GROUND-BASED LIDAR IMAGING AND SURFACE ANALYSIS OF LATE PLEISTOCENE AND QUATERNARY WAVE-CUT TERRACES AND HIGH-ANGLE FAULTS: CONSTRAINTS ON ACTIVE DEFORMATION IN THE ALVORD DESERT, SOUTHEASTERN OREGON

Ground-based LIDAR provides an accurate, cost effective means of imaging surface features useful in assessing the magnitude and rate of deformation within active settings like the Alvord Desert of southeastern Oregon. The Alvord Desert lies in a north-northeast trending fault bound basin bordered to the west by Steens Mountain, which forms the footwall to an east-facing fault system that accommodates at least 3.0 km of stratigraphic throw. The Alvord extensional fault system cuts 17-15 Ma basalt and Upper Miocene siliceous volcanic and volcanogenic sedimentary rocks and initiated after the extrusion of 7-4 Ma basalt flows. Late Pleistocene and Quaternary deposits are cut by high-angle fault scarps, and during the Late Pleistocene(?) and Quaternary, basin margins were incised by flights of wave-cut terraces in at least three high-stands of paleo-Lake Alvord. The morphology of scarps and wave-cut terraces can yield approximate ages of formation and differential wave-cut terrace elevations across the basin record changes in geoidal height. The vertical displacement rate on a single fault within the Alvord basin is 1.3 to 1.7 mm/yr over the last 10⁴ years and aggregate vertical displacement across the basin is substantially greater. In this analysis, wave-cut terrace and fault scarp images of as many as 1.5 x10⁷ points each were acquired at ranges up to 700 m using Riegl LPM-800HA reflectorless lasers. Point positions are located with a precision of 1.5 cm and 1 ppm of the baseline and acquired at a rate of 1000 points/second. Laser beam dispersion increases with baseline length at 13.0 cm per 100 m and produces progressively greater surface averaging with distance. Scans were acquired in several look-directions to improve surface resolution and coverage and merged using InnovMetrics Polyworks software. Images are registered in an earth-centered earth-fixed reference system using dual frequency Leica GPS receivers linked by FM radio. Point positioning and point cloud precision is better than 10 cm and surface averaging does not exceed 90 cm² and typically is less than 30 cm². Application of ground-based LIDAR imaging is feasible in many actively deforming regions where scarp-like features are formed and provides measure of surface displacements and estimates of surface-morphology ages over time-scales of 10³ to 10⁶ years.