2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 6
Presentation Time: 2:55 PM


OLDOW, John S., SINGLETON, Eron S. and WHIPPLE, Kim L., Geological Sciences, Univ of Idaho, Moscow, ID 83844-3022, oldow@uidaho.edu

The history and rates of deformation in the Alvord basin of southeast Oregon was possible only by integration of a broad spectrum of digital geological and geophysical data. The NNE-trending faults of Alvord extensional basin forms part of the western Great Basin that was initiated after 7 to 4 Ma. Although seismically quiescent, the basin contains active faults with Holocene displacement that accommodate up to ~2.0 mm/yr displacement determined from wide aperture continuous GPS sites. The Alvord basin is localized along the axis of a late Miocene NNE-trending regional fold that involved 17 to 5 Ma volcanic and volcaniclastic rocks. The basin is underlain by a system of right-stepping en echelon half-grabens up to 1.5 km deep bound by faults dipping 45° to 60°. The faults form sub-basins linked by a northerly-trending displacement transfer fault system cored by a basement high. Using potential field models of basin morphology and surface geology, reconstruction to the pre-extensional geometry indicates vertical displacement on bounding faults of over 3.0 km. Together with geologic timing constrains, the aggregate displacement provides an estimate of long term (106 years) vertical and horizontal displacement rates of 1.1 to 0.9 mm/yr and 0.95 to 0.8 mm/yr, respectively. Late Pleistocene and Quaternary alluvial deposits within the basin are incised by wave-cut terraces formed in highstands of paleo-Lake Alvord. Fault scarps crosscut highstand terraces on at least three active structures and exhibit 29.5 m of cumulative vertical displacement. The terraces correlate with 12,000 to 13,500 year old Lahontan wave-cut terraces in northwestern Nevada and yield an integrated vertical displacement rate of between 2.2 to 2.5 mm/yr. This vertical displacement rate was accommodated by 45° to 60° dipping faults and corresponds to an integrated horizontal motion of 1.3 to 2.5 mm/yr over the last 12,000 to 13,500 years. Within limits imposed by geologic and geophysical data, the deformation rates within the Alvord basin are remarkably similar over 106, 104, and 10 year time scales. Deformation rates are arguably steady state and may suggest that commonly cited disparities in geodetically and geologically determined deformation rates determined elsewhere may be more a reflection of rate uncertainty than tectonic process.