Description
Two-dimensional (2-D), Backus-Gilbert inversion of the onshore EMSLAB (ElectroMagnetic Study of the Lithosphere and Asthenosphere Beneath the Juan de Fuca Plate) magnetotelluric (MT) data along the 200-km-long Lincoln Line in western Oregon has yielded optimally smooth geoelectric sections. The land portion of the Lincoln Line traverses the edge of the North American plate that is being underthrust by the Juan de Fuca plate system. Multiple inversions were performed on the apparent resistivity and impedance phase data using two different techniques. The first method used only data approximating the transverse magnetic (TM) mode, while the second technique used averaged values of both the transverse electric (TE) and TM modes. A new tensor decomposition of the impedance data from two stations on the Lincoln Line provided estimates of potential near-surface distortion from three-dimensional (3-D) inhomogeneities. The 3-D forward modeling of the Willamette Valley furnished estimates of potential near-surface distortion associated with this major geomorphic feature. The 2-D inversions revealed four conductive anomalies in the depth range of 5 to 40 km. A slightly conducting (< 100 Siemens (S)) zone is centered at 30 to 35 km depth under the Coast Range and may be the top of the subducting Juan de Fuca plate. A root-like protrusion of relatively high conductivity is present at 70 to 80 km from the coast to a depth of 20 km under the Willamette Valley. A prominent conducting zone of several hundred S is detected at a depth of 30 to 35 km under the very resistive (> 1000 ohm-m) Western Cascades. Because of the shallow depth of the latter two conductive anomalies, they are not the eastward dipping Juan de Fuca Plate which must be deeper beneath the Willamette Valley and the Western Cascades. However, these anomalies could be indirect indicators of subduction zone processes. A conductive anomaly east of the Cascades is not well constrained by the Lincoln Line data and could be related either to 3-D effects or to extensional processes associated with the Basin and Range Province. The tensor decomposition analysis identified an intermediate-scale distortion in the Western Cascades not imaged by the 2-D inversions. The tensor decomposition method also indicated that the Pacific Ocean, the accretionary sediments beneath the continental margin, and the Willamette Valley are not producing 3-D electric field distortions in the middle to long period MT results. The 3-D forward modeling showed no evidence that the Willamette Valley was a major regional 3-D distortion feature. However, the 3-D results do not preclude 3-D distortion in the observed data from local surficial inhomogeneities.
