Crust and Upper Mantle Electrical Conductivity Beneath The ... - IRIS

2011 EarthScope National Meeting May 17-20, 2011
Crust and upper mantle electrical conductivity beneath the Yellowstone Hotspot Track
Anna Kelbert 1 , Gary D. Egbert 1 and Catherine deGroot-Hedlin 2
(1) College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR, USA
(2) Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, USA
We have performed a set of three dimensional inversions of magnetotelluric data in the Snake River
Plain and Yellowstone areas. We used a total of 91 sites from Earthscope MT Transportable Array,
covering much of Idaho and Wyoming, southern Montana, eastern Oregon and northern Nevada, in
addition to 34 sites from an earlier long-period MT survey, collected in two denser profiles along and
across the eastern Snake River Plain (SRP). Data for 14 periods from 7.3 secs to 5.2 hours were inverted
for 3D inverse conductivity models on a grid with horizontal resolution of 10 km. We obtained a class of
models that fit the data adequately and were therefore able to identify the features that were
recovered robustly. After removal of outliers in the data, our preferred model (Figure 1) fits both the
impedances (with 5% error floors) and the vertical magnetic transfer functions to an RMS of 1.8.
The images reveal the presence of a large, interconnected conductive body beneath the Eastern and
central SRP. Consistent with the surface-wave [Pollitz & Snoke 2010; Obrebski et al. 2011] and ambient
noise [Gao et al., 2011] tomography, we resolve highly anomalous upper mantle beneath the
Yellowstone hotspot track at lithospheric depths. However, in contrast to the seismic wave velocities
which stay negative to 200-250 km depth, the electrical conductivity anomaly peaks in the uppermost
mantle around 40-80 km depth. There, conductivities of the order of 0.1 S/m or higher are present.
These conductivities are too high to be explained by maximum 1% partial melt content usually
estimated in this area [e.g., Leeman et al. 2009], unless we also allow for the presence of volatiles such
as water, or alternative explanations such as carbonatite melts. Our resolution of the deeper
asthenosphere is limited below the SRP, but the conductivities stay elevated throughout the upper
mantle, with at least 0.02 S/m beneath the Wyoming craton in the Yellowstone vicinity. The seismically
imaged thermal anomaly [Obrebski et al., 2011; Smith et al., 2009; Yuan & Dueker, 2005] interpreted as
the Yellowstone plume is likely to be poorly resolved by the MT data, which are much more strongly
impacted by partial melt and fluids present at shallower depths.
The lithospheric anomaly extends to at least 200 km southwest of Yellowstone, roughly parallel to the
direction of North America absolute motion. The anomaly connects to the near-surface in several
1

2011 EarthScope National Meeting May 17-20, 2011
locations along and to the North of the SRP, as well as directly beneath the Yellowstone caldera. There,
highly conductive (~ 1 S/m) shallow anomalies are to be found. This leads us to believe that the complex
lithospheric feature beneath the SRP represents a partially molten magma reservoir, possibly rich in
volatile constituents, that feeds the Yellowstone hotspot. Additionally, in several locations beneath the
Eastern SRP very high conductivities (a few S/m) are imaged at or near the base of the lower crust.
These can probably be explained by a combination of partial melt, and highly saline fluids exsolved
during magmatic underplating.
Figure 1: Inferred electrical conductivity distributions for representative depths in the lower crust and uppermost mantle.
Two profiles of higher station density (grey lines) along (bottom left) and across (bottom right) the eastern SRP are also
shown. Crosses indicate their point of intersection; Yellowstone caldera’s Sour Creek Dome is indicated by the letter Y.
Our model suggests that the state of the uppermost mantle beneath the eastern SRP needs to be reevaluated
in terms of melt content and possibly the presence of free volatiles. The modest temperature
anomaly estimated for the Yellowstone plume [Leeman et al. 2009; Adams & Humphreys 2010] even in
conjunction with up to 1% partial melt does not provide an adequate explanation for the high
conductivities imaged at lithospheric depths beneath the Yellowstone hotspot track.
References:
Adams, D. C., and E. D. Humphreys (2010), New constraints on the properties of the Yellowstone mantle
plume from P and S wave attenuation tomography, Journal of Geophysical Research, 115(B12),
B12311--. [online] Available from: http://dx.doi.org/10.1029/2009JB006864
2

2011 EarthScope National Meeting May 17-20, 2011
Gao, H., E. D. Humphreys, H. Yao, and R. D. van der Hilst (2011), Crust and lithosphere structure of the
northwestern U.S. with ambient noise tomography: Terrane accretion and Cascade arc
development, Earth and Planetary Science Letters, 304(1-2), 202-211,
doi:10.1016/j.epsl.2011.01.033. [online] Available from:
http://linkinghub.elsevier.com/retrieve/pii/S0012821X11000598 (Accessed 22 March 2011)
Leeman, W. P., D. L. Schutt, and S. S. Hughes (2009), Thermal structure beneath the Snake River Plain:
Implications for the Yellowstone hotspot, Journal of Volcanology and Geothermal Research, 188(1-
3), 57-67, doi:DOI: 10.1016/j.jvolgeores.2009.01.034. [online] Available from:
http://www.sciencedirect.com/science/article/B6VCS-4VP667P-
1/2/83f669bdbd6998a076c04e868606531f
Obrebski, M., R. M. Allen, F. Pollitz, and S.-H. Hung (2011), Lithosphere-asthenosphere interaction
beneath the western United States from the joint inversion of body-wave traveltimes and surfacewave
phase velocities, Geophysical Journal International, no-no, doi:10.1111/j.1365-
246X.2011.04990.x. [online] Available from: http://doi.wiley.com/10.1111/j.1365-
246X.2011.04990.x (Accessed 28 March 2011)
Pollitz, F. F., and J. A. Snoke (2010), Rayleigh-wave phase-velocity maps and three-dimensional shear
velocity structure of the western US from local non-plane surface wave tomography, Geophysical
Journal International, 180(3), 1153-1169, doi:10.1111/j.1365-246X.2009.04441.x. [online] Available
from: http://doi.wiley.com/10.1111/j.1365-246X.2009.04441.x (Accessed 25 January 2011)
Smith, R. B., M. Jordan, B. Steinberger, C. M. Puskas, J. M. Farrell, G. P. Waite, S. Husen, W.-L. Chang, and
R. OʼConnell (2009), Geodynamics of the Yellowstone hotspot and mantle plume: Seismic and GPS
imaging, kinematics, and mantle flow, Journal of Volcanology and Geothermal Research, 188(1-3),
26-56, doi:DOI: 10.1016/j.jvolgeores.2009.08.020. [online] Available from:
http://www.sciencedirect.com/science/article/B6VCS-4X5JSS3-
1/2/29d05cc90f7cef363c4df04b687b56a6
Yuan, H., and K. Dueker (2005), Teleseismic P-wave tomogram of the Yellowstone plume, Geophysical
Research Letters, 32, 7304-+, doi:10.1029/2004GL022056.
3