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1. Expedition 391 Scientific Prospectus: Walvis Ridge Hotspot

   https://doi.org/10.14379/iodp.sp.391.2020  

   Sager, W. ; Hoernle, K. ; Petronotis, K. ( April 2020 , Scientific prospectus)

   null (Ed.)

   Walvis Ridge (WR) is a long-lived hotspot track that began with a continental flood basalt event at ~132 Ma during the initial opening of the South Atlantic Ocean. WR stretches ~3300 km to the active volcanic islands of Tristan da Cunha and Gough, and it was originally paired with Rio Grande Rise (RGR) oceanic plateau. Because of the duration of its volcanism and the length of its track, the Tristan-Gough hotspot forms the most pronounced bathymetric anomaly of all Atlantic hotspots. Its age progression, chemistry, and connection to flood basalts point to a lower mantle plume source, projected to be the hypothesized plume generation zone at the margin of the African large low shear-wave velocity province. The hotspot interacted with the Mid-Atlantic Ridge (MAR) during its early history, producing WR and RGR through plume-ridge interaction. Valdivia Bank, a WR plateau paired with the main part of RGR, represents heightened hotspot output and may have formed with RGR around a microplate, disrupting the expected hotspot age progression. After producing a relatively uniform composition from ~120 to ~70 Ma, WR split into three seamount chains with distinct isotopic compositions at about the time that the plume and MAR separated. With ~70 My spatial zonation, the hotspot displays the longest-lived geochemical zonation known. Currently at ~400 km width with young volcanic islands at both ends, the hotspot track is far wider than other major hotspot tracks. Thus, WR displays global extremes with respect to (1) width of its hotspot track, (2) longevity of zonation, (3) division into separate chains, and (4) plume-ridge interaction involving a microplate, raising questions about the geodynamic evolution of this hotspot track. Understanding WR is critical for knowledge of the global spectrum of plume systems. To test hypotheses about mantle plume zonation, plume activity around a microplate, and hotspot drift, we propose coring at six locations along the older ridge to recover successions of basaltic lava flows ranging in age from ~59 to 104 Ma. Samples will help us trace the evolution of geochemical and isotopic signatures as the hotspot track became zoned, offering vital clues about compositional changes of the plume source and important implications for understanding the origin of hotspot zonation. Dating will show the age progression of volcanism both at individual sites and along the ridge, testing whether WR formed as a strictly age-progressive hotspot track and whether Valdivia Bank formed as a plume pulse, extended volcanism around a microplate, or possibly even a continental fragment. Paleomagnetic data will track paleolatitude changes of the hotspot, testing whether hotspot drift or true polar wander, or both, explain changes in paleolatitude. 

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2. Nature and Origin of Magnetic Lineations Within Valdivia Bank: Ocean Plateau Formation by Complex Seafloor Spreading

   https://doi.org/10.1029/2023GL103415  

   Thoram, S. ; Sager, W. W. ; Gaastra, K. ; Tikoo, S. M. ; Carvallo, C. ; Avery, A. ; Del Gaudio, Arianna V. ; Huang, Y. ; Hoernle, K. ; Höfig, T. W. ; et al ( July 2023 , Geophysical Research Letters)

   Valdivia Bank (VB) is a Late Cretaceous oceanic plateau formed by volcanism from the Tristan‐Gough hotspot at the Mid‐Atlantic Ridge (MAR). To better understand its origin and evolution, magnetic data were used to generate a magnetic anomaly grid, which was inverted to determine crustal magnetization. The magnetization model reveals quasi‐linear polarity zones crossing the plateau and following expected MAR paleo‐locations, implying formation by seafloor spreading over ∼4 Myr during the formation of anomalies C34n‐C33r. Paleomagnetism and biostratigraphy data from International Ocean Discovery Program Expedition 391 confirm the magnetic interpretation. Anomaly C33r is split into two negative bands, likely by a westward ridge jump. One of these negative anomalies coincides with deep rift valleys, indicating their age and mechanism of formation. These findings imply that VB originated by seafloor spreading‐type volcanism during a plate reorganization, not from a vertical stack of lava flows as expected for a large volcano.

    

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3. Longest continuously erupting large igneous province driven by plume-ridge interaction

   https://doi.org/10.1130/G47850.1  

   Jiang, Qiang ; Jourdan, Fred ; Olierook, Hugo K.H. ; Merle, Renaud E. ; Whittaker, Joanne M. ( October 2020 , Geology)

   null (Ed.)

