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Dynamics of the Upper Mantle in Light of Seismic Anisotropy

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10 Dynamics of the Upper Mantle in Light of Seismic Anisotropy Thorsten W. Becker1,2 and Sergei Lebedev3 ABSTRACT Seismic anisotropy records continental dynamics in the crust and convective deformation in the mantle. Deci- phering this archive holds huge promise for our understanding of the thermo-chemical evolution of our planet, but doing so is complicated by incomplete imaging and non-unique interpretations. Here, we focus on the upper mantle and review seismological and laboratory constraints as well as geodynamic models of anisotropy within a dynamic framework. Mantle circulation models are able to explain the character and pattern of azimuthal ani- sotropy within and below oceanic plates at the largest scales. Using inferences based on such models provides key constraints on convection, including plate-mantle force transmission, the viscosity of the asthenosphere, and the net rotation of the lithosphere. Regionally, anisotropy can help further resolve smaller-scale convection, e.g., due to slabs and plumes in active tectonic settings. However, the story is more complex, particularly for continental lithosphere, and many systematic relationships remain to be established more firmly. More integrated approaches based on new laboratory experiments, consideration of a wide range of geological and geophysical constraints, as well as hypothesis-driven seismological inversions are required to advance to the next level. 10.1. INTRODUCTION unless noted otherwise. Anisotropy is a common property of mineral assemblages and appears throughout the Anisotropy of upper mantle rocks records the history of Earth, including in its upper mantle. There, anisotropy mantle convection and can be inferred remotely from seis- can arise due to the shear of rocks in mantle flow. As such, mology. Seismic anisotropy refers to the orientational it provides a unique link between seismological observa- dependence of propagation velocities for waves traveling tions and the evolution of our planet. However, given at different azimuths, or a difference in velocities for the need to resolve more parameters for an anisotropic waves that are polarized in the horizontal or vertical plane than for an isotropic solid, seismological models for ani- such as Love and Rayleigh waves, respectively. “Anisot- sotropy are more uncertain, and the interpretation and ropy” without any qualifier shall here refer to the seismic link to flow necessarily non-unique. kind caused by an anisotropic elastic stiffness tensor Our personal views of seismic anisotropy have oscil- lated from a near-useless can of worms to the most useful 1Institute for Geophysics, Jackson School of Geosciences, constraint on convection ever, and we strive to present a The University of Texas at Austin, Austin, USA more balanced view here. Anisotropy matters for all 2Department of Geological Sciences, Jackson School of layers of the Earth, and there exist a number of excellent Geosciences, The University of Texas at Austin, Austin, TX, USA reviews covering the rock record, seismological observa- 3Dublin Institute for Advanced Studies, Dublin, Ireland tions, and laboratory constraints (e.g., Nicolas and Mantle Convection and Surface Expressions, Geophysical Monograph 263, First Edition. Edited by Hauke Marquardt, Maxim Ballmer, Sanne Cottaar and Jasper Konter. © 2021 American Geophysical Union. Published 2021 by John Wiley & Sons, Inc. DOI: 10.1002/9781119528609.ch10 259 0005049850.3D 259 20/4/2021 10:56:50 AM 260 MANTLE CONVECTION AND SURFACE EXPRESSIONS Christensen, 1987; Silver, 1996; Savage, 1999; Mainprice, et al., 1969). It can be shown that the azimuth, φ, depend- 2007; Skemer and Hansen, 2016; Romanowicz and ence of P wave speed anomalies, δv, for small seismic ani- Wenk, 2017), as well as comprehensive treatments in text- sotropy at location x can be approximated by books (e.g., Anderson, 1989). Also, most of what was said δv φ, x ≈ A0 x + A1 x cos 2φ + A2 x in the overview of Long and Becker (2010) remains rele- (1) φ x φ x φ vant. However, here we shall focus our discussion on the sin 2 + A3 cos 4 + A4 sin 4 upper mantle within and underneath oceanic plates, the (Backus, 1965). The simplest form of azimuthal anisot- seemingly best understood part of mantle convection. ropy is due to the 2φ terms alone, and the corresponding We will highlight some of the insights afforded by seismic 180∘ periodic pattern seen in Morris et al.’s (1969) results, anisotropy within a convective context, and discuss for example (Figure 10.1). Based on such patterns, Hess selected open questions and how to possibly answer them. (1964) concluded that the oceanic lithosphere and upper- most mantle must have undergone convective flow and 10.2. OBSERVATIONS OF SEISMIC made the connection to seafloor spreading (Vine and ANISOTROPY Matthews, 1963). Anisotropy beneath continents was also detected, using A range of seismic observations show the presence of ani- both refraction and quarry-blast data (e.g., Bamford, sotropy in the Earth. In