THERMAL STRUCTURE OF THE TRANSITION ZONE BENEATH YELLOWSTONE AS REVEALED BY RECEIVER FUNCTION IMAGING

The existence of a mantle plume beneath the Yellowstone hotspot is controversial. The primary evidence for a plume origin derives from the 2-3 cm/yr progression of volcanism parallel to North American plate motion and elevated ³He/⁴He ratios. However, most current tomographic models show no low velocity anomaly below 240 km depth. The study of mantle discontinuities beneath this region provides insight into the thermal structure of the mantle. Seismic data primarily from the Continental Dynamics Yellowstone and Snake River Plain broadband PASSCAL arrays provides a 500 km diameter dense array approximately centered upon the hotspot. A common conversion point stacking method of radial and tangential receiver functions provides images of transition zone discontinuity topography. If a mantle plume is driving the Yellowstone hotspot, the elevated temperatures associated with the plume conduit will deflect the 410 and 660 km discontinuities in opposite directions due to their opposite Clapeyron slopes; thus decreasing the thickness of the transition zone. Preliminary results show 30 km of variation in transition zone thickness, indicating substantial thermal heterogeneity. The region of thinnest transition zone (220 km) is located 100 km west of the Yellowstone caldera. Better constraint on the thermal structure of the transition zone will be provided through correlation of transition zone topography with new tomographic velocity models.