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Mantle Structural Geology from Seismic Anisotropy

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Mantle Structural Geology from Seismic Anisotropy Mantle Petrology: Field Observations and high Pressure experimentations: a Tribute to Francis R. (Joe) Boyd © The Geochemical Society, Special Publication N° 6, 1999 Editors: Yingwei Fei, Constance M. Bertka, and Bjorn O. Mysen Mantle structural geology from seismic anisotropy PAUL SILVER1, DAVID MAINPRICE2, WALID BEN ISMAÏL2, ANDRÉA TOMMASI2, and GUILHEM BARRUOL2 1Department of Terrestrial Magnetism, 5241 Broad Branch Road, N.W., Washington, DC 20015, U.S.A. 2Laboratoire de Tectonophysique, ISTEEM, CNRS UMR 5558, Université Montpellier II, 34095 Montpellier cedex 05, France. Abstract--Seismic anisotropy is a ubiquitous feature of the subcontinental mantle. This can be inferred both from direct seismic observations of shear wave splitting from teleseismic shear waves, as well as the petrofabric analyses of mantle nodules from kimberlite pipes. The anisotropy is principally due to the strain-induced lattice preferred orientation (LPO) of olivine. The combined use of these mantle samples, deformation experiments on olivine, and numerical modeling of LPO, provides a critical framework for making inferences about mantle deformation from observed seismic anisotropy. In most cases there is a close correspondence between mantle deformation derived from seismic observations of anisotropy, and crustal deformation, from the Archean to the present. This implies that the mantle plays a major, if not dominant role in continental deformation. No clear evidence is found for a continental asthenospheric decoupling zone, suggesting that continents are probably coupled to general mantle circulation. to seismologists. They are also very simple and INTRODUCTION robust measurements that are now made routinely. Their utility in constraining subcontinental mantle The mantle nodules that are brought to the surface deformation is analogous to the use of isotropic by kimberlitic eruptions form the foundation for travel times as a means of studying variations in inferring the geological properties of the mantle temperature and composition. As integral subcontinental mantle. These rocks provide a unique functions of the anisotropy along the path of the means of constraining the composition, petrology, shear wave, they provide valuable information about temperature, pressure, age, and physical properties flow and deformation in the upper mantle. While of the subcontinental mantle. JOE BOYD has been measuring the splitting parameters is a reasonably the leader in recognizing and exploiting the valuable straightforward seismological task, working out the information that these rocks contain. More recently, relationship between anisotropy and mantle these same rocks have also been used to constrain deformation is somewhat less so, as it involves the degree and style of deformation in the knowledge from a variety of disciplines. The basis subcontinental mantle, through a comparison of the for this relationship is the development of lattice petrofabrics in these rocks and the observations of preferred orientation (LPO) in upper mantle rocks seismic anisotropy that are produced by this fabric. containing the anisotropic mineral olivine. One is There has been a rapidly growing interest in seismic interested in using anisotropy to map mantle anisotropy as a way of constraining the deformation deformation as a way of testing dynamic models of of the continental upper mantle. There are two basic mountain building or mantle flow characteristics problems that can be addressed with this approach: associated with the past and present motion of the deformation of the mantle portion of continental plates. To go from the deformation generated by a plates, and the mantle flow field beneath these particular tectonic process to the prediction of plates. The first problem represents a natural splitting parameters, one needs to: i) presume the extension of structural geology into the mantle and deformation field that is developed by that tectonic down to the base of the plate. The second problem process (e.g., mountain building), ii) determine the addresses the nature of the mantle flow field just resulting LPO of upper mantle aggregates from finite below the plates, down to the transition zone. A strain and calculate the aggre g ate elastic constants, major stimulus for the recent focus on the study of and iii) calculate the seismological manifestation of mantle deformation is the rapid increase in the anisotropy for this medium, incorporating the number of observations related to anisotropy. This appropriate geometry. has come from the establishment, over the last With these relationships in place, it is possible decade, of a global network of broadband to predict the characteristics