TWO-STAGE OPENING OF THE NORTHWESTERN BASIN AND RANGE IN EASTERN OREGON

New data document onset of extensional deformation in the Northwestern Basin and Range (NWBR) within 3 Ma of eruption of the Steens Basalt at 16.5 Ma. Extension-related subsidence before 13 Ma and continuing until 7 Ma formed the Crane Basin (CB), a series of half grabens located to the east of the Harney Basin (HB) and Burns, OR on the footwall block of the Steens fault. New geologic mapping (1:24,000), stratigraphic sections, and ⁴⁰Ar/³⁹Ar dating of the CB rocks provide control on the structural and depositional history of the basin. Basin fill consists of volcaniclastic sediments, tuffs, and lava flows. Intra-basin key marker beds include the 9.63 Ma age of the Devine Canyon tuff, in addition to newly acquired dates of a ~12.5 Ma age for a newly-discovered air fall ash, a 7.25 Ma Drinkwater basalt age, and a 1.93 Ma Voltage basalt age. Basal CB strata onlap tilted Steens Basalt. An angular unconformity marks the contact between tilted CB strata and the Drinkwater basalt. Vertical fault separation and tilt of the Drinkwater basalt is greater in the west, on the edge of the HB, than it is in the east in the CB. Together, these observations indicate that extensional deformation initiated prior to 12.5 Ma and was localized in the CB until ~7 Ma (STAGE 1). After ~7 Ma deformation shifted westward to the HB, although a lack of evidence for faulting <7 Ma suggest that the HB deformational event was short-lived (STAGE 2). Moreover, these new data demonstrate a greater magnitude and degree of extension to the east in the CB than in the HB to the west. A similar contrast in the magnitude of extension can be seen more than 200 km along-strike to the south. The western limit during the first extensional period in the NWBR occurred along and to the east of an “extensional front” defined by the CB, Steens Mountain, and Pueblo Mountain. A continuous and larger degree of deformation in the CB characterize the first extensional stage (>13 to 7 Ma) as indicated by thickening of syntectonic strata towards half graben-bounding faults and the greater degree of bedding tilt, respectively. Minor, short lived extension to the west in the HB during the second stage may have been triggered by the widespread basaltic volcanism between 8 and 7 Ma in the High Lava Plains of east-central Oregon.