Implications for continental structure and evolution from seismic anisotropy

Silver, Paul G.; Chan, W. W.

doi:10.1038/335034a0

Cited by 473 publications

(281 citation statements)

References 30 publications

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“…3) perpendicular to the mechanically weak directions (from Fig. 6) as expected from the fossil anisotropy model [12]? Shown is the percentage of fast axes that match the weak direction to within 15 ‡ (black) and, where di¡erent, to within 22.5 ‡ (gray), based on the long-wavelength (A; data from Fig.…”

Section: Discussionmentioning

confidence: 61%

Seismic and mechanical anisotropy and the past and present deformation of the Australian lithosphere

Simons

Hilst

2003

Earth and Planetary Science Letters

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We interpret the three-dimensional seismic wave-speed structure of the Australian upper mantle by comparing its azimuthal anisotropy to estimates of past and present lithospheric deformation. We infer the fossil strain field from the orientation of gravity anomalies relative to topography, bypassing the need to extrapolate crustal measures, and derive the current direction of mantle deformation from present-day plate motion. Our observations provide the depth resolution necessary to distinguish fossil from contemporaneous deformation. The distribution of azimuthal seismic anisotropy is determined from multi-mode surface-wave propagation. Mechanical anisotropy, or the directional variation of isostatic compensation, is a proxy for the fossil strain field and is derived from a spectral coherence analysis of digital gravity and topography data in two wavelength bands. The joint interpretation of seismic and tectonic data resolves a rheological transition in the Australian upper mantle. At depths shallower than V150^200 km strong seismic anisotropy forms complex patterns. In this regime the seismic fast axes are at large angles to the directions of principal shortening, defining a mechanically coupled crust^mantle lid deformed by orogenic processes dominated by transpression. Here, seismic anisotropy may be considered 'frozen', which suggests that past deformation has left a coherent imprint on much of the lithospheric depth profile. The azimuthal seismic anisotropy below V200 km is weaker and preferentially aligned with the direction of the rapid motion of the Indo-Australian plate. The alignment of the fast axes with the direction of present-day absolute plate motion is indicative of deformation by simple shear of a dry olivine mantle. Motion expressed in the hot-spot reference frame matches the seismic observations better than the no-net-rotation reference frame. Thus, seismic anisotropy supports the notion that the hot-spot reference frame is the most physically reasonable. Independently from plate motion models, seismic anisotropy can be used to derive a best-fitting direction of overall mantle shear. ß

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Section: Discussionmentioning

confidence: 61%

Seismic and mechanical anisotropy and the past and present deformation of the Australian lithosphere

Simons

Hilst

2003

Earth and Planetary Science Letters

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“…Instead of two separate split peaks on the radial component, the pulse width causes the arrival of the radial Ps phase to broaden and to vary in arrival time. The transverse phases are smaller and closer together and look more like the typical derivative of the radial function that is observed in long-period splitting studies [e.g., Silver and Chan 1988;Vinnik et al 1989]. With typical noise, the transverse energy would be difficult to observe.…”

Section: Characteristics Of Receiver Functions Traveling Through Anismentioning

confidence: 95%

Lower crustal anisotropy or dipping boundaries? Effects on receiver functions and a case study in New Zealand

