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What Makes a Tectonic Plate?

The lithosphere-asthenosphere boundary beneath continents: The gradient in seismic velocity across the lithosphere-asthenosphere boundary is key to constraining the physical and chemical properties that create differences in mechanical strength between the lithosphere and the deeper mantle.

The concept of a strong lithosphere that translates as a relatively coherent layer over a weak asthenosphere is fundamental to models of plate motions, tectonics, and mantle convection. However, much remains to be learned about the physical and chemical properties that create rheological differences between the lithosphere and asthenosphere. We have used Sp and Ps scattered waves to image mantle discontinuities beneath North America and Australia.

In the tectonically active western U.S., large portions of the Phanerozoic eastern U.S., Phanerozoic eastern Australia and the adjacent edge of the Australian craton, prominent Sp phases from a negative velocity contrast were found at depths of 50-130 km, consistent with the lithosphere-asthenosphere boundary depth range from surface wave tomography. These phases imply significant (4-10%) velocity drops over depth ranges of 30-40 km or less, and thus cannot be simply explained by a lithosphere-asthenosphere boundary that is governed purely by temperature. Rather, they imply that the asthenosphere is hydrated with respect to a drier, depleted lithosphere or contains a small amount of partial melt.

In contrast, no significant negative Sp phase was found at the base of the thick cratonic lithosphere in either continent, implying that the cratonic lithosphere-asthenosphere velocity gradient is distributed over more than 50-70 km in depth. This gradient may be purely thermal in origin, although gradational changes in composition or melt content cannot be ruled out. A negative discontinuity internal to the cratonic lithosphere was observed at depths of 50-115 km. The depth of this boundary is comparable to the thickness of oceanic and younger continental lithosphere. This discontinuity may date from the formation of the cratonic lithosphere, or it could reflect later alteration of the cratonic lithosphere by melting and metasomatism, perhaps as the top of a melt cumulate layer.

Lithosphere-asthenosphere boundary papers:

Ford, H. A., K. M. Fischer, D. L. Abt, Catherine A. Rychert, L. T. Elkins-Tanton, The lithosphere-asthenosphere boundary and cratonic lithospheric layering beneath Australia from Sp wave imaging, Earth Planet. Sci Lett.., 300, 299-310, doi:10.1016/j.epsl.2010.10.007, 2010.

Abt, D. L., K. M. Fischer, S. W. French, H. A. Ford, H. Yuan, and B. Romanowicz, North American lithospheric discontinuity structure imaged by Ps and Sp receiver functions, J. Geophys. Res., 115, B09301, doi:10.1029/2009JB006914, 2010a.

Fischer, K. M., H. A. Ford, D. L. Abt, C. A. Rychert, The lithosphere-asthenosphere boundary, Ann. Rev. Earth Planet. Sci., 38, 551-575, doi:10.1146/annurev-earth-040809-152438, 2010.

Yuan, H., B. Romanowicz, K. M. Fischer, and D. L. Abt, 3-D shear wave radially and azimuthally anisotropic velocity model of the North American upper mantle, Geophys. J. Int., 184, 1237-1260, DOI: 10.1111/j.1365-246X.2010.04901.x, 2011.

Rychert, C. A., P. M. Shearer, K. M. Fischer, Scattered wave imaging of the lithosphere-asthenosphere boundary, Lithos, 120, 173–185, doi:10.1016/j.lithos.2009.12.006, 2010.

Till, C. B., L. T. Elkins-Tanton, and K. M. Fischer, A mechanism for low-extent melts at the lithosphere-asthenosphere boundary, Geochem. Geophys. Geosyst., 11, Q10015, doi:10.1029/2010GC003234, 2010.
Rychert, C. A., S. Rondenay, and K. M. Fischer, P-to-S and S-to-P Imaging of a Sharp Lithosphere-Asthenosphere Boundary beneath eastern North America, J. Geophys. Res., 112, B08314, doi:10.1029/2006JB004619, 2007.

Rychert, C. A., K. M. Fischer, and S. Rondenay, A sharp lithosphere-asthenosphere boundary imaged beneath eastern North America, Nature, 436, 542-545, doi:10.1038/nature03904, 2005.

Further Studies of the Continental Lithosphere:
We have also been studying other aspects of the structure and dynamics of the continental lithosphere, including mantle anisotropy and its implications for asthenospheric flow, crustal structure and its relationship to tectonic history, and sub-lithospheric mantle discontinuities.

