The Cascadia Paradox: Mantle flow and slab fragmentation in the Cascadia subduction system

Long, Maureen D.

doi:10.1016/j.jog.2016.09.006

Cited by 41 publications

(53 citation statements)

References 119 publications

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“…The fast axis orientations also exhibit complex short‐wavelength patterns and correlate well with the trajectories of tectonic provinces. For instance, at a depth of 50 km, model

{US}_{32}

includes a west‐east fast axis orientation perpendicular to the trench of the Cascadia Subduction Zone, reflecting the development of 2‐D poloidal‐mode mantle flows induced by the descending Juan de Fuca and Gorda slabs (Currie et al, ; Hall et al, ; Long, ). This strong anisotropy persists down to 100 km.…”

Section: Resultsmentioning

confidence: 99%

Azimuthal Anisotropy of the North American Upper Mantle Based on Full Waveform Inversion

Zhu

Yang

2020

JGR Solid Earth

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A new azimuthal anisotropy model for the North American and Caribbean Plates, namely, US32, is constructed based on full waveform inversion and records from the USArray and other temporary/permanent networks deployed in the study region. A total of 180 earthquakes and 4,516 seismographic stations are employed in the inversion to simultaneously constrain radially and azimuthally anisotropic model parameters: L, N, Gc, and Gs, within the crust and mantle. Thirty‐two preconditioned conjugate gradient iterations have been utilized to minimize frequency‐dependent phase discrepancies between observed and predicted seismograms for three‐component short‐period (15–40 s) body waves and long‐period (25–100 s) surface waves. Model US32 exhibits complicated variations in anisotropic fabrics underneath the western and eastern United States, especially at depths shallower than 100 km. For instance, the fast axis orientations in model US32 suggest the presence of trench‐perpendicular mantle flows underneath the Cascadia Subduction Zone and also follow the strikes of the Snake River Plain, the Ouachita Orogenic Front, and the Grenville and Appalachian Orogenic Belts. The amplitudes of azimuthal anisotropy reduce to around 1% at depths greater than 200 km, and the orientations are subparallel to the global plate motion directions to the east of the Rocky Mountain, except for large discrepancies in central and eastern Canada. At a depth of 700 km, the fast axes change along the trajectory of the Farallon slab underneath the Great Lakes region and Gulf of Mexico, which might indicate the development of 2‐D poloidal‐mode mantle flows perpendicular to the strike of the sinking slab within the uppermost lower mantle. Comparisons between model US32 with a western U.S. model from ambient noise tomography and SKS splitting measurements demonstrate a relatively good agreement for the fast axis orientations, considering the usage of different data sets and imaging techniques. However, the absolute magnitude of azimuthal anisotropy in model US32 might be underestimated, especially at greater depths, given the poor agreement on the amplitudes of predicted and observed SKS splitting times. At the current stage, the agreement among different azimuthal anisotropy models at global and continental scales is still poor even for the United States with a dense station coverage.

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“…The fast axis orientations also exhibit complex short‐wavelength patterns and correlate well with the trajectories of tectonic provinces. For instance, at a depth of 50 km, model

{US}_{32}

Section: Resultsmentioning

confidence: 99%

Azimuthal Anisotropy of the North American Upper Mantle Based on Full Waveform Inversion

Zhu

Yang

2020

JGR Solid Earth

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“…Another intriguing feature of the Cascadia subduction zone is the puzzling nature of the surrounding mantle flow field, particularly the extent of toroidal versus entrained mantle flow in response to slab rollback (for a recent review on the topic of the “Cascadia Paradox,” see Long, ). In subduction dynamics, mantle flow is considered to follow an entrained flow pattern when it is coupled with and moving in the same direction as the downgoing slab.…”

Section: Introductionmentioning

confidence: 99%

“…In this scenario there is a component of subslab mantle flow that is trench parallel, as opposed to purely trench normal in the entrained case. (We refer the reader to Figure 3 of Long () if further explanation is required). One of the most direct methods by which to map the mantle flow patterns is by studying seismic anisotropy (i.e., the directional dependence of seismic wave speed).…”

