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Chapter 20. Fabric of the Mantle

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Chapter 20. Fabric of the Mantle Chapter 20 Fabric of the mantle A nisotropy this edition. There was a very large section on anisotropy since it was a relatively new concept And perpendicular now and now to seismologists. There was also a large section devoted to the then-novel thesis that seismic transverse, Pierce the dark soil and as velocities were not independent of frequency, they pierce and pass Make bare the and that anelasticity had to be allowed for in esti­ secrets of the Earth's deep heart. mates of mantle temperatures. Earth scientists no longer need to be convinced that anisotropy Shelley, Prometheus Unbound and anelasticity are essential elements in Earth physics, but there may still be artifacts in tomo­ Anisotropy is responsible for large variations in graphic models or in estimates of errors that seismic velocities; changes in the orientation of are caused by anisotropy. At the time of the mantle minerals, or in the direction of seismic first edition of this book - 1989 - the Earth waves, cause larger changes in velocity than can was usually assumed to be perfectly elastic and be accounted for by changes in temperature, isotropic to the propagation of seismic waves. composition or mineralogy. Plate-tectonic pro­ These assumptions were made for mathemati­ cesses, and gravity, create a fabric in the mantle. cal and operational convenience. The fact that Anisotropy can be microscopic - orientation of a large body of seismic data can be satisfacto­ crystals - or macroscopic - large-scale lamina­ rily modeled with these assumptions does not tions or oriented slabs and dikes. Discussions of prove that the Earth is isotropic or perfectly velocity gradients, both radial and lateral, and elastic. There is often a direct trade-off between chemistry and mineralogy of the mantle must anisotropy and heterogeneity, and between fre­ allow for the presence of anisotropy. Anisotropy quency dependence and depth dependence of is not a second-order effect. Seismic data that are seismic velocities. An anisotropic structure can interpreted in terms of isotropic theory can lead have characteristics, such as travel times, normal­ to models that are not even approximately cor­ mode frequencies and dispersion curves, that are rect. Slab anisotropy can cause artifacts in tomo­ identical, or similar, to a different isotropic struc­ graphic models. A wealth of new infonnation ture. A layered solid, for example, composed of regarding mantle structure, history, mineralogy isotropic layers that are thin compared to a seis­ and flow is becoming available as the anisotropy mic wavelength will behave as an anisotropic of the mantle is becoming better understood. solid - the velocity of propagation depends on direction. The effective long-wavelength elastic Introduction constants depend on the thicknesses and elastic properties of the individual layers. The reverse is In the first edition of Th eory of the Earth also true; an anisotropic solid with these same there were sections that are largely missing in elastic constants can be modeled exactly as a ORIGIN OF MANTLE ANISOTROPY 257 stack of isotropic layers. The same holds true for isotropic inversions of Love and Rayleigh waves an isotropic solid permeated by oriented cracks (pseudo-isotropic inversions). This is not a or aligned inclusions. This serves to illustrate the valid procedure. There is a trade-off between trade-off between heterogeneity and anisotropy. anisotropy and structure. In particular, the very Not all anisotropic structures, however, can be low upper-mantle shear velocities, 4.0-4.2 km./s. modeled by laminated solids. found by many isotropic and pseudo-isotropic The crystals of the mantle are anisotropic, inversions, are not a characteristic of models and rocks from the mantle show that these crys­ resulting from full anisotropic inversion. The P­ tals exhibit a high degree of alignment. There is wave anisotropy makes a significant contribution also evidence that crystal alignment is uniform to Rayleigh wave dispersion. This must also be over large areas of the upper mantle. At man­ allowed for in tomographic surface wave inver­ tle temperatures, crystals tend to be easily recrys­ sions, but seldom is. tallized and aligned by the prevailing stress and Since intrinsic anisotropy requires both flow fields. But there may also be large-scale fab­ anisotropic crystals and preferred orientation. ric in the upper mantle, caused by orientation of the anisotropy of the mantle contains informa­ subducted slabs or dikes, for example. tion about the mineralogy and stress gradients. The effects of anisotropy are often subtle and, For example, olivine, the most abundant upper­ if unrecognized, are usually modeled as inhomo­ mantle mineral, is extremely anisotropic for both geneities, for example, as layering or gradients. P-wave and S-wave propagation. It is readily ori­ The most obvious manifestations of anisotropy ented