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Mantle Flow in Subduction Systems from the Global Pattern of
Shear Wave Splitting above and below Subducting Slabs
Maureen Long (Yale University), Paul Silver (Carnegie Institution of Washington)
The character of the mantle flow field in subduction zone regions remains poorly understood, despite its importance for our
understanding of subduction dynamics. Observations of seismic anisotropy, which manifests itself in shear wave splitting, can
shed light on the geometry of mantle flow in subduction zones, but placing constraints on anisotropy in various parts of the
subduction system (including the overriding plate, the mantle wedge, the subducting slab, and the sub-slab mantle) remains
challenging from an observational point of view. In order to identify dynamic processes that make first-order contributions
to the pattern of mantle flow in subduction zones, we analyze a global compilation of shear wave splitting measurements for
a variety of ray paths, including SK(K)S and teleseismic S phases as well as local S and source-side splitting from slab earthquakes.
We have compiled shear wave splitting measurements from subduction zones globally to produce estimates of average
shear wave splitting parameters – and their spatial variation – for the mantle wedge and the sub-wedge region for individual
subduction segments [Long and Silver, 2008]. These estimates are then compared to other parameters that describe subduction.
The sub-wedge splitting signal is relatively simple and is dominated by trench-parallel fast directions in most subduction zones
worldwide (with a few notable exceptions). Average sub-wedge delay times correlate with the absolute value of trench migration
velocities in a Pacific hotspot reference frame, which supports a model in which sub-slab flow is usually trench-parallel and is
controlled by trench migration [Long and Silver, 2009]. Shear wave splitting patterns in the mantle wedge are substantially more
complicated, with large variations in local S delay times and complicated spatial patterns that often feature sharp transitions
between trench-parallel and trench-perpendicular fast directions. We find a relationship between average wedge delay times
and the ratio of the trench migration velocity and the convergence velocity. This supports a model in which a trench-parallel
flow field (induced by trench migration) interacts with a 2-D corner flow field (induced by downdip motion of the slab) in the
mantle wedge, with the relative influence of these flows being governed by the relative magnitude of convergence velocity and
trench migration velocity in a Pacific hotspot reference frame.
References
Long, M. D., and T. W. Becker (2010), Mantle dynamics and seismic anisotropy, Earth Planet. Sci. Lett., in press.
Long, M. D., and P. G. Silver (2008), The subduction zone flow field from seismic anisotropy: A global view. Science, 319, 315-318.
Long, M. D., and P. G. Silver (2009), Mantle flow in subduction systems: The sub-slab flow field and implications for mantle dynamics. J.
Geophys. Res., 114, B10312.
Acknowledgements: This work was supported by the Carnegie Institution of Washington and by NSF grant EAR-0911286.
Sketch of constraints on subduction zone anisotropy from shear wave splitting
measurements (figure from Long and Becker, 2010, after the compilation of
Long and Silver, 2008, 2009). The anisotropic signals of the wedge and subslab
regions are shown separately. Red arrows indicate average fast directions
for the sub-slab splitting signal from SKS, local S, and source-side teleseismic S
splitting measurements. The associated average sub-slab delay times are shown
in red. Blue arrows indicate average fast directions for wedge anisotropy from
local S splitting. In regions where multiple fast directions are shown, splitting
patterns exhibit a mix of trench-parallel, trench-perpendicular, and oblique
fast directions.
Sketch of the model proposed by Long and Silver (2008) for mantle flow in subduction
zones controlled by trench migration.
II-190 | 2010 IRIS Core Programs Proposal | Volume II | Upper Mantle Structure and Dynamics