LATE CENOZOIC EXTENSION AND STRUCTURAL EVOLUTION OF THE ALVORD BASIN, SOUTHEASTERN OREGON

The Alvord basin in southeast Oregon forms part of the northern boundary of the western Great Basin. The NNE-trending extensional basin is ~10 km wide and is bound on the west by the eastern flank of Steens Mountain, which rises over 1.5 km in a distance of 5 km. The basin is localized along the axis of a NNE-trending regional fold that involved 17 to 5 Ma volcanic and volcaniclastic rocks. The fold had an amplitude of 1.6 km and half-wavelength of ~40 km and was characterized by a steep western limb and shallowly dipping eastern limb. The transition from contraction to extension is constrained by an angular unconformity between steeply dipping (folded) Tertiary rocks and flat-lying 4-5 Ma basalt flows exposed along the western margin of Steens Mountain. Extensional structures cut earlier folds and formed after deposition of the 4-5 Ma basalt. The Alvord basin is underlain by a system of right-stepping en echelon half-grabens bound by faults dipping between 40° to 60°. Synthetic and antithetic faults internally divide the basin and are localized both on the margins and along the axis of the topographic depression. The Steens-Alvord region is seismically quiescent but records Holocene activity on faults that locally cut 12 Ka lake terraces. The subsurface basin geometry is constrained by over 200 km of gravity and magnetic profiles acquired at a station spacing of 300 m or better and surveyed using dual frequency GPS. The weakly asymmetric half-grabens face east and contain accumulations of poorly consolidated sediments up to 1.5 km deep, primarily along the western flanks of sub-basin structures. Using modeled basin morphology and surface geology, reconstruction to the pre-extensional geometry provides vertical and horizontal displacement rate estimates integrated over 4 to 5 m.y. of 1.1 to 0.9 mm/yr and 0.95 to 0.8 mm/yr, respectively. From the Cascade Range to the Owyhee Plateau, the Alvord basin accommodates ~70% of the total horizontal displacement, and the integrated long term horizontal displacement is nearly 3 mm/yr less than the ~4 mm/yr of differential displacement observed geodetically across the western Great Basin farther south. The disparity in rates suggests that either displacement has increased during the late Neogene or that the onset of extension in the Modoc Plateau and Steens-Alvord region began at ~1.3 Ma.