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410 Discontinuity
Three-Dimensional Geometry of the Juan de Fuca/farallon Slab
Gary Pavlis (Department of Geological Sciences, Indiana University, Bloomington)
New understanding on the three-dimensional geometry of the western US subduction system is emerging by viewing multiple
imaging results in a fully three-dimensional framework. The process is comparable to interpreting 3D seismic reflection data, but
is complicated in USArray by the scale, which makes spherical geometry important. The figure shown is a first attempt at a joint
interpretation of P tomography models by Burdick et al. [2009], Sigloch et al. [2008], and recently produced scattered wave images
of Pavlis [submitted]. The scattered wave images show dipping features above the 410 km discontinuity I have interpreted as defining
a shear zone at the boundary layer between the subducting slab and the overriding North American plate. The model shown
in this figure is a kinematic model showing flow lines and constant time lines based on the current relative motion of the Juan
de Fuca and North American plates. The surface is interpreted from data, but the time lines assume no longitudinal strain in the
descending slab. The visualization shows the predicted edge of the slab window from the San Andreas based on the no longitudinal
strain approximation and using the current trace of the San Andreas as an anchor point. The new scattered wave images and the
reasonable geometry of this slab model provide strong support for the model of the Farallon slab described by Sigloch et al. [2008].
References
Burdick, S., C. Li, V. Martynov, T. Cox, J. Eakins, T. Mulder, L. Astiz, F. L. Vernon, G. L. Pavlis, and R. D. van der Hilst, 2009, Model update
December 2008; upper mantle heterogeneity beneath North America from P-wave travel time tomography with global and USArray transportable
array data, Seismol. Res. Lett., 80(4), 384-392, doi: 10.1785/gssrl.80.4.638.
Pavlis, G. L., submitted, Three-dimensional Wavefield Imaging of Data from the USArray: New Constraints on the Geometry of the Farallon
Slab, Geosphere, in review.
Sigloch, K., N. McQuarrie, and G. Nolet, 2008, Two-stage subduction history under North America inferred from multiple-frequency tomography,
Nature Geosci., 1, 458-462, doi:10.1038/ngeo231.
Acknowledgements: This work was supported by the National Science Foundation under CMG-0327827. It also benefited from advanced computing
resources provided by the National Science Foundations TeraGrid program at Indiana University.
Burdick et al (2009)
P tomography model
Edge of Slab
Window
P to S scattered
Wave Image
Juan de Fuca/Farallon lab model in 3D visualization framework. This is a still image from a 3D visualization scene used to overlay tomography models and results of a
recently developed full 3D, scattered wave imaging method. Coastlines and state boundaries in magenta and rivers shown in blue provide a geographic reference. A
section through the Burdick et al. (2009) P model and a section through the scattered wave image are shown to illustrate data that went into the interpretation. The
slab model shows flow lines and constant time surface based on the NUVEL1 model of relative plate motion. The time ticks are based on a no strain approximation.
The surface is constrained to daylight at the trench and pass through points 100 km below all active volcanoes in the Pacific Northwest. A estimate of the western
edge of the slab window based on the no strain approximation is shown as the blue line passing through California and Arizona.
II-178 | 2010 IRIS Core Programs Proposal | Volume II | Upper Mantle Structure and Dynamics