Seismic anisotropy beneath Cascadia and the Mendocino triple junction: Interaction of the subducting slab with mantle flow

Eakin, C. M.; Obrebski, Mathias; Allen, R. M.; Boyarko, D. C.; Brudzinski, M. R.; Porritt, R. W.

doi:10.1016/j.epsl.2010.07.015

Cited by 69 publications

(78 citation statements)

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“…On the global scale the subslab anisotropy pattern appears to be dominated by trench‐parallel fast directions [ Long and Silver , , ]. There are notable exceptions, however, such as Cascadia [ Currie et al ., ; Eakin et al ., ], Alaska [ Christensen and Abers , ; Hanna and Long , ], and the Chilean flat slab [ Anderson et al ., ], which also display trench‐perpendicular ϕ beneath the slabs similar to what we observed beneath the Peruvian flat slab. These regional exceptions are all occurring beneath slabs with relatively shallow dips, as noted by Long and Silver [2009] and Song and Kawakatsu [].…”

Section: Discussionsupporting

confidence: 77%

Complex anisotropy beneath the Peruvian flat slab from frequency‐dependent, multiple‐phase shear wave splitting analysis

Eakin

Long

2013

JGR Solid Earth

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[1] Flat or shallow subduction is a relatively widespread global occurrence, but the dynamics remain poorly understood. In particular, the interaction between flat slabs and the surrounding mantle flow has yet to be studied in detail. Here we present measurements of seismic anisotropy to investigate mantle flow beneath the Peruvian flat-slab segment, the largest present-day region of flat subduction. We conduct a detailed shear wave splitting analysis at a long-running seismic station (NNA) located near Lima, Peru. We present measurements of apparent splitting parameters (fast direction ϕ and delay time δt) for SKS, ScS, and local S phases from 80 events. We observe well-defined frequency dependence and backazimuthal variability, indicating the likely presence of complex anisotropy. Forward modeling the observations with two or three layers of anisotropy reveals a likely layer with a trench-normal fast direction underlying a layer with a more trench-oblique (to trench-subparallel) fast direction. In order to further constrain the anisotropic geometry, we analyzed the source-side splitting from events originating within the slab measured at distant stations. Beneath the flat-slab segment, we found trench-normal fast splitting directions in the subslab mantle, while within the dipping portion of the slab further to the east, likely trench-subparallel anisotropy within the slab itself. This subslab pattern contradicts observations from elsewhere in South America for "normal" (i.e., more steeply dipping) slab conditions. It is similar, however, to inferences from other shallowly dipping subduction zones around the world. While there is an apparent link between slab dip and the surrounding mantle flow, at least beneath Peru, the precise nature of the relationship remains to be clarified.

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Section: Discussionsupporting

confidence: 77%

Complex anisotropy beneath the Peruvian flat slab from frequency‐dependent, multiple‐phase shear wave splitting analysis

Eakin

Long

2013

JGR Solid Earth

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“…To the east of the HLP, in western Montana and eastern Idaho, there is a transition from E‐W fast directions to NE‐SW directions, which are roughly parallel to the absolute motion of the North American plate and are likely due to shear in the asthenosphere caused by plate motion [ Fouch and West , 2008]. To the south of the HLP and the Mendocino Triple Junction, splitting patterns are more complex, leading to suggestions of flow around the southern edge of the Juan de Fuca slab [e.g., Eakin et al , 2010], Juan de Fuca slab‐driven toroidal flow throughout much of the western U.S. [e.g., Zandt and Humphreys , 2008; Fouch and West , 2008], and mantle downwelling associated with a well‐documented minimum in delay times in central Nevada [ West et al , 2009].…”

Section: Seismologic Constraintsmentioning

confidence: 99%

“…Black bars are from the studies of Fouch and West [2008] and Long et al [2009]. Gray bars indicate results from other authors, as compiled by Eakin et al [2010].…”

Section: Seismologic Constraintsmentioning

confidence: 99%

Mantle dynamics beneath the Pacific Northwest and the generation of voluminous back‐arc volcanism