   Abstract Large igneous provinces (LIPs) typically form in one short pulse of ∼1–5 Ma or several punctuated ∼1–5 Ma pulses. Here, our 25 new 40Ar/39Ar plateau ages for the main construct of the Kerguelen LIP—the Cretaceous Southern and Central Kerguelen Plateau, Elan Bank, and Broken Ridge—show continuous volcanic activity from ca. 122 to 90 Ma, a long lifespan of >32 Ma. This suggests that the Kerguelen LIP records the longest, continuous high-magma-flux emplacement interval of any LIP. Distinct from both short-lived and multiple-pulsed LIPs, we propose that Kerguelen is a different type of LIP that formed through long-term interactions between a mantle plume and mid-ocean ridge, which is enabled by multiple ridge jumps, slow spreading, and migration of the ridge. Such processes allow the transport of magma products away from the eruption center and result in long-lived, continuous magmatic activity. 

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4. A Late Cretaceous‐Eocene Geomagnetic Polarity Timescale (MQSD20) That Steadies Spreading Rates on Multiple Mid‐Ocean Ridge Flanks

   https://doi.org/10.1029/2020JB020034  

   Malinverno, A. ; Quigley, K. W. ; Staro, A. ; Dyment, J. ( August 2020 , Journal of Geophysical Research: Solid Earth)

   Magnetic anomalies over mid‐ocean ridge flanks record the history of geomagnetic field reversals, and the width of magnetized crustal blocks can be combined with absolute dates to generate a Geomagnetic Polarity Timescale (GPTS). We update here the current GPTS for the Late Cretaceous‐Eocene (chrons C33–C13, ~84–33 Ma) by extending to several spreading centers the analysis that originally assumed smoothly varying spreading rates in the South Atlantic. We assembled magnetic anomaly tracks from the southern Pacific (23 ship tracks), the northern Pacific (35), the southern Atlantic (45), and the Indian Ocean (51). Tracks were projected onto plate tectonic flow lines, and distances to magnetic polarity block boundaries were estimated by fitting measured magnetic anomalies with a Monte Carlo algorithm that iteratively changed block model distances and anomaly skewness angles. Distance data from each track were then assembled in summary sets of block model distances over 13 ridge flank regions. We obtained a final MQSD20 GPTS with another Monte Carlo algorithm that iteratively perturbed ages of polarity chron boundaries to minimize the variability of spreading rates over all ridge flanks and fit an up‐to‐date set of radioisotopic dates. The MQSD20 GPTS highlights a major plate motion change at 50–45 Ma, when spreading rates decreased in the Indian Ocean as India collided with Eurasia while spreading rates increased in the South Atlantic and Northern Pacific and the Hawaii‐Emperor seamount chain changed its orientation.

    

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5. Izanagi-Pacific ridge subduction revealed by a 56 to 46 Ma magmatic gap along the northeast Asian margin

   https://doi.org/10.1130/G46778.1  

   Tsung-Jui Wu, Jeremy ; Wu, Jonny ( August 2019 , Geology)

   Abstract Recent studies have debated the timing and spatial configuration of a possible intersection between the Pacific-Izanagi spreading ridge and the northeast Asian continental margin during Cretaceous or early Cenozoic times. Here we examine a newly compiled magmatic catalog of ∼900 published Cretaceous to Miocene igneous rock radioisotopic values and ages from the northeast Asian margin for ridge subduction evidence. Our synthesis reveals that a near-synchronous 56–46 Ma magmatic gap occurred across ∼1500 km of the Eurasian continental margin between Japan and Sikhote-Alin, Russian Far East. The magmatic gap separated two distinct phases of igneous activity: (1) an older, Cretaceous to Paleocene pre–56 Ma episode that had relatively lower εNd(t) (−15 to + 2), elevated (87Sr/86Sr)0 (initial ratio, 0.704–0.714), and relatively higher magmatic fluxes (∼1090 km2/m.y.); and (2) a younger, late Eocene to Miocene post–46 Ma phase that had relatively elevated εNd(t) (−2 to + 10), lower (87Sr/86Sr)0 (0.702–0.707), and a lower 390 km2/m.y. magmatic flux. The 56–46 Ma magmatic gap links other geological evidence across northeast Asia to constrain an early Cenozoic, low-angle ridge-trench intersection that had profound consequences for the Eurasian continental margin, and possibly led to the ca. 53–47 Ma Pacific plate reorganization. 

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