of seismic anisotropy seismographs, and perhaps more importantly, from for a particular tectonic process. (iii) is the realm of the deployment of portable instruments, which seismology, and within it are the various allow for the collection of anisotropy data in seismological manifestations of a specified particular regions of geologic interest. anisotropic medium. (ii) is the domain of Shear-wave splitting observations are the most petrophysics/mineral physics and rock mechanics, unambiguous manifestation of anisotropy available and involves such issues as the single-crystal 79 80 P. Silver et al. properties of upper mantle minerals, the rheological shear wave φ and the delay time δt between the behavior of mineral aggregates under finite strain arrival time of the fast and slow shear waves (Fig. 1). (including recrystallization), and the quantification From (1) it is found that δt is proportional to path of strain indicators, such as those based on length L according to the approximate expression δ δ microtexture. One of the most important sources of for small anisotropy: t=L Vs/<Vs> where <Vs> is δ information concerning the petrophysical the isotropically averaged shear velocity and Vs is constraints is the suite of kimberlitic mantle the dimensionless intrinsic anisotropy, which is a nodules; as we will see, these mantle samples play a function of n (SILVER and CHAN, 1991). As we will crucial role in the interpretation of seismic see, φ is related to the orientation of the mantle anisotropy. The linking together of seismic strain field. Splitting in teleseismic shear waves anisotropy and LPO constitutes the forward problem such as SKS and S (See Fig. 1) is particularly useful of determining the anisotropic properties of upper in assessing the mantle’s role in geological mantle deformation. We next seek to ascribe a processes, because of the excellent lateral resolution physical process to the observed anisotropy, in (about 50km) for the periods typically used i n which case it is necessary to predict the deformation splitting analyses. Thus small geologic domains, field that the process creates, and this corresponds to and the boundaries between domains can be carefully (i) above. The generation of this deformation or flow sampled. The vertical resolution is not good, since field has been traditionally the domain of mantle the anisotropy could, in principle, reside anywhere dynamics, applied both to mantle convection and between the core-mantle boundary (CMB) and the plate (usually continental) deformation. There has surface in the case of SKS (Fig. 1). As shown below, recently been renewed interest among however, the dominant source of anisotropy in geodynamicists in making anisotropic predictions shear-wave splitting is most probably in the upper of mantle flow models as an important way of mantle beneath the recording station. testing such models (BUTTLES and OLSON, 1998; There are presently over 400 observations of CHASTEL et al. 1993; BLACKMAN et al., 1996; splitting in teleseismic shear waves (SAVAGE, BLACKMAN and KENDALL,1997; FOUCH et al., 1999; 1999). These represent a combination of permanent KINCAID and SILVER, 1996; DAVIS et al., 1997; stations as well as a large number of portable TOMMASI et al., 1996; TOMMASI, 1998). This is a experiments where splitting parameters have been healthy trend that will make the maximum use of the obtained in areas of specific geologic interest. constraints that anisotropy provides. We will There have been two recent reviews (SILVER, 1996; consider later two simple physical processes that SAVAGE, 1999) that will provide the reader with a have straightforward anisotropic characteristics. general overview of the shear wave splitting data set. The highlights are given here. There are CONTINENTAL ANISOTROPY: SUMMARY OF splitting observations from essentially all SHEAR WAVE SPLITTING OBSERVATIONS. continents, although the two with the most observations are North America and Eurasia. The In an isotropic, homogeneous medium there are mean value of splitting delay times is about 1 s two types of body waves, a P and an S wave; in an (SILVER, 1996). Assuming about 4% intrinsic anisotropic medium, there are three, a quasi-P wave anisotropy, this corresponds to about a 100 km and two quasi-shear waves. For weak anisotropy thick layer of anisotropic mantle. The range is from (the case encountered in the Earth), the P and S wave barely detectable (less than 0.5 s) to 2.4 s. There are displacement directions are still, in most cases, only a few measurements above δt=2 s. There are nearly parallel and perpendicular, respectively, to also about 10% of the data set where splitting was the propagation direction, as for an isotropic not detected. This means that anisotropy is a near- medium. Shear-wave splitting refers to the fact that ubiquitous feature of
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