SPATIAL VARIATIONS OF MANTLE TRANSITION ZONE THICKNESS BENEATH WESTERN UNITED STATES

The thickness of the mantle transition zone (MTZ), which is bounded by the so-called 410-km (d410) and 660-km (d660) discontinuities, is generally considered as an in-situ indicator of temperature anomalies in the vicinity of the discontinuities. Here we report results from common-conversion-point stacking of P-to-S converted phases from the discontinuities to systematically map the spatial distribution of MTZ thickness from the western coast to approximately 100-degree west. A total of about 200,000 radial receiver functions from USArray and all the other permanent and portable seismic networks in the study area are used in the stacking. We use the P-wave anomaly model of Burdick et al. (2012) and the S-wave model of Obrebski et al. (2012) to correct for apparent topography variations of the discontinuities.

Main features of the resulting MTZ discontinuity depths and associated thickness include:

1). The 'lithospheric drip' in northern Nevada corresponds to a 20-km uplift of the d410 and a depression of the d660 with similar magnitude, suggesting that the low temperature anomaly extends through the MTZ;

2). Relative to the continental interior to its east, the Yellowstone volcanic area is associated with a narrow (about 150 km in diameter) depression of the d410, and a broad (about 400 km) uplift of the d660, resulting in a thinning of the MTZ of about 20 km. These observations imply that Yellowstone is associated with upwelling of hot material originated from the lower mantle;

3). The area with thinned MTZ beneath Yellowstone has an elongated shape, extending toward the WSW direction which is subparallel to the absolute plate motion direction of the North American plate. The spatial relationship between the mantle upwelling and its surface expression could suggest that the lithosphere moves slower than the underlain asthenosphere.