SPATIAL AND TEMPORAL PATTERN OF NEOGENE AND QUATERNARY FAULT SLIP-RATES IN THE ALVORD EXTENSIONAL BASIN, NORTHWESTERN GREAT BASIN, USA

The Alvord basin of southeastern Oregon lies along the northern boundary of the Great Basin and is bounded by Pliocene to Holocene faults that cut late Tertiary volcanic and sedimentary rocks, Quaternary sediments, and shorelines of the ancient pluvial Lake Alvord. Middle Pliocene lacustrine rocks were folded prior to formation of the Alvord extensional basin, which initiated after ~3 Ma. Although seismically quiescent, continuous GPS velocities indicate west-northwest differential motion across the basin of 1.75± 0.3 mm/yr, and the youngest activity on basin-bounding faults occurred before 2 to 4 ka. The steeply dipping faults (~60º) form a complex array of structures that bound and internally segment the basin, which has depths of 1.5 to 2.5 km. Pleistocene to Holocene fault displacement is recorded by offset of two sets of shoreline terraces; a lower set of 5 shorelines with ages of about 12-17 ka, and a higher set of 3 shorelines with ages between 130 to 350 ka. Cumulative fault displacement across the basin is ~5 km, and ground-based LiDAR (Light Detection and Ranging) imaging indicates that the younger and older lake terraces have cumulative offsets of 48.4 ± 0.5 m and 80.1 ± 0.6 m, respectively. Shoreline offsets document differential displacement on the faults during at least 17 periods of movement between episodes of lake desiccation. The locus of displacement was transferred along strike amongst basin-bounding faults via relay structures, but ~35% of the total offset was distributed on structures within the basin. Deformation rates vary with time interval. Faults have average displacement rates since the middle Pliocene (~1.0 mm/yr) comparable to geodetic rates, suggesting that long-term far-field displacement and strain accumulation are constant. Late Pleistocene rates are dramatically slower (0.13 to 0.36 mm/yr) suggesting either a slower far-field displacement or that the locus of deformation migrated within the basin and/or to locales outside of the basin fault system. During the latest Pleistocene to middle Holocene, displacement rates were elevated (3.5 mm/yr) and may mark an earthquake cluster. The pattern of surface ruptures on individual strands of the Alvord basin fault system indicate that strain release and recurrence interval for earthquakes varied spatially within the fault system.