by recrystallization in the ambient stress are: field. Olivine-rich outcrops show a consistent pre­ ferred orientation over large areas. In general, the (1) [s h ear-wave splitting] or birefringence seismically fast axes of olivine are in the plane - the two polarizations of S-waves arrive at of the flow, with the a-axis, the fastest direc­ different times; tion, pointing in the direction of flow. The b­ (2) [azimut h a l ani sot ropy] - the arrival axis, the minimum velocity direction, is gener­ times, or apparent velocities of seismic waves ally normal to the flow plane, or vertical. Pyrox­ at a given distance from an event, depend on enes are also very anisotropic. The magnitude of azimuth; and the anisotropy in the mantle is comparable to (3) an apparent discrepancy between Love that found in ultramafic rocks (Figure 20.1). Soft waves and Rayleigh waves [Love Rayleigh layers or oriented fluid-filled cracks also give an discrepan cy]. apparent anisotropy. Much seismic data that are used in upper-mantle modeling are averages over Even these are not completely unambiguous indi­ several tectonic provinces or over many azilnuths. cators of anisotropy. Effects such as P to S conver­ Az imut h a l a nisotropy may therefore be aver­ sion, dipping interfaces, attenuation, and density aged out, but differences between vertical and variations must be properly taken into account. horizontal velocities are not. There is now growing acceptance that much of the upper mantle may be anisotropic to the prop­ agation of seismic waves. Origin of mantle anisotropy It has been known for some time that the discrepancy between ma n tle Ray- leigh Nicholas and Christensen (1987) elucidated the and Love waves could be explained if the reason for strong preferred crystal orientation vertical P and S velocities in the upper man­ in deformed rocks. First, they noted that in tle were 7-8% less than the horizontal veloci­ homogenous deformation of a specimen com­ ties. The Love-Rayleigh discrepancy has survived posed of minerals with a dominant slip system, to the present, and average Earth models have the preferred orientations of slip planes and SV in the upper mantle less than SH by about slip directions coincide respectively with the ori­ 3% . Some early models were based on separate entations of the flow plane and the flow line. 258 FABRIC OF THE MANTLE 01.0 igneous rocks preferred orientation can be caused by grain rotation, recrystallization in a 0.8 nonhydrostatic stress field or in the presence of a thermal gradient, crystal setting in magma 0.6 chambers, flow orientation and dislocation­ 0.4 controlled slip. Macroscopic fabrics caused by banding, cracking, sill and dike injection can also 0.2 cause anisotropy. Eclogites and basalts are much ~ -"' 0 less anisotropic; anisotropy can therefore be used c.o> as a petrological tool. Plastic flow induces preferred orientations in rock-forming minerals. The relative roles of -.4 deviatoric stresses and plastic strain have been - .6 long debated. In order to assure continuity of a deforming crystal with its neighbors, five inde­ - .8 pendent degrees of motion are required (the Von - 1 .0 '---'---'---'---'-----'- --'-- --'---'--------' Mises criterion). This can be achieved in a crys­ 0 40 80 120 160 200 240 280 320 360 tal with the activation of five independent slip Azimuth systems or with a combination of fewer slip sys­ Azimuthal anisotropy of Pn waves in the tems and other modes of deformation. In silicates Pacific upper mantle. The unique anisotropy of only one or two slip systems are activated under the Pacific upper mantle has also been mapped with a given set of conditions involving a given tem­ surface waves (after Morris et a/. , 1969). perature, pressure and deviatoric stresses. The homogenous deformation of a dominant slip sys­ tem and the orientation of slip planes and slip Simple shear in a crystal rotates all the lines directions tend to coincide with the flow plane attached to the crystal except those in the slip and the flow direction. plane. This results in a bulk rotation of crystals Mantle peridotites typically contain more so that the slip planes are aligned, as required than 65% olivine and 20% orthopyToxene. The to maintain contact between crystals. The crys­ high-P wave direction in olivine (Figure 20.2) is tal reorientations are not a direct result of the along the a-axis [1001], which is also the domi­ applied stress but are a geometrical requirement. nant slip direction at high temperature. The low­ Bulk anisotropy due to crystal orientation is est velocity crystallographic direction is [0101], therefore induced by plastic strain and is only the b-direction. which is normal to a common indirectly related to stress. The result, of course, slip plane. Thus, the pattern in olivine aggregates is also a strong anisotropy of the viscosity of is related to slip orientations. There is no such the rock, and presumably attenuation, as well as simple relationship with shear waves and, in elastic properties.
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