Graduate Thesis Or Dissertation
 

Two-stage opening of the northwestern basin and range in eastern Oregon : evidence from the Miocene Crane basin

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  • New data are presented that provide evidence for the onset of extensional deformation in the Northwestern Basin and Range within 1 million years after the eruption of the Steens Basalt at 16.5 Ma. New geologic mapping (1:24,000), stratigraphic sections, and ⁴⁰Ar/³⁹Ar dating of the Crane Basin rocks provide a control of the structural and depositional history of the basin. Extension-related horst and graben formation began at 15.5 Ma and continued until ~7 Ma. The Crane Basin, is one of a series of these half-grabens located to the east of the Harney Basin and Burns, Oregon on the footwall block of the Steens Mountain normal fault. Basin fill consists of volcaniclastic sedimentary rocks, tuffs, and lava flows. Key intra-basin marker beds include the 9.63 Ma Devine Canyon tuff and 11.5 Ma Visher Creek lava, in addition to a newly-discovered air fall ash with a ~12.5 Ma age, the 7.25 Ma Drinkwater basalt, and the 1.93 Ma Voltage basalt age. Crane Basin strata unconformably onlap tilted Steens Basalt. Angular unconformities exist in the Crane Basin strata, between the basal sedimentary rocks, overlying Devine Canyon tuff and capping Drinkwater basalt. Fault histories in the Crane Basin record similar rates of displacement from ~15.5 Ma to 9.63 Ma, divergent and heightened rates from 9.63 to 7.25 Ma and extremely slow rates after 7.25Ma. Together, these observations indicate that broad extensional deformation initiated following the eruption of the 16.5 Ma Steens Basalt and continuing until ~9.63 Ma (STAGE 1). After ~9.63 Ma deformation became heterogeneous and localized, migrating east to west, eventually shifting from the Crane Basin to the Harney Basin. The field relationships signal that the second deformational event was short-lived, ceased by ~7Ma (STAGE 2). Post 7.25 Ma the displacement on Crane Basin faults and the Harney Basin depositional record demonstrates that only minor fault deformation occurred. The findings of this study provide a previously undocumented resolution of the development stages of the margin of a continental extensional province. Moreover, these new data demonstrate a "temporal" link between magmatic and tectonic activity in Southeastern Oregon, suggesting that both deformation mechanisms jointly accommodated extensional strain at the margin of the developing Basin and Range Province.
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