Imaging Mantle Convection Processes Beneath the Western USA Using the EarthScope Transportable Array

Mei Xue and Richard M. Allen
University of California Berkeley

AGU Fall Meeting, San Francisco, December 2007.

High resolution velocity models beneath western USA can provide important clues to mantle convection processes in this tectonically active region, e.g., the subduction of the Juan de Fuca plate, the upwelling of the Yellowstone plume, and their possible interactions. In this study, we apply the tomography technique using the Transportable Array data complemented by regional networks data resulting in a total of 732 stations. In our preliminary models we use 57 earthquakes sources. We derived two preliminary Vs models and one preliminary Vp model using tangential, radial, and vertical components respectively. Our preliminary tomographic images show some common features which have been imaged before such as the high velocity anomaly beneath the Cascades and the low velocity anomaly beneath the Yellowstone National Park. However, the unprecedented dense station distribution allows us to see deeper and reveals some new features: (1) the imaged Juan de Fuca subduction system goes deeper than previously been imaged. It reaches more than 500 km depth in Washington and northern California while in Oregon it seems break off and is segmented, implying a possible interaction with the proposed Yellowstone plume; (2) immediately south of the Juan de Fuca subduction system, we image low velocity anomalies down to 400 km depth, coincident with the proposed location of the slab gap; (3) we image the low velocity anomaly beneath the northeast Oregon down to 300 km depth, deeper than has previously been imaged, which has been hypothesized as the depleted mantle after the eruption of the Columbia River flood basalts, a result of delamination of the Wallowa plutonic roots [Hales, et. al., 2005]; (4) we see the high velocity Pacific plate abutting against the low velocity North American plate along the trace of the San Andreas Fault System. These observations suggest we are only just beginning to image the complex interactions between geologic objects beneath the western USA.

© Richard M Allen