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Imaging the Shear Wave Velocity "Plumbing" beneath the
Northwestern United States with Rayleigh Wave Tomography: The
High Lava Plains vs Yellowstone
Lara Wagner (University of North Carolina at Chapel Hill), Donald Forsyth (Brown University), Matthew Fouch (Arizona
State University), David James (Carnegie Institution of Washington)
Since the mid-Miocene, the northwestern United States has
experienced extensive flood basalt volcanism, followed by the
formation of two time-progressive tracks of silicic volcanism:
the Yellowstone/Snake River Plains (YSRP) and the High Lava
Plains (HLP). The YSRP track progresses towards the northeast,
parallel to North American plate motion, and has therefore often
been attributed to a deep mantle plume source. However, the
HLP track progresses to the northwest over the same time frame
in a direction not consistent with any regional plate motion. The
causes of the mid-Miocene flood basalts and the tracks of the
YSRP and HLP are a matter of ongoing debate. We performed
Rayleigh wave phase velocity inversions and inversions for 3-D
shear wave velocity structure of the northwestern United States
using data collected from the High Lava Plains Deployment and
the EarthScope USArray Transportable Array (TA). The large
number of stations used in these inversions allows us to show
an unprecedented level of detail in the seismic velocity structures
of this tectonically complex area. Our velocity images indicate
that low S-wave velocities in the uppermost mantle do not
well match the track of HLP volcanism. While at the surface the
Newberry caldera appears to anchor the NW end of the HLP
hotspot track, the seismic results show that it lies in a separate,
north-south trending low velocity band just east of the Cascades
that is distinct from the main HLP trace. The eastern edge of this
low velocity band also correlates with a change in the magnitude
of shear wave splitting delay times, possibly indicating changes
in amount of partial melt present [Long et al., 2009]. In contrast,
the ultra-low S-wave velocities beneath the YSRP track extend
locally to at least 175 km depth and are by far the most prominent
seismic anomalies in the region. Along axis, the YSRP hotspot
track is characterized by a discrete low velocity channel in
the upper mantle that shallows, narrows and intensifies to the
northeast, but then deepens rapidly to the north beneath Yellowstone. Because Rayleigh wave tomography loses resolution below
~200 km, we cannot determine whether or not this anomaly is caused by a deep mantle plume source. However, the shallowing of the
low velocity anomaly to the northeast is consistent with a moving heat source coming from below 200 km depth.
References
Geologic map of the northwestern United States with -3% low velocity surface
contour shown in 3-D below the map: Shown are the locations of the
Blue Mountain Province (BMP), Steens Mountain (SM), and the Owyhee Plateau
(OP). Red triangles show Holocene arc volcanism. The black dashed line is
the 87Sr/86Sr = 0.706, usually taken to represent the edge of cratonic North
America. The boundaries of Basin and Range extension are shown with a purple
dashed line. Black contours indicate the age progression for rhyolites in the
High Lava Plains and the Snake River Plains. The yellow and blue contours in the
HLP track show our 7 Ma contours for High Lava Plains rhyolites.
Yellowstone/Snake River Plains low velocity zone: shown is the -3% surface contour
in perspective, looking from the north down towards the southeast. Shown
is the location of Yellowstone Caldera (blue triangle).
Wagner, L., D. Forsyth, M. Fouch, and D. James (2010), Detailed three dimensional shear wave velocity structure of the northwestern United
States from Rayleigh wave tomography. Earth Planet. Sci. Lett., in review.
Long, M. D., H. Gao, A. Klaus, L. S. Wagner, M. J. Fouch, D. E. James, and E. Humphreys (2009), Shear wave splitting and the pattern of mantle
flow beneath eastern Oregon, Earth Planet. Sci. Lett., 288, 359-369.
Acknowledgements: The High Lava Plains deployment was funded through NSF award EAR-0507248 (MJF) and EAR-0506914 (DEJ). LW’s
participation was supported by NSF award EAR-0809192 and DWF was supported by NSF award EAR-0745972.
2010 IRIS Core Programs Proposal | Volume II | Upper Mantle Structure and Dynamics | II-217