High-Resolution Seismic Reflection to Measure Displacement along Shallow, Hidden Faults In the Northern Basin and Range

Late Pleistocene to Holocene displacement on faults in the Alvord extensional basin of southeast Oregon, estimated from offsets of pluvial lake shorelines demonstrated that a minimum of ~72 m of vertical displacement occurred across the basin in the past 20,000 years. Of this, exposed faults along the western margin accommodate nearly 50% of the total displacement, while 20% is attributed to structures exposed in the eastern margin. This leaves roughly 30% (~21 m) that must be accommodated by buried structures in the center of the basin. In a cross-basin seismic reflection profile located approximately 12 km south of the lake-terrace study, we identified three major fault systems; two bounding the east and west portions of the basin, and a central fault creating eastern and western sub-basins. In addition to the major faults which are associated with surface scarps, we identified numerous mid-basin, buried normal faults that exhibit no surface expression. Some of these hidden faults show as much as 200 m of vertical offset at depths of less than 700 m. The shallowest horizon we could interpret continuously over a significant distance is located in the eastern sub-basin at a depth of 26-32 m below the surface. Over a horizontal distance of 4.3 km, we estimate that the shallow horizon has a minimum integrated vertical displacement of 16 ± 4 m along faults that have no surface expression. This magnitude is within estimated uncertainty of the displacement inferred in the lake terrace study. In addition to identifying these hidden structures, the seismic profile reveals a complex history with different fault systems accommodating the majority of deformation at different times. It is clear that properly accounting for displacement along hidden structures is critical to improving our understanding of the difference between geodetic and geologic strain rates.