Section: Introductionmentioning

confidence: 99%

SKS Splitting Beneath Mount St. Helens: Constraints on Subslab Mantle Entrainment

Eakin

Wirth

Wallace

et al. 2019

Geochem Geophys Geosyst

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Observations of seismic anisotropy can provide direct constraints on the character of mantle flow in subduction zones, critical for our broader understanding of subduction dynamics. Here we present over 750 new SKS splitting measurements in the vicinity of Mount St. Helens in the Cascadia subduction zone using a combination of stations from the iMUSH broadband array and Cascades Volcano Observatory network. This provides the highest density of splitting measurements yet available in Cascadia, acting as a focused “telescope” for seismic anisotropy in the subduction zone. We retrieve spatially consistent splitting parameters (mean fast direction Φ: 74°, mean delay time ∂t: 1.0 s) with the azimuthal occurrence of nulls in agreement with the fast direction of splitting. When averaged across the array, a 90° periodicity in splitting parameters as a function of back azimuth is revealed, which has not been recovered previously with single‐station observations. The periodicity is characterized by a sawtooth pattern in Φ with a clearly defined 45° trend. We present new equations that reproduce this behavior based upon known systematic errors when calculating shear wave splitting from data with realistic seismic noise. The corrected results suggest a single layer of anisotropy with an ENE‐WSW fast axis parallel to the motion of the subducting Juan de Fuca plate; in agreement with predictions for entrained subslab mantle flow. The splitting pattern is consistent with that seen throughout Cascadia, suggesting that entrainment of the underlying asthenosphere with the subducting slab is coherent and widespread.

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“…Evidence for poloidal/toroidal mantle flow is supplied by different geological, geophysical, and geochemical methods. Seismological and geodetic observations provide indirect constraints on short-term mantle circulations through present-day surface motions (e.g., Palano et al, 2017) and shear-wave splitting analysis (e.g., Díaz & Gallart, 2014;Long, 2016;Long & Silver, 2008). Magmatism and basalt geochemistry in terms of major and trace elements, as well as isotopic compositions, are also used to obtain information about mantle flow linked to subduction (Mullen & Weis, 2015).…”

Section: Introductionmentioning

confidence: 99%

Opposite Subduction Polarity in Adjacent Plate Segments

et al. 2018

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Active and fossil subduction systems consisting of two adjacent plates with opposite retreating directions occur in several areas on Earth, as the Mediterranean or Western Pacific. The goal of this work is to better understand the first‐order plate dynamics of these systems using the results of experimental models. The laboratory model is composed of two separate plates made of silicon putty representing the lithosphere, on top of a tank filled with glucose syrup representing the mantle. The set of experiments is designed to test the influence of the width of plates and the initial separation between them on the resulting trench velocities, deformation of plates, and mantle flow. Results show that the mantle flow induced by both plates is asymmetric relative to the axis of each plate causing a progressive merging of the toroidal cells that prevents a steady state phase of the subduction process and generates a net outward drag perpendicular to the plates. Trench velocities increase when trenches approach each other and decrease when they separate after their intersection. The trench curvature of both plates increases linearly with time during the entire evolution of the process regardless their width and initial separation. The interaction between the return flows associated with each retreating plate, particularly in the interplate region, is stronger for near plate configurations and correlates with variations of rollback velocities. We propose that the inferred first‐order dynamics of the presented analog models can provide relevant clues to understand natural complex subduction systems.

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The Cascadia Paradox: Mantle flow and slab fragmentation in the Cascadia subduction system

Cited by 41 publications

References 119 publications

Azimuthal Anisotropy of the North American Upper Mantle Based on Full Waveform Inversion

Azimuthal Anisotropy of the North American Upper Mantle Based on Full Waveform Inversion

SKS Splitting Beneath Mount St. Helens: Constraints on Subslab Mantle Entrainment

Opposite Subduction Polarity in Adjacent Plate Segments

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