tomographic imaging of its three- 1977). More recently, Pn and Sn waves propagating from dimensional distribution, anisotropy must normally be earthquakes have been used for mapping azimuthal ani- resolved simultaneously with the isotropic seismic velocity sotropy in the uppermost mantle, just beneath the Moho heterogeneity, typically greater in amplitude. Substantial (e.g., Smith and Ekström, 1999; Buehler and Shearer, non-uniqueness of the solutions for anisotropy can arise 2010), where they form a connection between shallow, (e.g., Laske and Masters, 1998), and other sources of uncer- crustal anisotropy and the deeper mantle observations tainties include the treatment of the crust (e.g., Ferreira such as from SKS splitting which we discuss next. et al., 2010) and earthquake locations (Ma and Masters, 2015). In fact, the very existence of intrinsic anisotropy (e.g., due to lattice preferred orientation (LPO) of aniso- 1. 0 tropic mantle peridotite minerals) as opposed to apparent anisotropy (e.g., caused by layering of isotropic material of .8 different wave speeds; e.g., Backus, 1962) has been debated (Fichtner et al., 2013; Wang et al., 2013). .6 At least regionally, the occurrence of anisotropy is, of course, not really in doubt since different lines of seismic .4 evidence for it are corroborated by observations from mantle rocks (e.g., Ben Ismail and Mainprice, 1998; Mainprice, 2007). However, accurate determination of .2 anisotropy is clearly not straightforward. Models based on data of different types, or even of the same type, are 0 often difficult to reconcile and only agree on large spatial KM/SEC V, δ scales. Improvements in data sampling and anisotropy –.2 analysis methods are therefore subjects of active research, aimed at yielding more accurate and detailed information –.4 on the dynamics of the lithosphere and underlying mantle. In order to ground the dynamics discussion, we first –.6 address the scales of resolution and distribution of seismic anisotropy coverage that are currently available to guide –.8 global mantle circulation assessment. –1.0 0 40 80 120 160 200 240 280 320 360 Pn 10.2.1. Anisotropy AZIMUTH Historically, the detection of P wave anisotropy just Figure 10.1 Velocity deviation for Pn from the below the Moho from refraction experiments in the mean (vP = 8.159 km/s) in the central Pacific Pacific Ocean was important in terms of establishing N and NW of Hawaii as a function of the existence of seismic anisotropy in the upper mantle propagation azimuth along with a 2φ fit (eq. 1). and linking it to plate tectonics (e.g., Hess, 1964; Morris Source: Modified from Morris et al. (1969). 0005049850.3D 260 20/4/2021 10:57:16 AM UPPER MANTLE DYNAMICS IN LIGHT OF SEISMIC ANISOTROPY 261 10.2.2. Shear Wave Splitting The popular shear-wave splitting method yields a direct indication of anisotropy in the Earth (e.g., Savage, 1999). In the presence of azimuthal anisotropy, a shear wave Outer-core-traversing waves such as SKS and SKKS are pulse traveling into an anisotropic layer will be separated often used for the splitting measurements because they into two orthogonal pulses, one propagating within the can yield information on receiver side anisotropy; source medium’s fast polarization plane (containing its “fast effects are excluded because of the P to S conversion upon axis,” or fast-propagation azimuth), and the other within exiting the core. The advantages of the method are its ease the orthogonally oriented, slow propagation plane. At a of use and its high lateral resolution. Figure 10.2 shows seismic station, those split pulses will arrive separated the current distribution of teleseismic shear wave splitting by a delay time, δt, that is proportional to the integral of measurements with fairly dense sampling in most of the anisotropy strength and path length, assuming a uniform actively deforming continental regions. anisotropy orientation within the anisotropic layer (e.g., The main disadvantage of SKS splitting is its poor ver- Silver and Chan, 1988; Vinnik et al., 1989). Such “splitting” tical resolution; anisotropy may arise anywhere along the is akin to optical birefringence and observed for local shear path. In the presence of one dominant anisotropic layer wave arrivals in the shallow crust (δt ≲ 0.2 s) where it (say, the asthenosphere) with azimuthal anisotropy, the mainly reflects anisotropy due to aligned cracks, whose splitting parameters (delay times and fast azimuth) will opening is controlled by tectonic stresses (Crampin and characterize this layer directly. However, if multiple layers Chastin, 2003). For teleseismic shear waves, δt 1.2 s, with different fast axes or more complex types of anisot- on average, and the splitting measurements can be related ropy are present, the net splitting will depend nonlinearly to whole-crustal and mantle anisotropy (Vinnik et al., on backazimuth and the depth-variable anisotropy (e.g., 1992; Silver, 1996). SKS splitting due to anisotropic fabric Silver and Savage, 1994; Rümpker and Silver, 1998; Salt- within the crust is typically ≲ 0.3 s, much smaller than that zer et al., 2000).
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