Savage¹

1998

J. Geophys. Res.

283

275

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Abstract. We examine the effects on receiver functions of transverse anisotropy and of dipping isotropic boundaries. Splitting of the Moho Ps phase predicts the anisotropy from a postulated 10-km-thick layer of highly anisotropic crustal material only when other phases can be well isolated and when allowance is made for the rotation of the incident energy out of the plane of energy propagation expected for isotropic models. We examine azimuthal variations of the synthetic radial and transverse receiver functions. For both transversely anisotropic layers and dipping isotropic boundaries, radial receiver functions are symmetric and transverse receiver functions are antisymmetric about a given back azimuth (the horizontal projection of the axis of symmetry or the dip direction). Differences include the following: (1) Transverse energy from dipping boundaries arrives at the time of the initial P phase no matter at what depth the dipping boundaries occur, while transverse energy arrives at the time of the initial P phase for anisotropic material only when the anisotropy is at the surface. (2) Transversely anisotropic systems with horizontal symmetry axes have waveforms with 180 ø periodicity as a function of back azimuth, while dipping symmetry axes or dipping boundaries just have 360 ø periodicity. A consequence of the symmetry is that stacking the initial P and Ps phases from transverse receiver functions for back azimuths that are 180 ø apart tends to decrease the arrivals from shallowly dipping isotropic boundaries and doubles the arrivals from anisotropic layers with horizontal symmetry axes. We examine receiver functions for station SNZO in New Zealand, above a subducting plate dipping at 15 ø beneath the station. Opposite polarities of transverse receiver functions separated by 180 ø in back azimuth suggest that either dipping isotropic layers or dipping axes of symmetry of anisotropic layers are present. The 15 ø dip of the subduction zone is insufficient to explain the energy on the transverse receiver functions by the isotropic structures previously determined from refraction and earthquake travel time inversion. However, modifying the isotropic models for the region by anisotropy determined via shear wave splitting measurements and refraction studies, allows many of the features of both the radial and transverse receiver functions to be explained. A relatively highly anisotropic layer about 7 km thick at the base of the crust with an axis of symmetry dipping at 20 ø -40 ø may be caused by metamorphosed oceanic crust.

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“…The actual quantity measured on individual seismograms in the splitting intensity, S, which is defined as the amplitude of the transverse component relative to the time derivative of the radial component; this quantity can be measured either by simple projection of the components or by a singular value decomposition (SVD) procedure (Chevrot 2000). In particular, the predicted radial and transverse components for a vertically propagating shear wave that has undergone passage through a single layer of anisotropy with a horizontal axis of transversely isotropic (TI) symmetry can be written at long period (dt ( T) as (Silver and Chan 1988;Vinnik et al 1989b;Chevrot 2000):…”

Section: The Cross-correlation Methodsmentioning

confidence: 99%

“…The splitting parameters, /, dt, are measured from seismic records; these correspond to the orientation of the fast quasi-S phase and the time delay between the fast and slow components, respectively. Since early studies by, e.g., Keith and Crampin (1977), Kosarev et al (1979), Ando et al (1983), Vinnik et al (1984), Fukao (1984), and Silver and Chan (1988) shear wave splitting has emerged as a popular tool for characterizing anisotropy in the Earth, most notably in the crust, upper mantle, and in the D 00 region directly above the core-mantle boundary (CMB). With the increasing availability of broadband seismic data, there are now hundreds of published studies that examine shear wave splitting and interpret it in terms of mantle anisotropy.…”

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confidence: 99%

Shear Wave Splitting and Mantle Anisotropy: Measurements, Interpretations, and New Directions

2009

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Measurements of the splitting or birefringence of seismic shear waves that have passed through the Earth's mantle yield constraints on the strength and geometry of elastic anisotropy in various regions, including the upper mantle, the transition zone, and the D 00 layer. In turn, information about the occurrence and character of seismic anisotropy allows us to make inferences about the style and geometry of mantle flow because anisotropy is a direct consequence of deformational processes. While shear wave splitting is an unambiguous indicator of anisotropy, the fact that it is typically a near-vertical path-integrated measurement means that splitting measurements generally lack depth resolution. Because shear wave splitting yields some of the most direct constraints we have on mantle flow, however, understanding how to make and interpret splitting measurements correctly and how to relate them properly to mantle flow is of paramount importance to the study of mantle dynamics. In this paper, we review the state of the art and recent developments in the measurement and interpretation of shear wave splitting-including new measurement methodologies and forward and inverse modeling techniques,-provide an overview of data sets from different tectonic settings, show how they help us relate mantle flow to surface tectonics, and discuss new directions that should help to advance the shear wave splitting field.

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Implications for continental structure and evolution from seismic anisotropy

Cited by 473 publications

References 30 publications

Seismic and mechanical anisotropy and the past and present deformation of the Australian lithosphere

Seismic and mechanical anisotropy and the past and present deformation of the Australian lithosphere

Lower crustal anisotropy or dipping boundaries? Effects on receiver functions and a case study in New Zealand

Shear Wave Splitting and Mantle Anisotropy: Measurements, Interpretations, and New Directions

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