Geodynamic Significance of Seismic Anisotropy of the Upper Mantle: New Insights from Laboratory Studies (original) (raw)
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Earth and Planetary Science Letters, 1999
Forward numerical models are used to investigate the effect of deformation regime on the development of olivine lattice-preferred orientations (LPO) and associated seismic anisotropy within continental deformation zones. LPO predicted to form by pure shear, simple shear, transpression, or transtension are compared to a database comprising ca. 200 olivine LPO from naturally deformed upper mantle rocks. This comparison suggests that simple shear or plane combinations of simple and pure shear are probably the dominant deformation regimes in the upper mantle. Seismic properties, calculated using the modeled olivine LPO, suggest that seismic anisotropy data may carry information on the deformation regimes active in the lithospheric mantle, although not all deformation regimes are characterized by a distinct seismic anisotropy signal. Transtensional deformation in continental rift systems should result in fast S-wave polarization and P-wave propagation directions oblique to the rift trend within the extended lithospheric mantle. Simple shear (wrench) or transpression in vertical deformation zones and pure shear (horizontal extension) result in similar seismic anisotropy. Simple shear or widening-thinning shear may, however, induce obliquity between seismic and magnetotelluric electrical conductivity anisotropy data. Similarly, it is not possible to distinguish between simple shear or lengthening-thinning shear (plane transpression) in horizontal deformation zones (thrusts) and pure shear (vertical contraction=horizontal extension). In all cases, the polarization direction of the fast split S-wave and the fast P-wave direction parallels the flow direction, but the anisotropy for both Pn-and S-waves is lower in horizontal structures than in vertical ones. Finally, several deformations show an isotropic response to SKS and=or Pn waves, suggesting that seismic isotropy does not necessarily imply absence (or heterogeneity) of deformation. There is a good agreement between model predictions and seismic anisotropy data in both transtensional and transpressional zones, suggesting coupled deformation of the crust and mantle. Oblique fast S-wave polarization directions in the East African rift, for instance, may result from an early transtensional deformation in the mantle lithosphere below the rift system. In contrast, most thrust belts display fast S-waves polarized parallel to the trend of the belt. One possible interpretation is that the upper mantle is decoupled from the crust in these areas.
Mantle structural geology from seismic anisotropy
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.
Effect of anisotropy on oceanic upper mantle temperatures, structure, and dynamics
Journal of Geophysical Research, 1997
Olivine and orthopyroxene crystals composing the oceanic upper mantle align under progressive simple shear strain. Because the thermal diffusivities (r) and viscosities of these minerals are anisotropic, mineral alignment affects vertical heat flow and upper mantle dynamics. The vertical thermal diffusivity of upper mantle peridotite decreases with progressive simple shear strain, leading to higher temperatures in the shallow upper mantle than predicted by an isotropic half:space cooling model. This, in turn, causes higher surface heat flow, shallower ocean basins, weaker asthenosphere, and slightly thinner lithosphere. Viscosity associated with an oriented simple shear strain (r/sh), such as that caused by plate motion, also evolves with progressive strain, though r/sh at high strains has not been characterized. Regardless of the evolution of r/.,.h with strain, the effects of thermal diffusivity anisotropy on upper mantle temperatures, surface heat flow, lithosphere thickness, asthenosphere viscosity and shear stress at the plate bottom remain evident. Shear heating elevates the geotherm by more than r anisotropy except in the youngest o,'ean and in cases with significant shear weakening or a slowly moving plate (less than -3 cm/yr).
