Receiver Function Imaging of the Lithosphere-Asthenosphere Boundary

Receiver Function Imaging of the Lithosphere-Asthenosphere Boundary Global map of the depth to the lithosphere-asthenosphere boundary imaged using Ps receiver functions. Color indicates depth. Triangles show the 169 stations used in this study. Station color corresponds to tectonic regionalization, after Jordan (1981). Tectonic regions colored as follows: Oceanic – black, Phanerozoic orogenic zones and magmatic belts – red, Phanerozoic platforms – cyan, Precambrian shields and platforms – green. Although the work of Jordan (1981) divides oceanic environments into three age groupings, a single oceanic bin, encompassing all ages, is used here, since sampling of this region is sparse.
The lithosphere-asthenosphere boundary, or LAB, is often defined seismically as the top of the low velocity zone underlying the higher velocities of the lithospheric lid. Surface-wave studies have shown the global extent of upper-mantle low-velocity zones, but do not have the vertical resolution to constrain the details of LAB depth and sharpness. Better depth resolution, at least in the vicinity of seismic stations, is provided by receiver-function studies of converted phases, although care must be taken to distinguish LAB signals from noise and crustal reverberations. The continued operation of the global seismic networks, by IRIS and other agencies, has provided a sufficient volume of data that receiver-function imaging of the LAB is now possible on a global scale. We have analyzed 15 years of global seismic data using P-to-S (Ps) converted phases from over 150 stations and imaged an interface that correlates with tectonic environment, varying in average depth from 95 ± 4 km beneath Precambrian shields and platforms to 81 ± 2 km beneath tectonically altered regions and 70 ± 4 km at oceanic island stations. Because the polarity of the Ps arrivals indicates a shear-velocity drop with depth, this interface is likely the LAB in most regions, although it may constitute another boundary beneath the cratonic interiors of continents where the LAB is expected to be much deeper. The high frequencies observed in the Ps arrivals require a sharp discontinuity, implying a change in composition, melting, or anisotropy, not temperature alone.
</p><p>Rychert, C. A., and P. M. Shearer, A global view of the lithosphere-asthenosphere boundary, Science, 324, 2009. </p><p>Rychert, C. A., P. M. Shearer and K. M. Fischer, Scattered wave imaging of the lithosphere-asthenosphere boundary, Lithos, 2010.
</p><p>Jordan, T. H., Global Tectonic regionalization for seismological data analysis, Bull. Seismol. Soc. Amer., 71(4): 1131-1141, 1981. </p><p>Acknowledgements: This research was supported by National Science Foundation awards EAR-0229323 and EAR-0710881.</p>


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