North Arch volcanic fields near Hawaii are evidence favouring the restite‐root hypothesis for the origin of hotspot swells

[1] The lithosphere-asthenosphere boundary (LAB) separating the rigid lid from the underlying weaker, convecting asthenosphere is a fundamental interface in mantle dynamics and plate tectonics. However, the exact depth and defining mechanism of the LAB interface remain poorly understood. The ocean plates are ideal for testing hypotheses regarding the nature of a plate since they make up 70% of Earth's surface area and have a relatively simple geological history. Seismically imaging the oceanic LAB at high resolution has proved challenging. Yet, several studies have recently increased resolution with provocative results. We summarize recent imaging of discontinuity structure beneath much of the Pacific using receiver functions from ocean floor borehole seismometers and land stations located at ocean-continent margins, SS precursors, and waveform modeling of multiple phases including multiple bounce S waves, ScS reverberations, and surface waves. Overall, there is much agreement among these different approaches about the reported depth of a negative discontinuity that occurs near the expected depth of the LAB. Some of the apparent discrepancies in depth are explained by the variation in sensitivity of seismic waves that sample structure at different wavelengths. Yet, when the results are considered together, no single age-depth relationship is illuminated. There are also puzzling discrepancies in where the discontinuity is detected, which again suggests greater complexity. Here we test the possibility that discrepant detection of a strong sharp discontinuity is caused by anisotropic structure. We stack SS waveforms with bounce points in the central Pacific into azimuthal bins. We use two methods, one that inverts for discontinuity structure based on subtle variations in the character of the SS waveform, and another that considers SS at higher frequency. We find azimuthal variation in the amplitude of the waveform, including a polarity reversal. We suggest that anisotropy is an important factor in imaging and constraining discontinuity structure of the oceanic plate, and must be carefully considered to constrain the age-depth dependence and defining mechanism of the oceanic lithosphere.

Section: Previous Resultsmentioning

confidence: 99%

The Pacific lithosphere‐asthenosphere boundary: Seismic imaging and anisotropic constraints from SS waveforms

Rychert¹,

Schmerr²,

Harmon³

2012

“…More likely, episodes of increased thermal plume anomalies explain the observed lithospheric thickness variations. Periods of increased plume temperatures or flux could cause increased melting depths that are reflected in thicker regions of depletion and dehydration, and also thicker tectonic plates (Ito, ; Yamamoto & Morgan, ). Alternatively, propagation of small scale convective instabilities could periodically increase upwelling (Martinez & Hey, ).…”

Section: Discussionmentioning

confidence: 99%

Seismic Imaging of Thickened Lithosphere Resulting From Plume Pulsing Beneath Iceland

Rychert

Harmon

Armitage

2018

Ocean plates conductively cool and subside with seafloor age. Plate thickening with age is also predicted, and hot spots may cause thinning. However, both are debated and depend on the way the plate is defined. Determining the thickness of the plates along with the process that governs it has proven challenging. We use S‐to‐P (Sp) receiver functions to image a strong, persistent LAB beneath Iceland where the mid‐Atlantic Ridge interacts with a plume with hypothesized pulsating thermal anomaly. The plate is thickest, up to 84 ± 6 km, beneath lithosphere formed during times of hypothesized hotter plume temperatures and as thin as 61 ± 6 km beneath regions formed during colder intervals. We performed geodynamic modeling to show that these plate thicknesses are inconsistent with a thermal lithosphere. Instead, periods of increased plume temperatures likely increased the melting depth, causing deeper depletion and dehydration, and creating a thicker plate. This suggests plate thickness is dictated by the conditions of plate formation.

“…The Sp receiver‐function study by Rychert et al () identifies a velocity interface at 100–150‐km depth that they identify with the onset of melting as the hot plume rises through the pressure‐dependent solidus. In geodynamical modeling, the idealized plume models of Ribe and Christensen (, ) and Yamamoto and Morgan () can explain many features of Hawaiian swell bathymetry and island volcanism. Later studies of plume bathymetry have confirmed the general consistency of the plume model in terms of buoyancy flux and dynamic topography (Crosby & McKenzie, ; King & Adam, ; Wessel, ).…”

Section: Paradigm Shootout: Plume Plutonism Versus Metasomatismmentioning

confidence: 99%

“…Diffuse magmatic infiltration that extends laterally for hundreds of kilometers across the Hawaiian hot spot swell would raise the temperature of the lithosphere toward its solidus, in conflict with seismic tomography of the swell region (e.g., Laske et al, ). In addition, it seems difficult to reconcile broad and diffuse magma ascent beneath the Hawaiian swell with the localized eruption of the North Arch basalts on the swell, as described by Yamamoto and Morgan ().…”

Section: Paradigm Shootout: Plume Plutonism Versus Metasomatismmentioning

confidence: 99%

Why Is Crustal Underplating Beneath Many Hot Spot Islands Anisotropic?

Park

Rye

2019

High‐frequency harmonic regression (2.0–4.0‐Hz cutoff) of receiver functions at two long‐running seismic observatories at mid‐Pacific hot spot islands confirms earlier detections of underplated material with seismic velocities intermediate to crust and mantle, and reveals it to be multilayered and anisotropic within ~30 km of the surface. Magmatic underplating beneath the oceanic Moho has been proposed to accompany basaltic melt that erupts at the seafloor and (eventually) atop a subaerial volcano. An alternate hypothesis is “metasomatic underplating” whereby crustal fractures developed during magma ascent allow seawater to infiltrate and to serpentinize the sub‐Moho mantle partially. Metasomatic underplating would lower seismic wave speeds, promote the buoyancy of the hot spot swell, and induce textural anisotropy as metamorphic expansion of olivine‐rich peridotite promotes a crack network along which serpentinization spreads. Differential expansion of mantle peridotite and crustal gabbro promotes cracks in the crust that offer new pathways for seawater to descend to the Moho, allowing metasomatic underplating to expand laterally and to contribute anisotropy to the underplated layer. Rare serpentinized mantle xenoliths confirm that crack textures can develop during serpentinization at depth. The discovery of iron‐oxidizing microbial mats on the seafloor flank of the Loihi volcano, and many locations of diffuse low‐temperature venting worldwide, is consistent with the circulation of metasomatic fluids with reducing chemistry, sourced from serpentinization at depth. Posteruptive uplift of Santa Maria Island (Azores), and asymmetry of the Hawaiian swell, suggests that underplating requires 2–4 Myr to complete, suggesting that fluid infiltration is slow, subject to cycles of blockage and fresh fracturing.