David Mainprice

An olivine fabric database: an overview of upper mantle fabrics and seismic anisotropy

We present a unique database of 110 olivine petrofabrics and their calculated seismic properties. The samples come from a variety of the upper mantle geodynamic environments (ophiolites, subduction zones, and kimberlites) with a wide range of micro-structures. A phenomenological relationship is established between P- and S-wave seismic anisotropy and the degree of crystal alignment (fabric strength). Seismic anisotropy increases rapidly at low fabric strength before reaching a near saturation level of 15 to 20% for P-waves and 10 to 15% for S-waves. Despite a large variation in the symmetry of fabric patterns, the average seismic properties of the different fabric, micro-structural and geodynamic settings have similar anisotropies in both magnitude and symmetry. Hence it would seem possible to determine some measure of fabric strength from seismic anisotropy if the dimensions of the anisotropic region are known, but not geodynamic environment or details of the petrofabric pattern. A simple pattern of seismic anisotropy characterises the average sample of the database, which has the following features: the polarisation plane of the fastest S-wave is parallel or sub-parallel to the foliation plane; the maximum shear wave splitting is parallel to the Y structural direction (in foliation plane and normal to the lineation); the maximum of the P-wave velocity is parallel to the high concentration of [100] axes, which is sub-parallel to the lineation. The [100] orientation distribution has the greatest influence on the P-wave seismic anisotropy. The [100] and [001] orientation distributions have the greatest influence on the symmetry of S-wave anisotropy, although the magnitude of anisotropy is influenced by the distribution of all three principal axes. Although the database only contains olivine petrofabrics, this statistical study clearly shows that seismic anisotropy can be used to deduce the orientation of the structural frame in the upper mantle.

Interpretation of SKS-waves using samples from the subcontinental lithosphere

Abstract The seismic properties of five South African kimberlite nodules have been calculated from petrofabric measurements. These garnet lherzolite and harzburgite composition nodules (56–80% olivine) are known from previous studies to have originated at depths of 120–170 km in the subcontinental lithosphere. Their in situ seismic properties have been calculated by extrapolating the elastic constants to the appropriate temperature (900–1050°C) and pressure (3.0–3.5 GPa) conditions. The average of the five samples has a maximum S-wave anisotropy ( δV s ) of 3.7% which is sufficiently high to explain previously reported teleseismic SKS delay times ( δt ) in continental shield areas of up to 1.7 s. The biggest delay time is observed when the foliation is vertical, the lineation horizontal, and the fastest S-wave polarized parallel to the lineation, i.e. an orientation for transcurrent motion. Delay times of over 1 s can only be generated by this orientation. A strong mantle-crust mechanical coupling is suggested for such situations. The potential use of seismic anisotropy as an indicator of strain in the lithosphere has been investigated using experimental and simulated fabric data for olivine, the most abundant phase (70%) in the upper mantle. The V p and V s seismic anisotropy increases with fabric strength and finite strain. Recrystallization tends to reduce fabric strength and seismic anisotropy, resulting in saturation values for experimental and simulated fabrics which correspond to approximately 50–60% strain or 8–9% δV s for pure olivine aggregates. A survey of 25 naturally deformed peridotites of oceanic origin suggests an average maximum S-wave anisotropy for the olivine component of 9%, or about 8% for a lherzolite or harzburgite rock composition when the orthopyroxene component is taken into account. The ophiolite samples are twice as anisotropic as the kimberlite nodules. If an average anisotropy value is representative of the lithosphere, then the SKS delay times represent variations in anisotropic layer thickness for delay times over 1 s as the orientation is constrained to be foliation vertical and lineation horizontal. The magnitude of S-wave anisotropy is less sensitive than V p to variations in olivine volume fraction in range 50–100%, and to deformation or fabric intensity, further suggesting that S-wave anisotropy is particularly apt for the determination of the anisotropic lithosphere thickness. For smaller delay times there is some trade off between structural orientation and thickness.

Dominant c slip in naturally deformed quartz: Implications for dramatic plastic softening at high temperature

A combined microstructural, X-ray texture goniometry and transmission electron microscopy study has been undertaken to document rare examples of c direction of slip in naturally deformed quartz. The presence of optically visible basal (0001) subgrain boundaries and strong concentrations of c axes parallel to the inferred shearing direction (close to the stretching lineation) are considered characteristic of c slip. Dominant c slip appears to be limited to high-temperature (>650 °C) and possibly hydrous conditions. The possibility of plastic softening associated with the relatively easy glide on this system is discussed.

