No Evidence For Strain Localization Above Strike-slip Faults in Northern Walker Lane, USA: Invalidation of the Elastic Dislocation Model?

The kinematics of the lithosphere are an important component to understanding solid Earth dynamics. Geodetically measured deformation of the Earth's surface contains valuable information about the structure and physical properties of the lithosphere. The surface deformation resulting from strain accumulation near locked faults is typically explained by a simple elastic screw-dislocation model (EDM), and long-term fault slip rates can be inferred from it. Here, we show that the EDM is unable to predict the GPS-measured surface deformation in the northern Walker Lane region of the Basin and Range province, USA. The Walker Lane, a part of the diffuse boundary between North America and the Pacific plates, is a zone of dextral shear east of the Sierra Nevada. Its northern section is characterized by sub-parallel, nearly vertical strike-slip Mohawk Valley, Honey Lake, and Warm Springs fault systems, which meets the standard conditions for application of the EDM. We carefully process the position-time series to remove equipment-related offsets, postseismic transients, outliers, and regional common mode before using MIDAS, a robust trend estimator, to obtain surface velocities. We find the resulting velocity profile across the region to be linear, with no visual indication of the existing faults. We attempt to fit the profile with the EDM and find that the model cannot predict the observations with a realistic locking depth of faults. The profile is, however, well-fit with a straight line, showing no evidence of strain rate localization at the locked faults. Furthermore, similarly obtained profiles across the central Walker Lane region, approximate latitudes 39.4°, 38.8°, and 38.4°, which do not contain any strike-slip faults, are also well-fit with a straight line. Models alternative to the EDM suggest that surface deformation could be driven either by farfield boundary forces or a strong underlying lower crust/upper mantle, with the elastic crust passively riding on top. These results caution against ubiquitous application of the EDM and that important information about the deformation of the lower crust/upper mantle can be inferred where the EDM does not fit the observations.