HR: 14:55h
AN: G33D-06    [Abstracts]
TI: Ground-Based LiDAR Determined Fault-Offsets of Paleo-Lake Terraces: Late Middle Pleistocene to Holocene Deformation Rates in the Alvord Extensional Basin, Southeastern Oregon
AU: * Oldow, J S
EM: oldow@uidaho.edu
AF: University of Idaho, Geological Sciences, Moscow, ID 83844-3022 United States
AU: Singleton, E S
G33D-06 AF: University of Idaho, Geological Sciences, Moscow, ID 83844-3022 United States
AB: Ground-based LiDAR combined with GPS positioning provides high-resolution measurement of faults offsetting wave-cut terraces formed along shorelines of the Pleistocene to Holocene Lake Alvord in southeastern Oregon. The lake was located within the Alvord extensional basin that produced a profound physiographic depression bound by Steens Mountain to the west and the topographically lower Tule Springs Rim to the east. The wave-cut terraces are incised into late Tertiary volcanic rocks and Quaternary sediments and formed during at least eight stillstands of the lake. The geomorphic expression of individual terraces varies with age and topography of the basin margin. Two sets of terraces are recognized, with the topographically higher and older Serrano terrace series consisting of three shorelines. Five topographically lower shorelines constitute the younger Alvord terrace series. Terrace elevations vary around the perimeter of the basin, and fault scarps crosscut stillstand shorelines on at least six structures. LiDAR images of the terraces and faults were acquired at seven sites along an east-west transect across the basin. The images, consisting of up to 3 x 10⁷ points, were acquired in three to five look-directions for footprints with dimensions ranging from 400 m to 700 m on a side. All scans were located with dual frequency GPS referenced to a base station occupied from 4 to 15 hours per day for 16 days. Transformation of scans to a common geospatial reference frame (NAD83) allowed combination with other digital resources and surface analysis in GIS. Uncertainty estimation is based on propagation of errors determined from GPS positioning, geoidal correction, transformation of point clouds to a common reference, and GIS surface analysis. The characteristic elevations of selected geomorphic surfaces were determined with standard errors of between 0.059 and 0.388 m. Cumulative vertical displacement across the faults measured on the Serrano series highstand is 80.1 ± 0.6 m and the vertical offset of the Alvord series highstand is 48.4 ± 0.5 m. The age of the Serrano terrace series is not directly determined. Based on correlation with the Eetza highstand of Lake Lahontan, exposed in northwestern Nevada 15 km south of the Alvord basin, the Serrano highstand is between 350 and 130 Ka. Similarly the Alvord terrace series are correlated with Sehoo shorelines of Lake Lahontan and are estimated as 13 to 8 Ka. The integrated vertical displacement rate for the Alvord highstand, with an age of 13 Ka, is 3.7 ± 0.5 mm/yr. In contrast, using the age range of 350 to 130 Ka, the Serrano highstand yields an integrated vertical rate of 0.23 to 0.62 ± 0.01 mm/yr. These vertical displacement rates were accommodated on faults dipping 60° and correspond to integrated horizontal rates of 2.25 ± 0.5 mm/yr since formation of the Alvord highstand and of 0.13 to 0.36 ± 0.01 mm/yr since the Serrano highstand. A horizontal velocity across the basin of 1.75 ± 0.6 mm/yr exits from two continuous GPS sites located 40 km northeast and 60 km southwest of the Alvord Desert. Within uncertainty, the geodetic rate and geologic rate over the last 10⁴ years are essentially the same, suggesting steady state deformation. The geologic rate integrated over the 10⁵ year time-scale, however, is lower and may indicate an increase in deformation rate and/or spatial migration of the locus of deformation since the Late Pleistocene.
DE: 1209 Tectonic deformation (6924)
DE: 1242 Seismic cycle related deformations (6924, 7209, 7223, 7230)
DE: 1295 Integrations of techniques
SC: Geodesy [G]
MN: Fall Meeting 2005