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Acknowledgments: This work was supported as part of the
Center for Catalytic Hydrocarbon Functionalization, an Energy
Frontier Research Center Funded by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences,
under award DE-SC0001298 (to R.A.P. and D.H.E). D.H.E.
thanks the BYU Fulton Supercomputing Lab. We also thank
S. Burt (BYU) for assistance with Tl NMR. Several provisional
and one nonprovisional patents have been filed (applications
PCT/US2014/018175, 61/768,715, 61/862,715, 61/862,723,
Materials and Methods
Figs. S1 to S9
Tables S1 to S3
Schemes S1 to S4
5 December 2013; accepted 19 February 2014
Changes in Seismic Anisotropy
Shed Light on the Nature
of the Gutenberg Discontinuity
Caroline Beghein,1 Kaiqing Yuan,1 Nicholas Schmerr,2 Zheng Xing1
The boundary between the lithosphere and asthenosphere is associated with a platewide
high–seismic velocity “lid” overlying lowered velocities, consistent with thermal models. Seismic
body waves also intermittently detect a sharp velocity reduction at similar depths, the Gutenberg
(G) discontinuity, which cannot be explained by temperature alone. We compared an anisotropic
tomography model with detections of the G to evaluate their context and relation to the
lithosphere-asthenosphere boundary (LAB). We find that the G is primarily associated with vertical
changes in azimuthal anisotropy and lies above a thermally controlled LAB, implying that the
two are not equivalent interfaces. The origin of the G is a result of frozen-in lithospheric structures,
regional compositional variations of the mantle, or dynamically perturbed LAB.
Plate tectonic theory describes a strong and rigid lithospheric “lid” that trans- lates coherently atop a weaker and more
deformable convecting asthenosphere. Determining the depth and pervasiveness of the interface between these two layers, known as the
lithosphere-asthenosphere boundary (LAB), is
key for understanding the formation, evolution,
and thermochemical properties of plates and associated tectonics. The exact compositional and
thermal mechanisms that control this rheological
division remain enigmatic, but seismological
imaging of anisotropy—the directional dependence of seismic wave velocity—across the upper
mantle provides an essential tool for interrogating the transition in material properties across
Seismological interrogations of the oceanic
upper mantle beneath the Pacific Ocean find evidence for a sharp drop in seismic velocity, known
as the Gutenberg (G) discontinuity (1), at 40-
to 100-km depth. The depth of the G roughly
coincides with the top of a low-velocity zone
(LVZ) and may be the seismological expression
of the LAB. However, correlating G depth with
plate age and distance to mid-ocean ridges has
not produced a unifying interpretation of the
relationship between the G and the LAB (2–6).
This has led to several alternative hypotheses for
the origin of the G, including partial melt lenses
in the asthenosphere (3), hydrogen depletion
of olivine from decompression melting beneath
mid-oceanic ridges (7, 8), frequency-dependent
attenuation effects reducing the shear modulus
in the presence of mantle hydration (9), and dy-
namical melt-producing processes to explain
the strong regional variations in G reflectivity
from SS precursor data (5, 10).
To improve our understanding of how isotropic and anisotropic velocity models relate
to the observations of seismic discontinuities,
we modeled the three-dimensional isotropic and
anisotropic structure of the upper mantle beneath
the Pacific Basin (Fig. 1) using a global data set
of surface wave phase velocity maps (11, 12).
The dispersive properties of surface waves make
them ideal to put depth constraints on seismic
anisotropy and velocity, and the use of higher-mode surface waves to model azimuthal anisotropy provides sensitivity throughout the upper
mantle (fig. S1). The detection of changes in
seismic anisotropy has been successfully used to
identify layering in the mantle, variations in LAB
depth beneath continents and oceans (13, 14),
and chemical stratification within the lithosphere
under the North American craton (14). Here we
focus on anisotropy under the Pacific Plate, which
is well sampled by surface waves and therefore
constitutes a natural laboratory to constrain the
evolution and cooling history of the oceanic lithosphere. The surface wave anisotropy results are
compared to a large data set of high-frequency SS
precursors that highlight the G (5).
Our models show a stratified upper mantle
under the Pacific Ocean and a correlation between the boundaries of these layers and the location of observed seismic discontinuities (Fig. 2).
The top layer (layer 1) is defined by a poor alignment between VSV fast axes direction and the
absolute plate motion (APM) (15), and the underlying layer (layer 2) by a better alignment
with the APM. Layer 1 is also characterized by
high seismic velocities away from ocean ridges
[4 to 5% with respect to our reference model (16)],
and its thickness increases with crustal age, similar to past surface wave studies (13, 17–19).
Furthermore, layer 1 is associated with 1 to 2%
radial anisotropy with VSV > VSH, and azimuthal
anisotropy amplitudes of 1 to 2%. This fast VSV
direction roughly follows the orientation of ocean
floor fracture zones at 50-km depth near ridges,
around 80-km depth for ocean ages between 80
and 120 million years ago (Ma), and at 100-km
depth under old oceanic plates (Fig. 1). Ocean
floor fracture zones are temporally stable features that record plate motion path and can thus
be used as proxy for the paleospreading directions. Layer 2 has lower S-wave velocity (–5%
relative perturbations), strong radial anisotropy
(5%) with VSH > VSV, and 3% azimuthal anisotropy with, by definition, fast axes subparallel
to the APM (<30° deviation from APM).
On the basis of the above seismological observations, we define the LAB in our models as
the dipping interface between these two layers.
The strong anisotropy of layer 2 suggests alignment of olivine fast axes with mantle flow direction associated with plate motion that can
occur in the deformable asthenosphere by dislocation creep (20) or diffusion creep (21). Olivine
lattice-preferred orientation (LPO) formed by
mantle flow–induced shear strain in the dislocation creep regime is consistent with a low-viscosity asthenosphere (22) and a flow channel
coincident with a low-velocity zone (23). The
thickness of our tomographically defined layer
1 increases with plate age, following the 900°
to 1100°C isotherms in a half-space cooling
(HSC) model (black lines, Fig. 2). Combined
with elevated seismic velocities, layer 1 is therefore consistent with cold lithosphere that has a
thermally controlled thickness and implies that
the LAB is a temperature-related phenomenon.
Furthermore, the alignment of the VSV fast axes
with the fossil spreading direction in layer 1 is
consistent with LPO and the frozen-in record
1Department of Earth, Planetary, and Space Sciences, University
of California, Los Angeles, 595 Charles Young Drive East, Box
951567, Los Angeles, CA 90095–1567, USA. 2Department of Geology, University of Maryland, College Park, MD 20742, USA.
*Corresponding author. E-mail: email@example.com