Viscoplastic self-consistent and equilibrium-based modeling of olivine lattice preferred orientations : Implications for the upper mantle seismic anisotropy

Anisotropy of upper mantle physical properties results from lattice preferred orientation (LPO) of upper mantle minerals, in particular olivine. We use an anisotropic viscoplastic self-consistent (VPSC) and an equilibrium-based model to simulate the development of olivine LPO and, hence, of seismic anisotropy during deformation. Comparison of model predictions with olivine LPO of naturally and experimentally deformed peridotites shows that the best fit is obtained for VPSC models with relaxed strain compatibility. Slight differences between modeled and measured LPO may be ascribed to activation of dynamic recrystallization during experimental and natural deformation. In simple shear, for instance, experimental results suggest that dynamic re-crystallization results in further reorientation of the LPO leading to parallelism between the main (010)[100] slip system and the macroscopic shear. Thus modeled simple shear LPOs are slightly misoriented relative to LPOs measured in natural and experimentally sheared peridotites. This misorientation is higher for equilibrium-based models. Yet seismic properties calculated using LPO simulated using either anisotropic VPSC or equilibrium-based models are similar to those of naturally deformed peridotites; errors in the prediction of the polarization direction of the fast S wave and of the fast propagation direction for P waves are usually <15°. Moreover, overestimation of LPO intensities in equilibrium-based and VPSC simulations at high strains does not affect seismic anisotropy estimates, because these latter are weakly dependent on the LPO intensity once a distinct LPO pattern has been developed. Thus both methods yield good predictions of development of upper mantle seismic anisotropy in response to plastic flow. Two notes of caution have nevertheless to be observed in using these results: (1) the dilution effect of other upper mantle mineral phases, in particular enstatite, has to be taken into account in quantitative predictions of upper mantle seismic anisotropy, and (2) LPO patterns from a few naturally deformed peridotites cannot be reproduced in simulations. These abnormal LPOs represent a small percent of the measured natural LPOs, but the present sampling may not be representative of their abundance in the Earths upper mantle.

Pressure sensitivity of olivine slip systems and seismic anisotropy of Earth's upper mantle

The mineral olivine dominates the composition of the Earths upper mantle and hence controls its mechanical behaviour and seismic anisotropy. Experiments at high temperature and moderate pressure, and extensive data on naturally deformed mantle rocks, have led to the conclusion that olivine at upper-mantle conditions deforms essentially by dislocation creep with dominant [100] slip. The resulting crystal preferred orientation has been used extensively to explain the strong seismic anisotropy observed down to 250 km depth. The rapid decrease of anisotropy below this depth has been interpreted as marking the transition from dislocation to diffusion creep in the upper mantle. But new high-pressure experiments suggest that dislocation creep also dominates in the lower part of the upper mantle, but with a different slip direction. Here we show that this high-pressure dislocation creep produces crystal preferred orientations resulting in extremely low seismic anisotropy, consistent with seismological observations below 250 km depth. These results raise new questions about the mechanical state of the lower part of the upper mantle and its coupling with layers both above and below.

Fault-induced seismic anisotropy by hydration in subducting oceanic plates

The variation of elastic-wave velocities as a function of the direction of propagation through the Earth’s interior is a widely documented phenomenon called seismic anisotropy. The geometry and amount of seismic anisotropy is generally estimated by measuring shear-wave splitting, which consists of determining the polarization direction of the fast shear-wave component and the time delay between the fast and slow, orthogonally polarized, waves. In subduction zones, the teleseismic fast shear-wave component is oriented generally parallel to the strike of the trench, although a few exceptions have been reported (Cascadia and restricted areas of South America). The interpretation of shear-wave splitting above subduction zones has been controversial and none of the inferred models seems to be sufficiently complete to explain the entire range of anisotropic patterns registered worldwide. Here we show that the amount and the geometry of seismic anisotropies measured in the forearc regions of subduction zones strongly depend on the preferred orientation of hydrated faults in the subducting oceanic plate. The anisotropy originates from the crystallographic preferred orientation of highly anisotropic hydrous minerals (serpentine and talc) formed along steeply dipping faults and from the larger-scale vertical layering consisting of dry and hydrated crust–mantle sections whose spacing is several times smaller than teleseismic wavelengths. Fault orientations and estimated delay times are consistent with the observed shear-wave splitting patterns in most subduction zones.