Long

Till

Druken

et al. 2012

Geochem Geophys Geosyst

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[1] The Pacific Northwest (PNW) has a complex tectonic history and over the past $17 Ma has played host to several major episodes of intraplate volcanism. These events include the Steens/Columbia River flood basalts (CRB) and the striking spatiotemporal trends of the Yellowstone/Snake River Plain (Y/SRP) and High Lava Plains (HLP) regions. Several different models have been proposed to explain these features, which variously invoke the putative Yellowstone plume, rollback and steepening of the Cascadia slab, extensional processes in the lithosphere, or a combination of these. Here we integrate seismologic, geodynamic, geochemical, and petrologic results from the multidisciplinary HLP project and associated analyses of EarthScope USArray seismic data to propose a conceptual model for post-20 Ma mantle dynamics beneath the PNW and the relationships between mantle flow and surface tectonomagmatic activity. This model invokes rollback subduction as the main driver for mantle flow beneath the PNW beginning at $20 Ma. A major pulse of upwelling due to slab rollback and upper plate extension and consequent melting produced the Steens/CRB volcanism, and continuing trench migration enabled mantle upwelling and hot, shallow melting beneath the HLP. An additional buoyant mantle upwelling is required to explain the Y/SRP volcanism, but subduction-related processes may well have played a primary role in controlling its timing and location, and this upwelling likely continues today in some form. This conceptual model makes predictions that are broadly consistent with seismic observations, geodynamic modeling experiments, and petrologic and geochemical constraints.

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“…Large lateral gradients of anisotropy, as observed in shear-wave splitting in the western United States (Wang et al, 2008;Zandt and Humphreys, 2008;Long et al, 2009;West et al, 2009;Eakin et al, 2010), consist of a broad wavenumber spectrum that should induce significant coupling among normal modes. Such coupling should create impulsive QL waves with a broad frequency content.…”

Section: Introductionmentioning

confidence: 98%

An Approximation to the General Galerkin Method of Computing Coupled-Mode Synthetic Seismograms

Rieger

Park

2014

Bulletin of the Seismological Society of America

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Quasi-Love (QL) scattered surface waves are the result of lateral gradients of seismic anisotropy in Earth's upper mantle. These polarization anomalies can be forward modeled in the context of normal-mode coupling. However, methods for computing coupled-mode synthetic seismograms, such as the general Galerkin method, are computationally intensive, especially in the case of 3D structure. In this report, we present an approximation of the general Galerkin method of computing coupled-mode synthetic seismograms. The approximation replaces the formal eigenvector decomposition of the elastic potential energy coupling matrix V in the general Galerkin method with a piecewise approach. The approximation uses the closed-form eigenvectors of 2 × 2 coupling matrices V kj , which represent the interactions between two individual singlets s k and s j , to form the eigenvectors of V. The approximation is benchmarked against the exact general Galerkin method using zonally symmetric anisotropic structure. We find the approximation successfully reproduces the QL waveforms with a slight underestimate of the amplitude. By limiting the eastwest extent of the zonally symmetric structure, we extend the approximation to a 3D structure. We find that the sharp lateral gradients in anisotropy created by the northsouth running boundaries of the 3D structure contribute to the observed scattering until they are outside the sensitivity zone as defined by Ritzwoller et al. (2002). This demonstrates the sensitivity of QL surface-wave scattering to off-path structure. We also explicitly confirm the path-integral asymptotics that in the limit of smooth offpath structure, QL surface-wave scattering can be represented via the path integral along the source-receiver great circle path rather than the volume integral over Earth.

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Seismic anisotropy beneath Cascadia and the Mendocino triple junction: Interaction of the subducting slab with mantle flow

Cited by 69 publications

References 50 publications

Complex anisotropy beneath the Peruvian flat slab from frequency‐dependent, multiple‐phase shear wave splitting analysis

Complex anisotropy beneath the Peruvian flat slab from frequency‐dependent, multiple‐phase shear wave splitting analysis

Mantle dynamics beneath the Pacific Northwest and the generation of voluminous back‐arc volcanism

An Approximation to the General Galerkin Method of Computing Coupled-Mode Synthetic Seismograms

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