Crustal Deformation in the Basin and Range: Recent Results from the BARGEN Network and Questions for the Future

As late as the mid-1990's, our state of understanding of crustal deformation within the northern Basin and Range was quite poor. Prior space geodetic studies indicated that the integrated deformation across this ∼1000-km-wide zone was 10--12~mm~yr^-1, with only one site (ELY) near the center of the province. By 1998, however, continuous GPS had described the major features of the horizontal velocity deformation field near lat. 40\deg{}~N, and by 2002 detailed horizontal velocity fields from both continuous and campaign GPS were available for both northern and central Basin and Range. Studies now focus on the physical mechanisms for both the broad-scale and detailed studies of deformation patterns as they relate to individual faults. These studies have highlighted a number of problems to be addressed by PBO, including: whether or not strain is localized along a small number of individual faults, versus evenly distributed across most Quaternary faults; the relationship between past and present strain and seismicity; the role of plate boundary forces versus gravitational forces in controlling the deformation field; and the contribution of historic earthquakes to the detailed deformation observed. With >7~yrs of continuous data from the BARGEN network, we are now in a position to begin evaluating two important avenues of research, including transients or accelerations in site motion, and the vertical component of deformation. A subset of BARGEN sites now shows temporal variations in velocity that are as high as 0.4~mm~yr^-2. Although we do not yet know whether these motions are related to tectonic processes, they have systematic properties, including regional coherence and displacements in accord with slip directions of nearby faults. Vertical velocities show a broad pattern of subsidence of ∼1~mm~yr^-1 with coherent spatial variations of +/- 2~mm~yr^-1. Even if the overall subsidence is in error on account of reference frame problems, the coherent variations of subsidence cannot be explained by any long-term, steady-state geophysical process such as thermal subsidence, erosion, or strain accumulation along faults.