Keywords: lithosphere–asthenosphere boundary seismic anisotropy LPO development The depth of the oceanic lithosphere–asthenosphere boundary (LAB), as inferred from shear wave velocities, increases with lithospheric age, in agreement with models of cooling oceanic lithosphere. On the other hand, the distribution of radial anisotropy under oceanic plates is almost age-independent. In particular, radial anisotropy shows a maximum positive gradient at a depth of ∼70 km, which, if used as a proxy, indicates an age-independent LAB depth. These contrasting observations have fueled a controversy on the seismological signature of the LAB. To better understand the discrepancy between these observations, we model the development of lattice preferred orientation (LPO) in upper mantle crystal aggregates and predict the seismic anisotropy produced by plate-driven mid-ocean ridge flows. The model accounts for the progressive cooling of the lithosphere with age and can incorporate both diffusion and dislocation creep deformation mechanisms. We find that an age-independent distribution of radial anisotropy is the natural consequence of these simple flows. The depth and strength of anisotropy is further controlled by the deformation regime – dislocation or diffusion creep – experienced by crystals during their ascent towards, and subsequent motion away from, the ridge axis. Comparison to surface wave tomography models yield constraints on rheological parameters such as the activation volume. Although not excluded, additional mechanisms proposed to explain some geophysical signatures of the LAB, such as the presence of partial melt or changes in water content, are not required to explain the radial anisotropy proxy. Our prediction, that the age-independent radial anisotropy proxy marks the transition to flow-induced asthenospheric anisotropy, provides a way to reconcile thermal, mechanical and seismological views of the LAB.
Pressure sensitivity of olivine slip systems and seismic anisotropy of Earth's upper mantle
Nature, 2005
The mineral olivine dominates the composition of the Earth's 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 1-4 . The rapid decrease of anisotropy below this depth has been interpreted as marking the transition from dislocation to diffusion creep in the upper mantle 5 . 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 highpressure 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.
Journal of Geophysical Research, 2000
Anisotropy of upper mantle physical properties results from lattice preferred orientation (LPO) of upper mantle minerals, in particular olivine. We use an anisotropic viscoplastic selfconsistent (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 recrystallization 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 peridotires. 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 peridotRes; 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 ø. 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 peridotRes 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 Earth's upper mantle.
Models of seismic anisotropy in the deep continental lithosphere
Physics of The Earth and Planetary Interiors, 1993
Seismological observations (SKS-wave polarizations, systematic P-residual variations, azimuthal dependence of P n- and surface-wave velocities or a dispersion of surface waves) are not consistent with isotropic, if laterally heterogeneous, upper-mantle structure. Therefore, an anisotropy should be considered as an a priori aspect of future large-scale studies of mantle structure. Most studies of anisotropy, however, have assumed horizontal or vertical axes of symmetry, but such orientations cannot explain bipolar patterns of spatial variations of P residuals, which we have observed at many seismological stations. On the basis of the petrophysical properties of real upper-mantle rocks we consider anisotropy formed either by hexagonal or by orthorhombic aggregates composed of olivine, orthopyroxene, and clinopyroxene. Rotations of the aggregates about vertical and horizontal axes allow us to find the three-dimensional orientations of symmetry axes that fit combinations of both P and S seismological observations in Central Europe and in western North America. The orientations with plunging symmetry axes (velocity extremes) seem to be consistent across large, spatially uniform tectonic units and change abruptly at important suture zones.
Global patterns of azimuthal anisotropy and deformations in the continental mantle
Geophysical Journal International, 1992
We present a summary of measurements of azimuthal anisotropy in the continental mantle based on the SKS technique and performed mostly with the active participation of the authors. The directions of polarization of the fast quasi-shear wave and the time delays between the quasi-shear waves are obtained at nearly 70 locations in all continents, except Antarctica. These data are interpreted in terms of lattice-preferred orientation of olivine which is caused by deformations in the mantle. The depth interval responsible for anisotropy is unknown but the data suggest that it may reach at least 300 km. The fast directions in SKS do not show clear correlation with the fast directions of the teleseismic P at the same seismograph stations. In the regions of present-day convergence the fast direction of anisotropy usually aligns with the plate boundary. This correlation implies that the direction of shortening is the same in the crust and the upper mantle. In the regions of rifting, the inferred direction of mantle flow usually aligns with the direction of extension in the crust. Outside the regions of recent tectonic activity we, most likely, observe a combined effect of frozen anisotropy in the subcrustal lithosphere and of recently formed anisotropy in the asthenosphere. On a global scale, in these regions there is a positive correlation between the absolute plate velocity directions and the fast directions of anisotropy. The correlation is especially strong in central and eastern parts of North America. A clear absence of any evidence of large-scale azimuthal anisotropy in the data of long-range refraction profiling for the upper 100 km of the mantle of that region implies that the effect in SKS is generated mainly at greater depths, in the asthenosphere. Orientation of olivine at these depths reflects recent and present-day flow in the mantle rather than processes of a distant geologic past.