Characterizing the 410 km Discontinuity Low‐Velocity Layer Beneath the LA RISTRA Array in the North American Southwest

Author Info

John J. Jasbinsek, California Polytechnic State University - San Luis ObispoFollow
Ken G. Dueker, University of Wyoming
Steven M. Hansen, University of Wyoming

Recommended Citation

Preprint version. Geochemistry Geophysics Geosystems, Volume 11, Issue Q03008, March 12, 2010, pages 1-17.

The definitive version is available at https://doi.org/10.1029/2009GC002836.

Abstract

Receiver functions recorded by the 54-station 920 km long Program for Array Seismic Studies of the Continental Lithosphere–Incorporated Research Institutions for Seismology Colorado Plateau/Rio Grande Rift Seismic Transect Experiment (LA RISTRA) line array display a pervasive negative polarity P to S conversion (P_ds) arrival preceding the positive polarity 410 km discontinuity arrival. These arrivals are modeled as a low-velocity layer atop the 410 km discontinuity (410-LVL) and are inverted for a velocity profile via a grid search using a five-parameter linear gradient velocity model. Model parameter likelihood and correlations are assessed via calculation of one- and two-dimensional marginal posterior probability distributions. The maximum likelihood model parameter values found are top velocity gradient thickness of 0.0 km with a 4.6% (−0.22 km/s) shear velocity reduction, a 19.8 km constant velocity layer, and bottom gradient thickness of 25.0 km with a 3.5% (+0.17 km/s) shear velocity increase. The estimated mean thickness of the 410-LVL is 32.3 km. The top gradient of the 410-LVL is sharp within vertical resolution limits of P to S conversion (km), and the diffuse 410 km velocity gradient is consistent with hydration of the olivine-wadsleyite phase transformation. The 410-LVL is interpreted as a melt layer created by the Transition Zone Water Filter model. Two secondary observations are found: (1) the 410-LVL is absent from the SE end of the array and (2) an intermittent negative polarity P₅₂₅s arrival is observed. We speculate that upper mantle shear velocity anomalies above the 410 km discontinuity may manifest Rayleigh-Taylor instabilities nucleated from the 410-LVL melt layer that are being shed upward on time scales of tens of millions of years.

Disciplines

Physics

Copyright

2010 American Geophysical Union.

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An edited version of this paper was published by AGU. Further reproduction or electronic distribution is not permitted.