French, S. W., K. M. Fischer, E. M. Syracuse, and M. E. Wysession, Crustal structure beneath the Florida-to-Edmonton broadband seismometer array, Geophys. Res. Lett., 36, L08309, doi:10.1029/2008GL036331, 2009.

Fischer, K. M., A. Li, D. W. Forsyth, and S.-H. Hung, Imaging three-dimensional anisotropy with broadband seismometer arrays, Seismic Earth: Array Analysis of Broadband Seismograms, AGU Geophysical Monograph, A. Levander and G. Nolet, eds., 99-106, 2005.

Li, A., D. W. Forsyth, and K. M. Fischer, Shear wave structure and azimuthal anisotropy beneath eastern North America from Rayleigh wave tomography, J. Geophys. Res., 108, 2362, doi:10.1029/2002JB002259, 2003.

Fischer, K. M., Waning buoyancy in the crustal roots of old mountains, Nature, 417, 933-936, doi:10.1038/nature00855, 2002.

Li, A., K. M. Fischer, S. van der Lee, and M. E. Wysession. Crust and upper mantle discontinuity structure beneath eastern North America, J. Geophys. Res., 107, doi: 10.1029/2001JB000190, 2002.

Fouch, M. J., K. M. Fischer, E. M. Parmentier, M. E. Wysession, and T. J. Clarke, Shear-wave splitting, continental keels, and patterns of mantle flow, J. Geophys. Res., 105, 6255-6275, 2000.

Li, A., K. M. Fischer, M. E. Wysession, and T. J. Clarke, Mantle discontinuities and temperature under the North American continental keel, Nature, 395, 160-163, 1998.

For additional papers please see publications list.

These studies involved waveform data from three deployments of broadband seismometers in eastern North America: the 28 station 2001-2002 Florida to Edmonton Broadband Seismometer Experiment (FLED), the 18 station 1995-1996 Missouri-Massachusetts Broadband Seismometer Experiment (MOMA), and the 4 station 1997-1999 NOMAD experiment in New York and Connecticut. The FLED and MOMA experiments were collaborations with Michael Wysession (Washington University, St. Louis), and all experiments used instruments from IRIS/PASSCAL. Analysis of data from these arrays and adjacent permanent stations have yielded a variety of results that range from quantification of crustal thickening beneath the Appalachian orogen to constraints on how the shape of the continental lithosphere interacts with deeper mantle flow.

Funding for this research was provided by the EarthScope and Geophysics Programs of the National Science Foundation.

Brown faculty collaborators:

Don Forsyth
Marc Parmentier

Other project collaborators:

Please see publication co-authors.

Mantle discontinuity depths estimated from Sp receiver functions in North America by Abt et al. (2010) (a) and in Australia by Ford et al. (2010) (b). The dots are colored for depth/amplitude and represent Sp piercing points that have been interpolated onto a fine grid and smoothed with a circular filter with a 30 km radius. (a) North America. Black inverted triangles indicate stations where the negative Sp phase is interpreted as the lithosphere-asthenosphere boundary (LAB), white inverted triangles are stations where the phase is interpreted as a mid-lithospheric discontinuity (MLD), and gray stations indicate ambiguity in the interpretation of the negative Sp phase. (b) Australia. Negative Sp phases at stations in Phanerozoic eastern Australia and just within the eastern margin of the Proterozoic craton (BBOO and stations to its east) are interpreted as the LAB. Negative Sp phases at most stations in the Proterozoic and Archean craton (station WRAB and the stations to its west) are interpreted as intralithospheric discontinuities. Figure from Fischer et al. (2010).

Three-dimensional view of the lithosphere-asthenosphere boundary and surface topography. The lower surface represents the location of the base of the lithosphere interpolated from migrated Ps and Sp waveforms (blue circles mark conversion points, grey paths are Sp and black paths are Ps). The Sp HRV data from northern back azimuths and the Ps from LBNH (grey circles mark conversion points) are not used to calculate the interpolated surface because of a depth discrepancy. Yellow arrow points in the direction of absolute plate motion; plate velocity is 2.5 cm/yr. From Rychert et al. (2007).

Broadband stations of the MOMA and FLED experiments (white) and other broadband stations in eastern North America (yellow).

Station MM05, Gleason, PA

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