms that have lower than 85% fit and visually checking for clear outliers. We migrate the receiver functions to depth and stack, using both single station stacking and common conversion point (CCP) stacking. We migrate the CCP stacks assuming both 1D (AK-135) and 3D velocity models throughout the region. Preliminary results from single-station stacks beneath the Thurston Island and Amundsen Sea regions suggest that the MTZ thickness is similar to the global average and the depth to the transition zone appears to be depressed, with average transition zone boundaries appearing around 430 and 680 km. If the MTZ is thinner than the global average, it may be an indication for high temperature thermal anomalies or a plume under West Antarctica that may help explain the history of extension and uplift there. These results could be useful for glacial isostatic adjustment and/or geothermal heat flux models that attempt to understand ice sheet history and stability. 

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4. Analysis of Local Seismic Events near a Large-N Array for Moho Reflections

   https://doi.org/10.1785/0220200087  

   Zeng, Qicheng; Nowack, Robert L. (October 2020, Seismological Research Letters)

   null (Ed.)

   Abstract Local seismic events recorded by the large-N Incorporated Research Institutions for Seismology Community Wavefield Experiment in Oklahoma are used to estimate Moho reflections near the array. For events within 50 km of the center of the array, normal moveout corrections and receiver stacking are applied to identify the PmP and SmS Moho reflections on the vertical and transverse components. Corrections for the reported focal depths are applied to a uniform event depth. To stack signals from multiple events, further static corrections of the envelopes of the Moho reflected arrivals from the individual event stacks are applied. The multiple-event stacks are then used to estimate the pre-critical PmP and SmS arrivals, and an average Poisson’s ratio of 1.77±0.02 was found for the crust near the array. Using a modified Oklahoma Geological Survey (OGS) velocity model with this Poisson’s ratio, the time-to-depth converted PmP and SmS arrivals resulted in a Moho depth of 41±0.6  km. The modeling of wide-angle Moho reflections for selected events at epicenter-to-station distances of 90–135 km provides additional constraints, and assuming the modified OGS model, a Moho depth of 40±1  km was inferred. The difference between the pre-critical and wide-angle Moho estimates could result from some lateral variability between the array and the wide-angle events. However, both estimates are slightly shallower than the original OGS model Moho depth of 42 km, and this could also result from a somewhat faster lower crust. This study shows that local seismic events, including induced events, can be utilized to estimate properties and structure of the crust, which, in turn, can be used to better understand the tectonics of a given region. The recording of local seismicity on large-N arrays provides increased lateral phase coherence for the better identification of precritical and wide-angle reflected arrivals. 

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5. Global Patterns in Cratonic Mid‐Lithospheric Discontinuities From Sp Receiver Functions

   https://doi.org/10.1029/2021GC009819  

   Krueger, Hannah E.; Gama, Isabella; Fischer, Karen M. (June 2021, Geochemistry, Geophysics, Geosystems)

   Abstract The goal of this study is to constrain the origins of layering in the seismic velocity structure within the cratonic mantle lithosphere (i.e. mid‐lithospheric discontinuities [MLDs]). For long‐lived stations in cratons worldwide, we calculated S‐to‐P converted phase receiver function stacks using time domain deconvolution and a k‐means algorithm to select robust, consistent receiver functions. Negative MLDs appear in only 50% of the receiver function stacks, indicating that negative MLDs are common but intermittent. The negative MLDs correspond to shear velocity drops of 1%–4%, which could be caused by layers of minerals created by metasomatism, although vertical layering in seismic anisotropy cannot be ruled out. In craton interiors, negative MLDs have a lower amplitude (<3% velocity drops) and can be explained by metasomatism of the original Archean mantle. Negative MLD amplitudes increase with decreasing upper mantle shear velocity (toward the outer margins of the cratons), but do not depend on the age of the craton. Thus, negative MLD amplitudes are not dominated by age‐related variations in the cratonic mantle composition, and, instead, are more strongly correlated with proximity to tectonic and metasomatic activity that occurred long after craton formation. Negative MLDs are less numerous among stations that have Paleoproterozoic and Archean thermotectonic ages, consistent with the view that shallow release of slab‐derived fluids during early “warm” subduction was less favorable for negative MLD formation. We also observe velocity gradients below 150 km at stations in craton boundaries and interiors, indicating the presence of seismic velocity changes at the cratonic lithosphere‐asthenosphere boundary and/or Lehmann discontinuity. 

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