Regional conductivity structure of Cascadia: preliminary results from 3D inversion of USArray transportable array magnetotelluric data

摘要

In conjunction with the USArray component of EarthScope, long period magnetotelluric (MT) data are being acquired in a series of arrays across the continental US. Initial deployments in 2006 and 2007 acquired data at 110 sites covering Washington and Oregon, and extending into western Idaho. The MT sites, distributed with the same nominal spacing as the USArray seismic transportable array (~75 km), produced data in the period range 10-10,000s of very good (2007) to excellent (2006) quality. We present initial results from processing and 3D inversion of this dataset. The most striking and robust features revealed by the inversion are extensive areas of high conductivity in the lower crust beneath the Northwest Basin and Range, and beneath the Cascade mountains, contrasting with very resistive crust in Siletzia (basement rocks in the Coast ranges, Willamette Valley and Puget Lowlands) and the Columbia Embayment. Significant variations in upper mantle conductivity are also revealed by the inversions, with the most conductive mantle beneath the Northeastern part of the array, and the most resistive corresponding to subducting oceanic mantle.