Seismic Anisotropy of the Deep Earth from a Mineral and Rock Physics Perspective

The seismic anisotropy of the deep Earth is reviewed as a profile from the upper mantle to the solid inner core at the centre of the Earth. The upper mantle is by far the most anisotropic region of the Earth, followed the D” layer above the core mantle boundary. In contrast it is shown that many minerals that are present in the upper mantle, transition zone, lower mantle, D” layer and solid inner core are elastically anisotropic to different degrees. The anisotropy of hydrous phases present in subduction zones is briefly introduced. The basic concepts of single crystal and polycrystalline elasticity and the extrapolation of elastic properties to high temperature and pressure are presented. The specific features of elastic wave propagation in an anisotropic medium of any arbitrary symmetry are illustrated using mantle and inner core phases. The roles of crystal preferred orientation, water and melt in producing seismic anisotropy in the upper mantle are discussed.

Development of shape and lattice preferred orientations: application to the seismic anisotropy of the lower crust

We review the physical basis of the development of fabrics in plastic and viscous flow and illustrate the typical fabrics formed by these processes in the main rock-forming silicates of the lower crust (feldspar, quartz, pyroxene and amphibole). The orientation process in plastic deformation where a single slip is dominant is recalled and the role of the constraint of neighbouring grains is emphasized. The fabric development of anisometric crystals in viscous flow is discussed as a function of the main controlling parameters: shear strain, aspect ratio and interference between crystals. The same sense of fabric asymmetry is introduced by plastic and viscous flow between the flow plane and the shape preferred orientation and hence coherent kinematic analysis can be undertaken in both modes of flow. In order to assess the role of such fabrics in the seismic laminations of the lower continental crust we have calculated the seismic P-wave properties of typical fabrics for hypothetical monomineralic and polymineralic rocks. The calculations show that the strongest anisotropies develop in monomineralic rock with values between 5 and 16%, compared with 5 and 8% for typical rock compositions. The strongest anisotropies for layered monomineralic rocks generated by fabrics is only 6% compared to the 14% suggested by model studies of the observed seismic laminations. We suggest that other effects, such as compositional layering and/or constructive interference of seismic waves are responsible for augmenting the apparent anisotropy.

C-SLIP IN QUARTZ FROM SUBSOLIDUS DEFORMED GRANITE

Abstract The slip behaviour of quartz from a granite and granitic veins during subsolidus high temperature conditions (700°–800°C) of deformation is studied by lattice preferred orientation (LPO) and microstructural techniques. LPOs with a point maximum of [ C ] axes parallel to the shear direction, which is close to the stretching lineation, are presented and interpreted with the aid of light and electron microscopy in terms of [ C ] as the dominant slip direction during plastic deformation. Microscopy of such samples reveals the presence of (0001) tilt boundaries and free [0001] Burgers vector dislocations in edge orientations. A change in LPO in a decreasing temperature environment from [ C ] axes parallel to the shear direction to [ C ] axes normal to the shear direction documents a fabric transition from the rare [ C ] slip to the more common a > slip fabric. It is concluded that conditions of high temperature and partial pressure of water are required to initiate dominant [ C ] slip during deformation at geological strain rates.

METHODS OF CALCULATING PETROPHYSICAL PROPERTIES FROM LATTICE PREFERRED ORIENTATION DATA

We consider the theoretical problems of calculating the physical properties of an aggregate from the constituent crystal properties and the lattice preferred orientation. The notion of a macroscopically homogeneous sample with an internally varing distribution of stress and strain fields is introduced to explain why further efforts have to be made to improve on the physically based Voigt and Reuss bounds. It is shown that the Voigt and Reuss bounds become increasingly separated with inceasing anisotropy, emphasising the need for better methods. The problem of highly anisotropic minerals is illustrated with polycrystals of plagioclase feldspar and biotite. Biotite is used to illustrate the mean velocity, the geometric mean and the self-consistent methods. The self-consistent method, which is generally accepted to give the best estimate, is almost identical to geometric mean recently introduced by Matthies and Humbert (1993) and similar to the arithmetic mean of the Voigt and Reuss bounds (Hill, 1952). The geometric mean has the powerful physical condition that the aggregate mean is equal to the mean of the inverse property (e.g. mean elastic stiffness and compliance). Despite its lack of theoretical justification the Hill average remains a useful estimate.